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"Oceanic Hydrates: more questions than answers"

by Jean Laherrere e-mail: jean.laherrere@wanadoo.fr

for Energy Exploration and Exploitation date May 3, 2000

Abstract

From an oil industry standpoint, methane hydrate is known as a major problem because it plugs casing and pipelines. From a media standpoint, hydrates provide an almost inexhaustible supply of articles concerning greenhouse effects, landslides, global warming and mysterious events such as the loss of aircraft in the "Bermuda Triangle". From a scientific standpoint, they provide much scope for academic research projects.

Oceanic hydrates have been recovered in some of the thousands of ODP/Joides boreholes, from which a total of over 250 km of core have been taken. Unfortunately, hydrates dissociate when brought on deck, and few samples were preserved for further analysis. Most of the oceanic hydrates are reported to be of biogenic origin, except where they overlie petroleum reservoirs, as in the Caspian Sea and Gulf of Mexico. The hydrates in the cores are found mostly as dispersed grains or thin laminae. Massive pieces of hydrate, greater than 10cm thick, have been found only at three sites. Downhole logs are unreliable indicators of hydrates due to cave-ins, and in many instances the inferred presence of hydrates depends on indirect evidence, such as seismic reflectors (BSR) or chlorinity changes in pore waters.

The oil industry requires much better evidence than this before attributing reserve status to a resource, yet in the case of hydrates, enormous deposits (such as recently declared in New Caledonia) are reported on the strength of no more than uncertain seismic information.

The gas hydrate stability zone (GHSZ) occurs in oceanic sediments over the first few hundred meters below the seabed. In this zone, any methane from organic material, including any seepages from below, is converted into solid hydrate, and is locked in place in the sediments. The origin of the methane is poorly understood, with even its biogenic origin being challenged.

Dissolved methane or free gas may precipitate at geological discontinuities such as faults, fractures and lithological boundaries, as well as at water salinity, temperature and pressure interfaces. In the past, the porosity in the GHSZ was thought to be dominantly filled by hydrate, thus providing a seal to gas, at and below the base of the stability zone. However, at the Blake Ridge, ODP Leg 164 found only minor porosity (maximum of about 5%) being filled by hydrate or gas. The recent Leg 172 in the same area failed to find any hydrates at all. A much higher concentration has been indicated in the Japan National Oil Company hydrate borehole in the Nankai Trough, although this is contradicted by other reports.

The Bottom Simulating Reflector (BSR) seismic reflector is caused mainly by gas bubbles at the base of the stability zone, which accordingly cannot act as a seal because the porosity is more than 95% filled by water, with the size of the pores and the gas bubbles being further factors. This is one reason why the BSR reflector does not correspond with the hydrate zones, as had been assumed. Cascadia, off Oregon, is one of the best places to investigate hydrates, as they crop out on the seafloor whereas on the Blake Ridge the first 200 m lack hydrates.

Prior to 1998, the resources of hydrates were often declared to be much greater than all known fossil fuels (coal, oil and natural gas). Ginsburg (1998) disputed such claims on the grounds that the hydrates are not continuously distributed vertically or horizontally. More recently, the USGS (Course 14, AAPG 2000) has drastically reduced its past estimates to a level where it is now claimed that hydrate accumulations may only rival the known reserves of conventional gas. These dispersed hydrate deposits may be better compared with dispersed oil and gas in petroleum systems, which are very much larger than the amounts contained in commercial reservoirs.

Many graphs on solubility of methane in water are computed from formulae, being rarely checked by experiments. Measurements in the laboratory seem to differ from field measurements in sediments. The solubility of methane in deep water is but poorly known, as few measurements have been taken, but it seems to be about a hundred times higher than in near surface-water. Methane released in deep water is dissolved in water, even when a large amount of methane is released. It cannot accordingly be the cause of any hazards. But little is known about the fate of the deep dissolved methane in upwelling seawater currents.

Methane hydrates are less dense than water when on the seafloor down to a certain depth, which is still unknown (2650 m for CO2 hydrate). So, extrusions of hydrate tend to float upwards, disappearing into the seawater. Log measurements in sediments report hydrates being denser than water, but direct measurements are lacking, and it would seem that such sediments are also subject to buoyancy pressure. Surficial pockmarks and mud volcanoes arise from gas expelled from overpressured, underconsolidated sediments — with or without hydrates being present.

Progress in understanding oceanic hydrates has not advanced much over the last twenty years because of the poor quality of measurements in soft sediments (cores, samples and logs), and because of the lack of calibration of seismic against a known oceanic hydrate system.

The chance of a viable production method being developed is slim because the oceanic hydrates are dispersed and occur in erratic patches. Only national oil companies in Japan and India are actively exploring for them.

Future progress may come from the deepwater exploration being undertaken by the oil industry using better tools, but oceanic hydrates seem to be similar in some respects to metallic nodules or gold in seawater–too dispersed to ever prove economic in most places. It is well said that they are a fuel for the future and likely to remain so.

 

 

Hydrates Everywhere

Hydrates are everywhere. Indeed we eat them in the form of carbohydrates as in sugar and starch found in fruit, vegetables and cereals. The study of hydrate has a long history. H. Davy discovered it in 1811, and M. Faraday established the chemical formula of hydrate of chlorine in 1823. During the 1930s, several gas pipelines were put into operation in cold climates, and it was found that methane hydrate formed in them, clogging the flow.

Methane hydrates were found in Siberia in 1964, and it was reported that they were being produced in the Messoyakha Field from 1970 to 1978. They were also reported in the Mackenzie delta (Bily 1974) and on the North Slope of Alaska (Collett 1983).

 

Oceanic Surveys

The International "Deep Sea Drilling Program" (DSDP), which was financed by several countries, started in 1968, and it organized cruises, called Legs, which investigated several Sites where closely spaced boreholes were drilled. It was extended in 1985, and renamed the Ocean Drilling Program (ODP), stimulated in part by an interest in hydrates. Russian research suggested that hydrates could occur at a depth of a few hundred meters below the seabed in deep water areas. Geophysicists simultaneously identified what was known as the Bottom Simulating Reflector (BSR) on deepwater seismic surveys (Markl, 1970, Shipley, 1979). It was soon assumed that the BSR marked the occurrence of hydrates, trapping free-gas below, and several Joides sites were designed to investigate the occurrences. These sites were planned by universities, not oilmen, although the latter were called in to advise on safety. (The author served on the Pollution Prevention and Safety Panel in the early 1980s). A total of 625 sites were drilled by the Glomar Challenger between 1963 and 1983 under the auspices of the DSDP, but it was decided not to drill through the BSR to avoid the risk of a blow-out. A further 500 sites have been drilled since by the drillship, the Joides Resolution.

Together, these programs have investigated a large number of sites in water depths up to 7000 m, the average being 3500 m. The ODP program concentrated in shallower waters but took more cores and, thanks to a safer drillship, penetrated the BSR. The details are as follows:

legs

number sites

number

holes

average waterdepth

cores

km

number

cores

core/

hole m

DSDP 1968-1983

1 to 96

624

1092

3527 m

92

84

ODP 1985-1998

100 to 177

469

1247

2624 m

161

26287

129

 

Together, as much as 250 km of cores have been recovered, with an average recovery of about 60%. For each site, an average of 3.4 nearby boreholes were drilled to investigate the immediate surroundings. Accordingly, it can be said that the first few hundred meters of the seabed in oceanic areas, covering some 360 M.km2 have been thoroughly explored. The Continental Shelves and the Slope (200-3000 m) cover respectively 7.5 and 15 percent of the oceanic area, together comprising some 80 M.km2

Publications

The cynic might say that the study of hydrates is tailor-made for academic research, insofar it can continue for a very long time without providing conclusive results, finding more questions than answers. It is furthermore a wide subject covering such matters such as fuel resources, transport, environmental hazards, global warming, turbidite formation, submarine slides and eruptions, drilling hazards, and even the Bermuda triangle mystery. One of the reasons for the inconclusive results is that hydrates decompose into water and methane on being brought to the surface, so it is difficult to study them in their original state.

Literally hundreds of papers have been written, with some authors contributing in abundance, but they are characterized by generalizations, speculations, quotations and references, rarely supported by useful facts. In fact, many such papers are designed to attract funding for still further research, often repeating what has already been done.

The Blake Ridge investigations are a case in point. They commenced with Leg 11 (Sites 102, 103 and 104), followed in turn by Leg 76, which allegedly found hydrate at Sites 533, and Leg 164. The paleoclimate Leg 172 drilled more holes on the Blake Ridge without finding any hydrates. A new 3D seismic survey is now proposed to reinvestigate the area.

Leg 170 returned in 1997 to the Costa Rica Trench, which had been already drilled on Leg 84 in 1982. Only one site out of five recovered hydrate in volcanic ash layers with 0.7 to 1% methane.

Cascadia was investigated by the ODP on Leg 146 in 1992, and by Geomar and Tecflux (Hydrate Ridge) in 1996. A further project is proposed either as Leg 198 in 2001or by a Portable Remotely Operated Drill operated by Tecflux. Cascadia seems the best area to search for hydrates. Since 1992, Tecflux, which is an international collaboration led by Geomar (University of Kiev Germany), the Oregon State University (USA) and the Monterey Bay Research Institute (MBARI, USA) has become one of the prime researchers on the subject.

