Uncertain resource size enigma of oceanic methane hydrates

PART I: This is first of a two-part series on oceanic methane hydrates. Part 1 focuses on current state of knowledge of methane hydrates. Part 2 will examine the reserve potential of oceanic methane hydrates.

Hydrates are everywhere. Indeed we eat many in the form of carbohydrates found in fruit, vegetables and cereals. Methane hydrate is different because methane is held inside the molecular lattice of water, in the proportion of 1:5.75. It is also known as clathrate from the Latin word for lattice. Methane hydrate can occur in a high-pressure environment, such as gas pipelines or oceanic sediments. When broken down, one unit volume of hydrate gives 150 units of methane and 0.85 of water.

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 placed in operation in cold climates, and it was found that methane hydrate, rather than ice, 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 Deep Sea Drilling Program (DSDP), which was extended in 1985 as the Ocean Drilling Program (ODP), stimulated an interest in hydrates. Russian research suggested that hydrates could occur at a depth of a few hundred meters below the seabed in deepwater 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 joint oceanographic institutions (JOIDES) boreholes were designed to investigate them. Universities, not oilmen, planned these boreholes, although the latter advised on safety.

A total of 625 sites were drilled by the Glomar Challenger between 1963 and 1983 under the auspices of the DSDP, but drilling through the BSR was not permitted, to avoid any chance of a blowout. A further 500 boreholes were drilled by the drillship JOIDES Resolution.

Combined, these programs investigated a large