JNOC drilled (Nov.16-Dec.1 1999) a borehole in 950 m of water finding 16 m of hydrates in the Nankai Trough (but the results are confidential). The same area had already been investigated by Leg 87, which observed the BSR but found no sign of hydrates. JNOC plans to drill another borehole soon, which may suggest that the results of the first were less than conclusive; and Leg 198 will return to Nankai in 2001 with a different objective. The research goes on; and while it never seems to deliver a concrete answer, it often does succeed in posing a new question.

On land in permafrost, Ginsburg, the Russian expert, disputed in 1993 the claims that the Messoyakha Field was producing natural hydrates, as quoted in US papers. A 1974 paper on hydrates in the MacKenzie Delta reported that hydrates were interbedded with gas and water, especially in borehole Mallik L-38. Another borehole at the same site was drilled in 1998, financed by JNOC, finding hydrates at depths between 897 and 1110 m beneath the base of the permafrost, which occurred at 640 m. It confirmed the early report, adding nothing new, apart from more cores. But it is interesting to compare land hydrates and oceanic hydrates, as they are completely different, particularly in regard to the reservoir.

The U.S.Department of Energy supported research from 1982 to 1991 to develop a full understanding of hydrates. This resulted in establishing the presence of hydrates in Alaska; studies of fifteen offshore hydrate basins; the development of production models for de-pressuring and thermal extraction; and the development of a Test Lab Instrument.

This funding ceased in 1993 as the USDOE administrators concluded that any viable hydrate production would be decades into the future, but work continued through a number of other institutions, including the USGS. Still another project has been unveiled in 1998, with identical objectives to that of 1982 by the US Department of Energy - Office of Fossil Fuels (DOE/FE-0378). It has resulted in the Methane Hydrate Research and Development Act of 1999 (HR 1753) with funding from 2000 to 2004. This act incidentally quotes an article in the Scientific American of March 1998 "The end of cheap oil" by Campbell & Laherrere. [sp]

Papers commonly state that it is impossible to assess the size of hydrate resources, but then go on to make estimates based on uncertain parameters, implausibly quoted to several significant digits. Previously, it was assumed that the porosity in the oceanic hydrate sequences were almost completely filled by hydrate, but then Leg 164 found that only 1-2% were filled. The range of uncertainty is very great.

Present recommendations on the potential of hydrates

The leading US hydrate experts E.D. Sloan, P.G. Brewer, T.S. Collett, W.P. Dillon, W.S. Holbrook, K.A. Kvenvolden, C.K. Paull published a paper in Feb 1999 "A Gas Hydrate Research Program", which included the following statement: "While gas hydrates occur in numerous offshore marine basins and in onshore areas underlain by permafrost, there are only vague ideas as to their abundance.

Unless progress is made in sensing and detecting gas hydrate, research regarding occurrence, distribution, and concentration will be severely impeded.  Gas hydrate research is an unusual challenge because traditional techniques of geologic sampling do not apply. We are currently dependent on geochemical proxies to measure amounts of gas hydrate, with techniques that we know are flawed because of uncertainties in the initial conditions.  Moreover, there are so few calibration sites that remote detection methods are poorly constrained."

"The most vital cross-cutting research issues are:

1. determining the distribution and concentrations of hydrates,

2. finding better ways to detect and to evaluate gas hydrate accumulations,

3. determining the effects of hydrate on sedimentary processes, and

4. defining the controls on hydrate masses in nature."

The leading Russian hydrate expert, G.Ginsburg, in his book "Submarine gas hydrates" 1998 (written in 1994 and revised) with Soloview states that: "a knowledge of where and how far gas hydrates are extended, what the way of their formation and dissociation is, what hydrate accumulations look like is the most significant and topical problem of the scientific deep sea drilling."

The leading Norwegian hydrate expert M.Hovland questions that hydrates could be commercial (1998).

In the United States, the Methane Hydrate Research and Development Act of 1999 (HR 1753) "requires the Secretary, in awarding such grants or contracts or entering into such cooperative agreements, to: (1) facilitate and develop partnerships among government, industry, and institutions of higher education; (2) undertake programs to develop basic information necessary for promoting long-term interest in methane hydrate resources as an energy source; (3) ensure that the data and information developed through the program are accessible and widely disseminated; (4) promote cooperation among agencies that are developing technologies that may hold promise for methane hydrate resource development; and (5) report annually to Congress on accomplishments. It authorizes appropriations for FY 2000 through 2004."

The research goes ever onward but seems incapable of answering the basic questions. A certain momentum is propelled by many vested interests that have staked their reputations on the elusive hydrate.

Gas hydrate stability zone GHSZ

Experimental studies in Norway and Russia have shown that natural gas hydrates are stable for up to two years when stored -15 to -5 °C at atmospheric pressure (Gudmundsson, 1996).

Numerous graphs of pressure (or water depth) versus temperature, have been drawn from theoretical computations where the gas hydrate stability zone is shown. But these values depend not only on pressure and temperature but also on the composition of the other components (guests or hosts) as in particular salinity, CO2, clay, and also the pore space. Based on these simplistic charts, and assuming a water temperature at sea floor being around 0 to 4 °C, the GHSZ in sediments could start from water depth of between 300 and 500 m. and attain a maximum thickness of 1000 m. But, experiments in natural conditions do not coincide with laboratory experiments or theory. Clennell, 1999, states: "Laboratory tests do not cover the range of pressures found in nature. We argue that none of the previous experimental studies ( Makogon, Sloan, Handa, Bondarev, Melnikov) can be applied directly to the marine subsurface". The presence of 10% of ethane in the gas mixture allows hydrate to be stable at 60 m and 6°C, when pure methane is stable only at 400 m (Lerche & Bagirov 1998).

There is an equilibrium between gas hydrate and gas dissolved in water, but the values are not clearly known (see below in the chapter on solubility).

Free gas is found below the GHSZ. Most papers assume that free gas occurs when the concentration of methane is above the solubility of methane in water (Haq 1998), but it is not at all clear, and the values are not known.

Historic Concept

Previously, it was assumed that the porosity in the oceanic hydrate sequences was almost completely filled by hydrate, but then Leg 164 found that only 1-2% was filled. The range of uncertainty is very great. But more important is the change of concept.

-Pre-1995 static and continuous concept:

From 1968 to 1983, the JOIDES Glomar Challenger drillship surveyed 96 Legs. Hydrates were assumed to be continuous both vertically and horizontally, and they were thought to fill the first hundred meters of sediments in the GHSZ as well as sealing free gas below. On this basis, the amount of methane in hydrates together with the underlying gas would reach huge proportions. It was assumed that the methane is biogenic and converted directly into hydrates, with no migration and flow.

The BSR was thought to be due to the higher velocity of the hydrates, and it was used to estimate the amount of hydrates, but its true significance was not investigated because it was feared that the assumed gas might cause a blow-out.

The largest massive hydrate occurrence was cored at Site 570: one meter was recovered from a possible three meters shown on logs.

Later, between 1985 and 1995, a superior drill-ship, the ODP JOIDES Resolution, (formerly the Sedco/BP 471) with equipment to deal with blow-outs was used to investigate the BSR in Legs 100 to 163. It is found that there was no correlation between the BSR and hydrates. This discovery completely undermined the basis for early estimates of the potential size of the resource.

-Post-1995 Dynamic and Discontinuous Concept

Leg 164 (Blake Ridge: N. & S. Carolina) in 1995 found that hydrates are discontinuous and fill only a few percent of the porosity. It transpires that the BSR is due to the lower velocity of the free gas below the GHSZ. It was concluded that the methane could have come from below, and the importance of the origin, fluid flows, and high pore pressure was recognized.

The evidence of seabed pockmarks, mud volcanoes, gas vents, gas plumes and occurrence of large seabed mounds, as in Norway, confirms a new dynamic concept of the origin of hydrates.

New organizations have started to investigate these phenomena, including GEOMAR in the Tecflux project on Cascadia (Oregon) using a remotely operated vehicle and drill.

Sloan et al . 1999, notes: "We need to investigate the dynamics of the hydrate/gas system - i.e., where, how, and at what rate does methane form, migrate, collect in, and exit from hydrate/gas deposits"

Japan became particularly interested in hydrate investigations and general oceanographic drilling. It is reported by "Gas Hydrate PPG", June 1998, that: "Plans for a two ship program with one ship equipped with a riser to be operated by Japan and another ship with JOIDES Resolution-like capabilities operated by the US are developing". JNOC drilled a hydrate exploratory borehole in 1999 in 950 m of water in the Nankai Trough, penetrating 1855 m of sediments, following another borehole (Mallik) drilled onshore in 1998 in the Mackenzie delta by the Geological Survey of Canada to sample permafrost hydrate.

 

Direct evidence of hydrates

-from cores

It is reasonable to expect that the many thousands of boreholes that have been drilled and cored in the oceanic seabed would have encountered ample indications of hydrates. But hydrates have been found in cores only along the Pacific trenches (Peru, Middle American trench, North California and Cascadia); Blake Ridge (NE of the Florida Peninsula); the Black Sea; the Okhotsk Sea; the Okushiri Ridge and the Nankai Trough in Japan. They have also been found in the petroleum producing basins of the Gulf of Mexico and the Caspian. These ten occurrences mean little when compared with the world’s Petroleum Systems, which number more than 500 as listed by USGS (97-463).

Ginsburg 1998 reports on the hydrates found in cores:

Hydrates found in cores

ODP:

Area

Country

Date

Leg

Site

Remarks

Middle American trench

Mexico

66

490 to 492

Volcanic ashes

Middle American trench

Guatemala

1979

67

496 to 498

Blake Ridge

US

1980

76

533

Middle American trench

Costa Rica

1982

84

565

Middle American trench

Guatemala

1982

84

568 ; 570

Gulf of Mexico

US

1983

96

618

Peru trench

Peru

1986

112

796

Okushiri Ridge

Japan

127

808

Nankai

Japan

131

892

Cascadia Oregon

US

1992

146

995 to 997

Blake Ridge

US

1995

164

1041

Middle American trench

Costa Rica

1997

170

Only 20 sites out of more than 1000 sites

 

Outside ODP: shallow coring (2m) for geochemical studies:

Area

Country

Date

Site

Gulf of Mexico

US

1983

GC 184,204,234,257,320, MC, GB 388

Okhotsk sea

FSU

1986

1395, 1423, 91-01-05, 07, 23, 91-02-39, 40, 41, 42, 44, 57

Caspian sea

FSU

1986;1988

Elm & Buzdag mud volcanoes

Black sea

FSU

1988

17, 18, 53, 57

Northern California

US

1991

Eel River 7 sites

Some of these finds were associated to mud volcanoes

But in fact there are only three major known occurrences of massive hydrates:

-14 cm were found at Site 498 (Leg 67 Guatemala)

-105 cm were recovered at Site 570 on Leg 84 off Guatemala (not far from an exploratory well), at a location where there was no BSR. On logs, the hydrate body is 3-4 m thick with an unknown lateral extent, there being no corresponding seismic reflection. This hydrate is evidently related to thermogenic methane.

-5-7-14 cm were found at Site 997A (Leg 164 Blake Ridge), but it was evidently a very local occurrence as no hydrates were found at same depth only 20 m away, at Site 997B. It confirms the view of Ginsburg (1998) that hydrate accumulations are restricted both in horizontal and vertical extent.

We may distinguish the massive hydrates that are related to faults, allowing the escape of thermogenic methane from depth, from the dispersed grains or thin laminae, which are of biogenic origin, resulting from microbial action.

Ginsburg has widely sampled the seabed off Russia, finding numerous hydrate occurrences in millimeter to centimeter thick layers. But these hydrates are discontinuous and are found in association with either lithological changes (volcanic ashes in the Middle American Trench) or fractures or faults. Homogeneous nucleation can occur only in larger pores or fractures..

The problem is that the ODP Pressure Core Sampler (PCS), which is supposed to retrieve cores at essentially in situ pressures, does not always work properly. Furthermore there is only one such unit in operation. Currently, no provision exists for making measurements on samples within the PCS or to extract samples at pressure. In 1997, the HYACE project began work with funding from the European Science Foundation to improve existing PCS tools.

"Gas Hydrate 1998 states: "Existing ODP technology does not provide any routine data that is readily interpretable about the in situ volumes of gas or gas hydrate in marine sediments. Unfortunately, because large pressure decreases occur during core recovery, most of the original gas is lost before the core reaches the deck of the JOIDES Resolution."

-JNOC well at Nankai Nov.1999

JNOC drilled an exploratory hydrate well in Nov 1999 in 950 m of water. But the results are confidential. Unpublished reports are contradictory. The first report said that the results were promising with 150 ft of hydrates in 20-30% porosity with 80 percent of the pore volume filled by hydrate. But a second report revealed that cores from three hydrates layers had a net pay of 16 m (a net/gross ratio of 28%) and the methane occupied 80% of the pore space -- the (remainder presumably being air or ethane?). Clearly, more than one interpretation is possible, and in the absence of proper published reports, it is difficult to determine the significance. Yet again, it is hoped to drill a follow-up borehole to check the observation, but since first well, costing $45 million, used most of the $50 million budget, it remains to be seen how much more of this research will be financed by government.

-from dredging or TV observation (remote operated vehicles)

The American-German project Tecflux has secured by grab large chunks (50 kg by 785 m) of hydrate from the seafloor off Oregon in an area where many gas vents are observed. More work is planned on a mound called the Hydrate Ridge.

We are left with the unavoidable conclusion from the colossal amount of research, which has been undertaken at huge expense, that that there is in fact no evidence for any massive hydrate deposits extending by more than 10 meters. Nevertheless, undeterred by this negative evidence, the researchers continue to seek support, now being forced to investigate indirect evidence.

Proxy (indirect) evidence of hydrates

Since coring failed to deliver any real evidence for substantial hydrate deposits, attention turned to indirect indications. These included: first, the famous and now discredited BSR seismic reflector; second, the presence of chloride component in the water; and lastly logs.

Twenty-nine of the hydrate occurrences listed by Ginsburg, 1998, are based on BSR evidence and now have to be rejected.

BSR (Bottom Simulating Reflector)

As already mentioned, the so-called BSR event typically occurs at a depth of between 200 and 600 m beneath the seabed reflector, which it parallels, cutting across other reflectors. It has been interpreted as the base of the zone at which hydrates form. Theoretically, it should deepen with water depth. It has been variously explained as being due to the higher velocity of hydrates or to the velocity contrast been hydrates and free gas or the free gas only.

There are discrepancies between velocities given by VSP (recorded with a geophone in the well) which is not very accurate and the Sonic log, which is commonly disturbed by cave-ins. The diameter of the hole very often exceeds the maximum range of the caliper.

The velocities at Leg 164 are a good example of poor quality and disagreements between VSP, Sonic and seismic surveys. Site 994 was drilled at a location lacking the BSR but close to Sites 995 and 997 with strong BSR. The three locations are similar and it is difficult to explain why the BSR disappears and to calibrate exactly the origin of the reflection. There are cases of BSR without hydrates and cases of hydrates without BSR.

Previously, it had been assumed that the hydrate cemented the sediments above the BSR, with free gas filling the pore-space below. The concentration of hydrate and the so-called free gas in mainly unconsolidated clays (with as much as 60% porosity) is assumed now to be about 1% of the porosity. In fact, the presence of only a few percent of gas in a reservoir can drastically lower its seismic velocity (Domenico 1976), so it is impossible to determine gas saturation by seismic means. Hydrates at such low concentration are unlikely to seal any gas. Free-gas occurs in fact as bubbles in water.

Most serious authors now agree that the BSR is due to the free-gas (1-5% of porosity) low velocity below the GHSZ. Hovland et al 1999 state: " "A transect of three sites (Hole 994 without a BSR, and Holes 995 and 997 with prominent BSR) was drilled across the Blake Ridge to investigate the association between gas hydrates and the BSR. Nothing recovered in the cores directly indicated a reason for presence or absence of the BSR. The lithology was uniform at the three sites with gas hydrates present in all the three holes between 200 and 450 mbsf." "The BSR is confirmed as a reflector associated with small volumes of freegas ".

This is why there can be a BSR without hydrates, and why there can be gas below the GHSZ but no hydrate in it. Accordingly, the use of the BSR as a hydrate indicator is highly unreliable. We may thus reject the recent report of a large deposit of hydrate offshore New Caledonia. Furthermore, using the BSR to estimate the volume of hydrates is absolutely unacceptable, as confirmed by Geomar, which reports that "Above all, seismics do not yet provide a quantification of gas hydrate occurrence".

In the past, it was assumed that dissolved gas in water ("Fizz water") gave a lower velocity, but experiments at the Houston Advanced Research Institute did not find any change. Accordingly, the BSR should correspond only to free-gas, presumably where the concentration of methane exceeds the solubility of methane in water. Free-gas bubbles should rise, if their size is less than the pore space, and concentrate at the top of the zone. Those crystals of hydrate, which are lighter than water and finer than the pore space, should do the same within the GHSZ, where the sediments are often the same as the underlying sequence, going also into the water. Unfortunately, in fine-grained clay sediments, as found for example at Site 995A, the pore-size distribution is around 0.5 micrometer, when gas bubbles seem to be in the order of ten micrometers. A satisfactory oil reservoir requires a pore throat is over 10 micrometers. In short, it means that there are no hydrate reservoirs in the Blake Ridge area.

There are several other possible explanations for the BSR reflector:

- it may be an artifact of the recording equipment, related to automatic gain control;

- it may represent a formation boundary, with the crossing events being artifacts caused by diffraction from faults or the seabed

- it may be a questionable pick

- it may represent diagenetic contrasts or compaction;

- it may represent opalisation;

- it may be a multiple of a water contrast (oceanic currents or sharp change of temperatures within the oceans, namely thermoclines) or the Sofar Channel (minimum of sound velocity in waters around 1000 m) which transmits sound on a very large distance (used by whales or submarines)

The literature contains much evidence for such alternative explanations or questionable picks for the BSR:

-A Barents Sea Bright Spot (Andreassen et al., 1990, Laberg et al., 1998) could be due to opal conversion, which is known to affect velocity (Tribble 1992; Westbrook, 1994; Lee 1996; Pecher 1997)

-Slides illustrated by Mienert 1998 fig 12 showing two BSRs; and Neben 1998 fig 8: short reflection lacking any BSR characteristic;

-Base of blanking (Schmuck 1993 fig 4&5);

-Discomformites and sedimentary changes as at Site 184 Leg 19, Bering Sea (Makogon 1981 fig 56);

-Diffraction, as on the USGS survey on Leg 164 (Blake Ridge) related to a nearby diapir

-Thermoclines (there are strong currents as the Gulf Stream or El Nino) within the ocean which can cause multiples, as seen for example on Leg 112 off Peru (Pecher et al 1996). However, the thermocline is probably an explanation only in special cases. It is worth mentioning in this connection the remarkable so-called SOFAR oceanic acoustic layer, used by dolphins and whales, and later the military, to communicate.

A detailed evaluation of all the many alternative explanations of the BSR is beyond the scope of this paper, but it is now recognized by most experts that it is related to the free-gas (1% of porosity) low velocity below the hydrate zone.

Others: log, chlorinity, and gas sample

Electric logs are used around the world in the petroleum industry to measure rock and fluid parameters, but they have to be calibrated against core material to develop the proper algorithms. In the case of hydrate research, it has not been possible to make the calibration because the hydrates disappear when brought to surface. Its characteristics are accordingly poorly known, in part because concentration is too low (1 to 10%). In the oil industry, most of the tools are calibrated to distinguish gas and water zones, but they are not capable of recording small concentrations. Furthermore, drilling in unconsolidated sediments, as is the case in most of the sediments where hydrates are reported, with poor drilling mud control, gives large cave-ins, large invasions, which make for unreliable log readings. The ODP 1998 states: "With respect to logging, ODP is on the trailing edge (not the leading edge) of logging technology. In comparison with industry, ODP typically operates in really soft sediment. Thus, the hole conditions are typically rather poor and the quality of the logs suffer".

The best core of massive hydrate was one meter at Site 570 but the logs give 4 m from sonic, 2.7 m from density and 0.6 m from electric logs. The discrepancies show how unreliable the data are. But, the new LWD (logging while drilling) technology may give better results.

The chloride concentration in pore water is also taken as a proxy sign of hydrate. But in fact the change is not so much in the amount as in the erratic behavior of the measurements (see graphs by Paull et al., Leg 164). Temperature is also used.

It is evident therefore that any occurrence of hydrate based on log, chloride or other sample studies cannot be regarded as more than highly speculative until confirmed by core evidence.

 

Free gas

It has been suggested that there is also a potential in the free gas below the GHSZ. But Claypool 1998 thinks that: "a significant continuous gas column cannot accumulate beneath the gas hydrate stability zone." and that: "conventionally completed wells in the free gas zone beneath the gas hydrate stability zone will produce only water"

 

Gas and fluid flows

As mentioned previously, Ginsburg & Soloview, 1998, relate shallow sub-bottom gas hydrates with fluid vents in the Caspian, Black and Okhotsk Seas, and the Gulf of Mexico. They note "GHSZ creates a geochemical barrier for upward-migrating methane. However, a quantity of dissolved methane and at least some free gas methane pass through this barrier." : "observations show that considerable part of filtering free gas can also pass to the hydrosphere through the hydrate stability zone".

Hyndman and Davis, 1992, believe that hydrates are deposited in convergent margins, coming from deep fluid flows from under-thrusted sediments.

Kvenvolden 1993 writes: "The active fluid-movement model is useful in accounting for gas hydrate in active- or convergent-margin settings, for example, the Cascadia margin and the Middle America-Peru- Chile trench systems of the Pacific ocean"

Geomar on the Hydrate Ridge (Cascadia) reports: "solid hydrate was observed to form within the gas samplers as well as on the camera itself, supporting the conclusion that methane is rapidly transported to the seafloor from beneath the BSR within discrete conduits, most likely separated from significant amounts of pore water. When discharged at the seafloor, some of the methane precipitates as hydrate and some continues to rise within the water column. Bubbles were observed with the ROV up to 50 meters above the seafloor." Tecflux: " Fluid expulsion as been observed to concentrate at surface faults, both compressional thrusts and deep-seated strike-slip faults":.

Numerous papers report on the presence of gas vents in many places. They are often related to mud volcanoes where water flows are large. So, fluid flows are very important to study the amount of gas transported. Most of the large flows occur in association with faults.

Mud volcanoes occur in many areas. The highest (700 m high) known mud volcano is on the Makran coast of Baluchistan (Pakistan). Although they occur in abundance in places like Azerbaijan, Pakistan, Trinidad and Colombia, most occur on the seabed. Mud volcanoes have roots that extend several kilometres underground. They actually represent "safety valves" for high underground pressures. They periodically give catastrophic outbursts of mud and gas, caused by the eruption of large volumes of gas (Hovland, 1999 web)

It is curious to find that in some cases these fluid flows and gas vents in deepwater are subject to not only tidal influences (Linke 1999) but also other short term variations, spanning weeks. Linke notes: "These temporal changes give a unique insight into the complex dynamics behind the hydrates and gas venting and must be understood before mechanisms responsible hydrate"

 

Geological setting:

The oil and gas producing basins, such as the Gulf of Mexico or the Caspian have not been considered in this article, which deals with oceanic hydrates. The Minerals Management Services (USDOI MMS) has recently determined that gas hydrates are to be covered in the normal hydrocarbon leases in the Gulf of Mexico.

Most of hydrates have been found in subduction basins along active margins, where a plate is over-ridden by another plate, creating an accretionary prism, as in the Pacific trenches: Peru, Middle American Trench, North California and Cascadia (offshore Oregon), Okushiri Ridge, Nankai Trough; and the Sea of Okhotsk. But hydrates have also been found in other settings: in the Black Sea, which is an intracratonic basin where hydrates are associated with mud volcanoes, and the Blake Ridge which is on a passive margin.

So far, the largest and most convincing hydrate occurrences are the US Blake Ridge and the US Hydrate Ridge (Cascadia). But they differ markedly in geological and physical terms. One area is a passive margin, the other an active margin. Hydrates crop out on the seafloor of the Cascadia with gas vents, whereas on the Blake Ridge they are found only from 200 m to 450 m, being absent from the first 200 m

Origin of gas

Methane is continuously produced at the surface of the Earth from the fermentation of a range of organic materials including termites, bovine digestion, rice paddies, marshes. It also comes from hydrocarbon source rocks during their chemical evolution from organic deposits on the seabed to deeply buried oil and gas deposits. In addition, it escapes from imperfectly sealed deeply buried hydrocarbon source-rocks and reservoirs. The so-called "eternal flames", which were worshiped in antiquity in several areas around the Middle East, are fuelled by methane. Methane is the most difficult hydrocarbon to seal, and most of caprocks, except massive salt, slowly leak methane. Gasfields are commonly subject to constant re-charge from the source-rock, which may partly compensate for the leakage.

The generation of gas from hydrocarbon source-rocks may be due to bacterial action giving what is termed biogenic gas, which may be distinguished by its isotopic signature from thermogenic gas, that is due to the heating of the source-rock on burial

"Molecular and isotopic compositions of methane from most recovered gas hydrate-samples indicate a microbial origin. Methane with a thermogenic signature has been seen at only three places--Gulf of Mexico, Caspian Sea, and the Middle America Trench" Kvenvolden 1993.

But the interpretation of the gas composition (mainly C13) is controversial. In the Western Siberia Basin, the largest gas accumulation on Earth, the gas was previously reported by some geochemists to be biogenic (98% methane), but now a new theory (Littke, et al. 1999) proposes that it is thermogenic, having been transported long distances (500 km), dissolved in water flows from the southern oil basins.

If molecular and isotopic compositions are unreliable, the origin of the gas flows should give an answer if correctly studied.

Methane, being a very simple chemical component, is known to occur on certain planets, including Europa, and in comets. It is also found in crystalline rocks and in volcanic gases, but in this case might be derived from brines in sedimentary sequences involved in subduction, as volcanic belts are commonly associated with subduction close to trenches.

We conclude that methane may be of abiogenic, biogenic or thermogenic origin.

 

Solubility of methane in seawater

Minshull.1989 states: "The stability of gas hydrate depends not only on the pressure and temperature conditions but also on the concentration of natural gas present in the sediment, which must exceed the solubility of gas in seawater".

Most papers are vague or ambiguous on the issue of solubility. The units of measurement differ from author to author (ppm per volume or per weight, mole fraction, mole/L, nmol/ml sediments, nmol (incorrectly written in mM), L/L, nl/L, cm3/g, cf/b, g/L), giving the amount of gas to water or sediments. The equivalence or ambient conditions are rarely stated.

The following equivalents are tentatively assumed by the author, who is not a chemist, in the absence of any good practical manual:

1 mole H20 = 18 g, 1 liter of water contains 1000/18 moles, 1 mole CH4 = 16 g, 1 mole gas (perfect) at 0°C and 1 bar occupies 22.4 L and contains 6 10E23 molecules

if M=number of moles of CH4 per liter and F = mole fraction multiplied by 1000 (mole fraction = number of moles of the component versus the total number of moles of the solution)

F= 1000 M/(1000/18 +M)

M=1000 F/(18*(1000-F))

when mole/L = 1, mole fraction *k =17.7

when mole fraction *k =1, mole/L = 0. 056

1 mole/L = 22.4 L/L = 125 cf/b

1 cf/b = 0.18 L/L

1 ppm = 45 nmol/L

The solubility of methane in seawater is around 0.85 the solubility in fresh water. Many papers do not report to the solubility at atmospheric pressure, giving it against certain pressure and temperature values. So far, no complete plot of the solubility of methane versus water depth has been developed as a means of determining the solubility at the seafloor. However Figure 1 is a compilation of available data from Bonham, Handa, Katz, Ginsburg, Mac Auliffe, Clennell and Louisiana State University

Figure 1:

 

 

Most of graphs are computed from formulae, but few are calibrated from experiments in situ.

Clennell et al., 1999 state: "Laboratory tests do not cover the range of pressures found in nature. We argue that none of the previous experimental studies ( Makogon, Sloan, Handa, Bondarev, Melnikov) can be applied directly to the marine subsurface."

Figure 1 shows a dramatic increase in the solubility of methane with pressure, which may explain the absence of giant gasfields in high pressure, high temperature (HPHT) conditions. This evidence suggests that gas at high pressure is mainly dissolved in water. It follows that where some free gas is present, the amount of water is limited, meaning it turn that the reservoir is small.

The Louisiana State University, in a recent article on leak detection in deepwater pipelines, reports that the solubility of methane can increase up to 150 times, from 0.68 cf/bw (0.12 L/L) in shallow water to 24-100 cf/bw (4-18 L/L) under deepwater pressures and temperatures. The mean value is not far from Bonham 1978, who gives 50 cf/bw (9 L/L) for 5000 m (Handa gives 7 L/L, but Clennell gives 3.3 L/L). Salinity and temperature can explain some discrepancies. These conclusions are in agreement with the values given on geopressured gas brines of the Gulf Coast, which have produced around 30 cf/bw (5 L/L). But Ginsburg 1999 shows in figure 9 that the solubility of methane for the first hundred of meters of sediments stays around 2 L/L, notwithstanding a range of water depth from 500m to 6000m (the temperature decreases only from 5°C to 2°C). This seems to conflict with the increase in solubility with pressure: at 6000m the solubility should be four to eight times greater.

There is a major discrepancy between the solubility of methane in seawater as given by the hydrate experts and the pipeline and gas experts. The former say that solubility factors determine the hydrate stability, whereas the latter state that 0.5 Tcf can be dissolved in a cubic mile of deep water. Site 994 found at 569 m below the seabed (3400 mbsl) a concentration of 5 L methane/L water, which was considered too high to be true, yet it corresponds with the solubility proposed by Bonham, the gas expert, for these conditions. On the Hydrate Ridge, the values reported by the AGU 1999 are far apart: 40 micromol CH4/L total sediment in sediments (40 000 nmol/L = 0,9 mL/L) by Heeschen (1999) at gas sites (and 3000 nmol/L at sites without degassing), 0.05 mL/L water in a methane plume by Torres (1999) and a concentration of 250% saturation in surface waters by Rehder (1999). These diverse readings underline the necessity to use the same unit, and they also demonstrate the lack of synthesis on the whole water column. Rehder (personal communication) gives the following data: the equilibrium of methane concentration in water with today’s atmospheric methane content (1.8 ppm) is between 2 and 3.5 nmol/L, depending mainly on temperature. Near the Equator, 2 nmol/L is a good value, in Antarctic waters it reaches the 3.5 nmol/L. In Cascadia, the saturation is 3 nmol/L, so 7 nmol/L is 250% saturation (or 150% super-saturation).

In California, methane concentrations of natural hydrocarbon seepages off Coal Oil Point near Santa Barbara (Washburn & Clark 1998) exceed 2000 ppm of the total dissolved gas extracted from seawater, i.e. 90 micromol/L; or twice the values of Heeschen. The value reaches 3000 ppm at 50 m depth.

In the Beaufort Sea in Alaska, (Lorenson & Kvenvolden 1995) methane concentrations in the upper 3m of ice-free water never exceeded 18 nM and are commonly between 6 and 14 nM.

Hovland et al 1999 report "Methane contents in the GHSZ 0.04 —0.84 moles of methane per liter of pore-water, corresponding to a gas hydrate saturation level of 9% of the pore space. Beneath the GHSZ a measured gas content of about 2 moles per liter was observed which gave a calculated gas saturation of about 12% of the pore volume."

This short inventory shows that it is difficult for non-chemists to have a clear picture of the methane concentration in the seawater, both for the saturation concentration and the concentration at different sites.

Andersson 1997 (Norwegian University of Science and Technology) who collected 28 papers on solubility concluded: "more experimental data is needed in the area of low temperatures (0 to 25°C) and intermediate pressures (0 to 90 bar) in order get reliable correlations for predicting solubility in this area".

 

Density

About 15 percent of the weight of gas hydrate is gas, and 85 percent is water, meaning that the overall density is about 950 kg/cubic meter. These measurements vary with the gas composition and the pressure and temperature at the time the hydrate is formed (Gudmundsson 1997). So, methane hydrate is normally lighter than water and should rise up to the surface. In the Hydrate Ridge, the result is a periodic release of large chunks of hydrate from the sea floor. They float to the surface and leave behind a chaotic topography of mounds and depressions of up to a meter in elevation (Suess, 1999). Brewer has produced CO2 hydrate by releasing CO2 both at 1000 m depth, when it floated, and at 3600 m, when it sank. There is a project to sequester CO2 by releasing it in liquid form at a depth of 3000 where it will be converted into hydrate, staying on the seafloor. Laboratory experiments show that the neutral point of buoyancy for CO2 hydrate is 2650 m. But the neutral point of methane hydrate is not reported.

 

Shallow water flows and high pressure

Hovland mentions the importance of "shallow water flows" (SWF) in deepwater sediments. Such flows are associated with under-compacted and over-pressured sediments, as found also in onshore drilling when excess water causes theover-pressure. Woolsey J.R.1999 in his USHR statement on hydrates, reports: "in 60 blocks of the GOM, SWF typically occur in water depths exceeding 1700 feet and originate in sand deposits located 1,000-2,000 feet below the sea floor. It is not a phenomenon peculiar to the Gulf of Mexico. SWF present the greatest obstacle to deep water drilling worldwide. The flows are very possibly associated with the dissociation of gas hydrates that had formed in the pore spaces of the sand bodies."

It seems that SWFs can occur in the GHSZ, but it does not mean that SWFs are due to hydrate conversion, as the only evidence of combined SWFs and hydrates is reported in the permafrost of the MacKenzie Delta and not in the Gulf of Mexico. In the deepwater Gulf of Mexico the Ursa oilfield, a platform with nine wells, had to be abandoned because of SWFs, and replaced by a new platform at a location without SWF.

Ruppel 1997 states: "However, the link between overpressure and gas hydrate distributions has not yet been fully explored and clearly represents a critical issue for the safety and success of future exploration and recovery programs. "

Pockmarks, gas vents, gas plumes, hydrate outcrops and mounds

Gas vents have been observed in many places on the seafloor, but the bubbles disappear fairly quickly on their way to the surface. Gas plumes also disappear before reaching the surface, as for example is the case on the Blake Ridge (Paull 1995), where a plume escapes from a pockmark at around 2000 m water depth, but disappears around 100 m above sea floor, being dissolved in the water. It is likely that this gas comes from the free-gas escaping from a fault.

Crater-like depressions were found in the Barents Sea in 1985 (Solheim 1993) in a water depth of around 300m. They are about 10 to 25 m wide and one m deep, and were interpreted as gas eruptions associated with hydrates, but proof was lacking. Gas eruptions or mud volcanoes occur in the oceans as on land. Pockmarks are craters in the seabed formed either as scar from hydrate buoyancy, as on the Hydrate Ridge or from the expulsion of gas and/or water from sediments. On the South Hydrate Ridge, active seepages are confined to two regions within an area measuring approximately 800 x 400m (Brown et al., 1999). These pockmarks occur worldwide, in the ocean at all depths, in lakes, and probably on the planet Mars.

In addition to craters, many seafloor mounds are attributed to hydrates. These mounds are built by organisms living on hydrocarbon seepages

Mounds (carbonate knolls?) due to active thermogenic methane seepage were found by Hovland et al., 1994 in 650-1000 m water in the Porcupine Basin off Ireland, where they were within GHSZ but no BSR is present, and in the Vulcan Basin, off Australia. It is noteworthy that another particular characteristic of the special Oregon margin is the ubiquitous presence of diagenetic carbonates, associated with oxidation of methane-rich fluids (Carson et al.,1994; Kulm and Suess, 1990).

Oxidation

Goldfinger et al., 1999, report; "Because methane is susceptible to oxidation through both microbial and inorganic reactions along its flow path, determining the fate of mobilized methane is critical for evaluating the role of gas hydrate in earth history and in global change." Sloan et al 1999: "Bacterial oxidation of the methane produces a bicarbonate anion, which binds with the calcium present in seawater to produce the carbonate. The morphology of the carbonate precipitates may suggest the intensity and style of fluid expulsion; nodular or slab-like carbonate are associated with diffuse fluid flow, whereas chimneys or "donuts" indicate discrete fluid venting. Another particular characteristic of the Oregon margin is the ubiquitous presence of diagenetic carbonates associated with oxidation of methane-rich fluids."

On the Blake Ridge, there is an anaerobic oxidation with sulfate reduction over the first tens of meters of the sediment below the seabed (Borowski 1999)

In Cascadia, much of the loss of methane from the water column is due to oxidation rather than mixing (Whitcar, 1999). Evidently, living organisms oxide methane very quickly, as confirmed by de Angelis et al 1999, who state: "Clam and snail shells exhibited specific oxidation rates equivalent to the removal of 24 to 110 % available methane per square cm per day. Bacterial mats sampled from carbonate rock surfaces were observed to be capable of removing 16 to 86 % of ambient methane per square cm of surface per day"

Methane floating to the surface of the sea and into atmosphere

While much of the oceanic methane arriving at the seafloor is converted into oxide, carbonate, sulfide and organic matter, which is eaten by clams, snails, bacteria and others organisms, some of it is in the form of free gas, dissolved gas or hydrate and arrives at the seafloor, where it floats upwards. But how much reaches the surface is uncertain. Most of it should rise, being dissolved in the upwelling currents, because the solubility declines sharply at the surface. In should be noted, however, that the reported amount of methane coming from the oceans (10 Tg/a or 10 Mt/a)) is much less than from wetlands (100 Tg/a), enteric fermentation from cattle (80 Tg/a), being matched only by that from termites (10 Tg/a) (Neue, 1993)

sources of methane

Tg/year or Mt/year

natural:

wetlands

120

lakes, rivers

20

oceans

10

termites

10

total

160

anthropogenic

mining and petroleum

100

enteric fermentation=cattle

80

flooded ricefields

50

biomass burning

30

landfills

30

animal waste

30

domestic sewage

20

total man-made

340

total

500

It is significant that the amount of biogenic marsh gas (wetlands) or bovine methane (enteric fermentation) is about ten times that in oceans where methane is assumed to come mainly from hydrates.

Explanations may be that the methane from hydrates is either retained in the sediments, or converted at the seafloor, or that it is present only in small amounts. A survey in the Beaufort Sea found only a small concentration of methane in water and shallow sediments (USGS 95-70).

In his Congressional testimony, Haq 1998 said: "Another large uncertainty is the fate of methane released from hydrate sources in the water column. How much of this is dissolved in the water and what proportion is emitted to the atmosphere? " It is the big question.

Methane is a greenhouse gas, which is twenty times more effective than CO2, but its concentration is smaller. It increased from 0.8 ppm in 1840 to 1.7 ppm today (compared with CO2 which rose from 280 to 370 ppm). The methane growth correlates with population growth (1 to 6 billions) (EPA).

But atmospheric methane stopped growing in 1992 (Dlugokencky 1998) who forecasts a steady level at 1.8 ppm to 2010, compared with the 3.6 forecasted by IPCC (figure 2)

"The end of the rise simply means that methane supply and its removal by chemical reactions in the atmosphere had come into balance at levels more than double those of 200 years ago. Dlugokencky and colleagues believe the most likely explanation is that one or more of the eight or so major sources, which range from digestion by termites and cattle to decomposition in wetlands and landfills, slowed by some 10 million tons a year. Their leading candidate is a drop in the amount of methane escaping from Russian natural gas systems. Dlugokencky points out that the huge Siberian gas fields and distribution system have been notoriously leaky. After a disastrous gas explosion in 1989, efforts began to reduce leaks in the Russian system, and by 1992 engineers might have plugged enough leaks to have ended the methane rise, says Dlugokencky." (Kerr 1994)

Figure 2:

Numerous measurements show the leveling of atmospheric methane as shown by the trendline in figure 3 for the two sites ALT & SPO in respectively the northern and southern hemisphere

Figure 3:

However graph (figure 4) shows the correlation from 1840 to now of population and atmospheric methane, but the last measures diverge.

Figure 4:

 

Global Warming and submarines landslides

The proponents of hydrates have not been slow to join in the debate about global warming. It is claimed that changes in sea-level during past glaciations led to the release of methane from hydrates causing global warming. However, it has to be recognized that hydrates take time to decompose and their input has to be related with other causes of climate change. Raynaud (1998) found no evidence for catastrophic hydrate release and Collett (1997) said:

"We don't see evidence of a significant amount of methane being released today from hydrates. We see little to no evidence that methane has effected climate over 100 year time frames, and not even on 10 000 year frames."

 

Blow out

Several floating drilling rigs have been lost in shallow water when a gas pocket was unexpectedly penetrated before the blow out preventer was installed. The gas arriving quickly at the surface creates a very low density fluid, and the floating rig sinks in accordance with Archimedes Law. This explains why shallow seismic surveys are now routinely carried out prior to drilling to avoid the danger of shallow gas. This hazard is considered when drilling hydrates in deepwater, but it can be dismissed for two reasons. First, the decomposition of hydrates takes time and energy, which is not the case with a gas pocket; second, decomposed methane or freegas dissolves before reaching the surface. The Louisiana State University report states: "Due to the greatly increased solubility of methane and other hydrocarbon gases in seawater, even remotely operated vehicles which can operate at the depths of 1,500 to 10,000 ft. may have difficulty visually detecting a small leakage."

As is well known, there are many submarine volcanoes, which periodically erupt gas, despite the huge ambient pressure, but apart from the tsunami wave, nothing comes to the sea surface as all gases, mainly steam, are dissolved in the water.

Bermuda triangle

McIver 1982 proposed that hydrate could decompose at the sea floor and emit a gas plume, causing turbidites flows, or a glide planes for massive slides, and even the disappearance of ships and aircrafts. This explanation received much media coverage in connection with the Bermuda triangle mystery. Dillon called this theory a fairy tale on the basis of insurance company data showing no special hazard in this area and on laboratory studies simulating the conditions under which the "pock- mark" eruptions are believed to occur. Even with many different simulations, he was unable to produce any conditions likely to prove hazardous.

This conclusion is confirmed by the studies of the Louisiana State University (see above) which reports: "Unlike shallow water, potentially large volumes of natural gas can be dissolved in seawater at the pressures and temperatures surrounding these deepwater flowlines. For example, from 392 to 526 billion cubic feet of natural gas can be dissolved in one cubic mile of seawater at deepwater conditions. The Gulf of Mexico (GOM) contains about 700 000 cubic miles of seawater and can accommodate very significant amounts of dissolved gas before the pollution is detected at the surface." However this Bermuda mystery from hydrate is still proposed by Gruy 1998 and by Fischer (WO June 1999).

 

Resources

Gas hydrate is distributed heterogeneously. Everyone admits that much more needs to be established on its distribution before any realistic estimate of its volume can be made. Gas Hydrate PPG 1998 states "Existing ODP technology does not provide any routine data that is readily interpretable about the in situ volumes of gas or gas hydrate in marine sediments."

This has not however prevented many claims being made that it represents a huge potential hydrocarbon resource. Such estimates range greatly, further demonstrating the uncertainty (Prensky 1995).

Methane gas

Methane gas

Methane carbon

Reference

1015 m3

Pcf (1015 cf)

Tt (1018 g)

Oceanic

5-25

180-880

2.7-13.7

Trofimuk et al. (1977)

3.1

110

1.7

McIver (1981)

7 600

270 000

4100

Dobrynin et al. (1981)

17.6

620

11

Kvenvolden (1988)

19.5

690

11

MacDonald (1990)

26-139

930-4 910

Gornitz and Fung (1994)

Continental

0.057

2

0.03

Trofimuk et al. (1977)

0.014

0.5

0.008

Meyer (1981)

0.031

1.1

0.02

McIver (1981)

34

1 200

18

Dobrynin et al. (1981

(The obviously wrong values of continental methane carbon have been corrected).

 

The "Gas Hydrate Research Program" elaborated in 1999 by the leading US experts said: "considerable uncertainty and disagreement prevails concerning the world’s methane hydrate resources".

Mielke 1999 commented: "The world's currently known natural gas reserves are estimated at 5,000 trillion cubic feet. The amount of methane contained in the world's gas hydrate accumulations is enormous. Estimates of the amounts are speculative and range over three orders of magnitude from about 100,000 trillion cubic feet to 279,000,000 trillion cubic feet of gas. However, it is likely that most of the hydrate occurs in low concentrations and has no commercial potential." USDOE-Office of Fossil Energy: "Worldwide, estimates of the natural gas potential of methane hydrates approach 400 million trillion cubic feet -- a staggering figure compared to the 5,000 trillion cubic feet that make up the world's currently known gas reserves"

Geomar b stated " Geophysicists have used seismic signals to map gas hydrate layers below the sediment surface throughout the world's continental margins. Estimates of the amount of methane trapped in these layers have been very uncertain because scientists have been unable to measure the physical and chemical properties of this unique substance" but they added: "Japan has recently announced the initiation of a new 50-million-US-dollar effort to commercially produce methane from the trillions of cubic meters of hydrate gas found under the seabed surrounding Hokkaido" these trillions were not yet drilled! But JNOC stated in 1998: "However, nobody has shown any possible recoverable reserves. We have not yet developed any production technology for methane hydrate and so far we have no ideas for recovery factors. We hope to obtain some key factors for reserve estimation by the drilling of Nankai Trough in 1999."

It is obvious that some statements are wishful thinking, and some are spurious attempts to secure research funds.

In India the Financial Express announced on Nov.15, 1998:" The National Institute of Oceanography has mapped 6150 trillion cubic meters (200 000Tcf) of gas hydrates, a hitherto untapped source of energy in the country, all along the lower tip of the peninsula, stretching from the Bay of Bengal to the Arabian Sea."

But most of these University experts seem to forget the difference between resources and reserves. Reserves are the part of the resources existing in the ground that is expected to be produced in the future under certain technical and economical conditions. Whale oil was used before petroleum, there are still some resources of whale oil, but no more reserves. France will soon stop producing coal, still with some resources but no reserves.

Haq 1998 stated:" Many in the industry believe that the widely cited large estimates of methane in gas hydrates on the continental margins may be grossly overstated (e.g., Hovland and Lysne, 1998). Moreover, if the hydrate is mostly thinly dispersed in the sediment rather than concentrated, it may not be easily recoverable, and thus not cost-effective to exploit."

All of these estimates are extremely unreliable, most failing to indicate the area, thickness and concentration of the alleged deposits.

The estimates of Kvenvolden and MacDonald are given below (Kvenvolden 1998). It is noteworthy that MacDonald was obliged by recent data to reduce the hydrate saturation in the pore space from 10% to 1% and to obtain the same order he was forced to multiply by 2.5 the area, demonstrating again the wild speculation that attaches to the subject.

Author

Area

Thickness

Pore

Amount carbon

M.km2

meter

%

Tt

Kvenvolden &Claypool 1988

10

500

10

21

Mac Donald 1990

25

500

1

11

As we have seen, the BSR is not at all a reliable tool to estimate the volume of hydrates as it is not related to hydrates but to the free gas below the GHSZ.

Kvenvolden and McDonald’s estimates are a wild extrapolation from the seismic, without taking into account the real data from cores. It is wrong to assume that hydrates are homogeneously distributed, having a thickness of 500 m, when the two best hydrates locations at the Blake Ridge (Leg 164) and Cascadia gave a gross thickness of less than half. The ODP report states: "Preliminary analysis from Leg 164 drill sites on the Blake Ridge indicates that gas hydrate occupies 1% to 2% of the sediment volume in a zone that is 200-250 m thick".

The area of 25 M•km2 (half of the area of the continental slope from 200 to 3000m) is also out of range based on the results at the Blake Ridge and Cascadia covering about 0.1 M•km2.

The claim by the USGS that "the worldwide amounts of carbon bound in gas hydrates is conservatively estimated to total twice the amount of carbon to be found in all known fossil fuels on Earth" is preposterous, especially since it is widely reported in the media.

Kvenvolden estimates the distribution of organic carbon, excluding dispersed carbon in rocks and sediments, which equals nearly 1000 times the total amount, as follows in gigatons (Gt):

source

Gt

gas hydrates:

10 000

fossil fuels

5 000

soil

1 400

dissolved organic matter in water

980

land biota

830

peat

830

detrital organic matter

60

atmosphere

4

marine biota

3

The comparison is greatly flawed. The amount of hydrocarbons in oil and gas fields is estimated to be only about one percent of the generated hydrocarbons. Since the reported hydrate volumes cover disseminated deposits, it is necessary to multiply the corresponding oil deposits by at least a hundred to obtain a comparable disseminated value.

A recent estimate of the hydrocarbon endowment (Perrodon, Laherrere & Campbell, 1998) gives proper ranges for the ultimate recoverable resource, being a small part of the resources as follows

-liquids in Gb:

mini

mean

maxi

conventional oil:

1700

1800

2200

conventional gas liquids

200

250

400

non-conventional HC

300

700

1500

total liquids Gb

2300

2750

4000

-gas in Tcf:

mini

mean

maxi

Conventional gas

9000

10 000

13 000

non-conventional gas

1000

2500

8000

total gas Tcf

10 000

12 500

20 000

The most optimistic (and unrealistic) estimate of ultimate conventional and non-conventional oil is 10 000 Gb (1 500 Gt) compared with the above mean of 2 750 Gb for oil. The maximum hydrocarbon amount, including gas, is about 3000 Gt, which with a further 2000 Gt from coal gives a total for fossil fuels of about 5000 Gt. But fuel reserves have to be compared with hydrate recoverable resources and not with carbon disseminated in the sediments.

Combaz (1991) estimated that the total fossil organic matter in its scattered state amounts to 1016 tons (10 000 000 Gt) of organic carbon, when the fossil fuels concentrated stock reaches about 1014 tons (100 000 Gt) of carbon. The oceans produce annually 23 Gt/a of organic matter compared to 0.7 Gt/a for land. The 10 000 Gt estimate by Kvenvolden for hydrates is only one tenth of Combaz's estimate of the fossil fuels.

It is wrong to compare reserves and resources (volume in place), especially when the resource is not the volume in place in accumulations but dispersed in the sediments.

The amount of hydrocarbons in oil and gas fields is estimated at only about one percent of the generated hydrocarbons. Since the reported hydrate volumes cover disseminated deposits, it is necessary to multiply the corresponding oil deposits by at least a hundred to obtain a comparable disseminated value. [Jean omit as already said above]

Hovland 1998 asked: "Are there commercial deposits of methane hydrates in ocean sediments?" Ginsburg 1998 said that the"Kvenvolden’s values are overestimated and that hydrates are discontinuous either on the horizontal extent or in the vertical extent."

This is confirmed by the 14 cm of massive hydrate in 997A which disappears 20 m away in 997B 

But the most recent paper by Collett 2000 is the AAPG Course 14 on hydrates states that: "Published gas hydrates resource estimates are highly speculative, but the total amount of gas hydrate accumulations of the world may rival the volume of known conventional gas reserves." completely different from another AAPG Explorer article by Collett et al 2000 where is said that "Current estimates of the amount of gas in the world’s marine and permafrost gas hydrate accumulations are in rough accord at about 20 000 trillion cubic meters" (700 000 Tcf)

The "rough accord" has reduced in few months from 700 000 Tcf to 5 000 Tcf (world’s conventional remaining reserves).

In comparing hydrates with other fossil fuels, the time factor has also to be taken into account. It is hard to believe that hydrates contained in the first 600 m of oceanic sediments, which were laid down in less than 10 millions years, could hold twice more carbon than the fossil fuels from 6000 m of sediments, laid down over more than 500 millions years.

 

Volumetric Flaws

In assessing a new oil prospect, the oil industry evaluates the necessary parameters including the occurrence of source-rock; maturation; migration; reservoir development; trap and seal. But in the case of hydrates, the source, reservoir and seal are the same, comprising the 500 m of unconsolidated sediment beneath the seabed. Since hydrate is a solid, there is no possibility of migration. Furthermore, hydrate makes up only about 1% of the porosity, the rest being water, so the hydrates do not correspond to a seal. Free gas stays there, as it is the equivalent of hydrate in zone where hydrate is unstable. Leg 164 in the Blake Ridge found that the percentage of methane is about the same above and below the BSR.

The estimates of volume of hydrates assume that most of the available organic material is converted into hydrate as the percentage of total organic carbon (TOC) is around 1% when the percentage of hydrate is around 1% of porosity or 0.5% of the total volume.

US estimates of hydrates

Some authors claim that about only one percent recovery is enough to give huge reserves, but it has to be a significant recovery or nothing. Furthermore, there are serious doubts that the volume in place is reliably assessed

In 1995, the U.S. Geological Survey (USGS) completed its most detailed assessment of U.S. gas hydrate resources, with the in-place gas resource from 112 000 Tcf to 676 000 Tcf, with a mean value of 320 000 Tcf (given by Collett in his testimony before the US House of Representatives Committee on Resources Subcommittee on Energy and Mineral Resources on May 25, 1999 for HR 1753, but Woolsey in his testimony the same day gives a range of 100 000 Tcf-7 000 000 Tcf). Subsequent refinements of the data in 1997 have suggested that the mean should be adjusted slightly downward, to around 200 000 Tcf.

The assessment of the volume of hydrates is based on an area, a thickness of the hydrates with a concentration. The area is taken from the seismic extent of the BSR, but it is known that it is not related to hydrates but to free gas below the GHSZ. The thickness is taken as the reported occurrence again not from cores but from other proxy parameters as chlorinity and concentration taken from seismic or gas samples. The potential of the Blake Ridge is given by Sloan et al 1999: "Data indicate that 1-5% bulk sediment volume is filled with gas hydrate throughout an ~250M thick section.  Extrapolations of this assessment over the 24,000 km2 of the Blake Ridge suggest 38 - 80 TCM of gas exist in this area.  However, production of gas from the Blake Ridge is unlikely with existing technologies of the low permeabilities associated with these silt- and clay-rich samples." The report of Leg 164 gave similar values with 1-2% on 200-250 m on 26 000 km2 with a volume of 10 Gt of methane carbon.

Hydrates are assumed to be present from 200 to 450 m on an area of 25 000 km2, but unfortunately Leg 172 drilled 9 sites (1054 to 1062) on this area called "the world’s best-known marine gas hydrate occurrence" from waterdepth of 1050 to 4760 m and not one found hydrates, despite the fact that Site 1061 penetrated 350 m of sediments.

It means that the occurrence of hydrates was vastly overestimated for the best explored hydrate area, and world estimates are even worse based on much less evidence.

Comparison with other non-conventional gas sources

There are several other sources of non-conventional gas, which are much better known and accessible than hydrates (Perrodon et al., 1998). For example, Bonham (1982) estimated that there was as much as 50 000 Tcf in geo-pressured brines in the Gulf Coast. This is much larger than the 1 300 to 7 000 Tcf attributed to the Blake Ridge hydrates by the USGS, and is obviously a much more reliable resource. Simon 1996 states: "A later "official" estimate, made in the midst of the congressional debate on energy in the same year 1977, by Dr. Vincent E. McKelvey, who was then director of the U.S. Geological Survey, was that "as much as 3,000 to 4,000 times the amount of natural gas the United States will consume this year may be sealed in the geo-pressured zones underlying the Gulf Coast region".

There is more in Russia. Zorkin & Stadnik (1975) estimated the amount of gas dissolved in water at around 35 000 Tcf for West Siberia and as much again for the Caspian. Geopressured brines have been tested in the Gulf Coast and found uneconomic. That said, there remain technical difficulties in producing gas from these sources, including the disposal of the brines, but the problems must be small compared with those of oceanic hydrates

 

Prospects for future production

So far, there is no commercial hydrate production and none is planned. The reports of production in the Messoyakha Field have been challenged, as already noted. To produce hydrates it would be necessary to first find a concentrated deposit and then apply thermal or depressurizing energy or solvents to release the gas. Many wild ideas have been voiced. For example, Iseux, 1991 proposed hydraulic fracturing, ignoring the unconsolidated nature of the sediments, and the injection of hot solvent. Islam, 1994 proposed electromagnetic heating of hydrates in permafrost. The absurdity of such ideas is evident when deepwater conventional oilfields require presently flow rates per well of around 10 000 b/d to be economic.

Haq 1998 stated: "Many in the industry believe that the widely cited large estimates of methane in gas hydrates on the continental margins may be grossly overstated (e.g., Hovland and Lysne, 1998). Moreover, if the hydrate is mostly thinly dispersed in the sediment rather than concentrated, it may not be easily recoverable, and thus not cost-effective to exploit. One suggested scenario for exploitation of such dispersed resources is excavation (open-pit style) rather than through drilling, which is environmentally a least acceptable option."

Mielke, 1999 " it is likely that most of the hydrate occurs in low concentrations and has no commercial potential.1 The goal of a research program would be to find locations where the methane hydrates are sufficiently concentrated to warrant commercial interest, in addition to proving the technological feasibility and safety of their production."

The fact is that there is no commercial interest in the oceanic hydrates (any more than there is in the oceanic dissolved gold, despite it being the largest gold resource in the world). Chevron testifying to a US Senate Committee in 1998 correctly stated that hydrates occur in low concentrations and have no commercial potential. Krason 1999 reports that Shell Gas Hydrate Team stated: "the lack of a geological-geophysical-petrophysical rock model precludes the possibility of determining the vertical and lateral distribution of gas hydrate deposits as well as the potential volume of trapped gas".

Gasprom (Yalushev) presented a very pessimistic opinion on production of gas from hydrates. In 1988 Makogon forecasted that hydrate will in 1997 cover 1 % of FSU gas production, but so far it has contributed nothing

Oil companies are involved in hydrate projects only in Japan and India, which are countries with limited indigenous oil and gas who face the growing cost of imports and their governments are willing to subsidize the research.

Sassen 1998 stated optimistically that "I think that eventually hydrate companies are going to replace oil companies". Such a dream is a long way from being realised.

Net energy

Economists have a very optimistic approach on the oil reserves, as they believe that when price increases, the reserves increase also, as it happens for minerals as copper. When copper price is low, only rich deposits are produced and when price is high, poor deposits are produced and reserves grow. But oil is a liquid not an ore. A prospective structure is filled with either oil or water; it is all or nothing. Furthermore what is important is not the cost of producing compared to the price but how much energy is needed to produce this fuel compared to the energy of the fuel. The net energy should be positive or the energy return on energy investment (EROI) should be over 1. For example ethanol produced in the USA from corn needs 70% more energy than it contains (Pimentel 1998): it is economic only because subsidy. No one knows how to produce hydrate, still less the net energy, as Allison 2000 USDOE rightly said: "one of the most important questions is the net energy balance."

 

Hydrate for transportation

The most valid interest in hydrates seems to be as a method for transporting the methane from the remote gasfields, which may be cheaper than liquefying the natural gas

The main finding of Professor Gudmundsson in Norway is that gas hydrates may be stored and transported at atmospheric pressure if the temperature is kept at approximately 15 degrees below the freezing point of water.

Gudmundsson 1996 "Capital costs of natural gas hydrate (NGH) and liquefied natural gas (LNG) are compared, for the transport of 400 MMscf/d (about 4 billion Sm3 per year) of natural gas for a distance of 5500 km. The NGH chain was found to cost about twenty-five percent less than the LNG chain. Studies in Norway have shown that frozen hydrates at atmospheric pressure remain stable long enough to make large-scale and long-distance transport of natural gas in the form of hydrate feasible." Also a slurry oil/hydrate is planned to be transported by tanker.

 

Conclusions

Sloan et al 1999 comment: "One of the most critical scientific problems is the need for accurate determination of the occurrence, distribution, and concentration of gas hydrates in nature.  While gas hydrates occur in numerous offshore marine basins and in onshore areas underlain by permafrost, there are only vague ideas as to their abundance. Unless progress is made in sensing and detecting gas hydrate, research regarding occurrence, distribution, and concentration will be severely impeded"

Methane hydrates are well known to the oil industry as a material that clogs pipelines and casing. They are also present in permafrost areas and in the oceanic sediments where the necessary temperature and pressure conditions for stability occur. Oceanic hydrates have been proven by core or dredging or visual observation on the seafloor only in around ten basins outside producing petroleum basins, such as the Gulf of Mexico or the Caspian, which should be excluded of the assessment of oceanic hydrates. The best places to explore for hydrates are first Cascadia (offshore Oregon), and second the Blake Ridge (offshore N&S Carolina). The Cascadia seafloor is full of seepages, but the first 200 m of sediments on Blake Ridge is without hydrates.

Claims for the widespread occurrence in thick oceanic deposits are unfounded. The thickest interval recovered from a total of 250 000 m of core from 2300 ODP/DSDP boreholes was one meter with an unknown extent. Mostly, they occur as dispersed grains and laminae. Indirect evidence from seismic reflectors (BSR), seismic direct hydrocarbon indicators, logs and free gas samples is unreliable and highly speculative. The BSR can have several origins, the most likely being the so-called free-gas below the hydrate stability zone where gas is not trapped but has the same concentration as hydrate, namely 1% of the porosity.

A former concept that hydrate is a static, biogenic, continuous, huge resource is changing to a new one that sees it as a dynamic, overpressured, discontinuous, and unreliable resource. Gas and fluid flows are more and more seen to be involved with hydrates, moving through faults, but the origin is difficult to find.

The solubility of methane in water and the density of hydrate are still badly known.

Because of the heterogeneity and the mainly millimetric to centimetric occurrences of hydrates, it is difficult to speak about accumulations or to envisage a way to produce them.

Oil companies are now contributing to deepwater investigates, but only the national companies of Japan and India have expressed any serious interest, because of their serious strategic position. Past academic research dedicated to hydrates has brought in more questions than answers. It seems that new Europeans (Germany and Norway) or Japanese actors may be bringing in new concepts, tools and ideas; so time will tell.

It is important to have a global view and to rely more on experiments than in computations, and give more weight to the first digit than to the last decimal. Too many papers are theoretical and ignore facts. It is time to answer simple questions as: where did the methane come from, how, and under which state, does it move through the GHSZ to the seafloor, to the seawater, to the sea surface and to the atmosphere. When these very important scientific questions are answered, then it will be possible to see in a few promising places if there is any producible accumulation and then how to produce it.

Meanwhile it is better to search for more truths and other alternatives for dealing with the coming transition from abundant to decreasing energy availability.

 

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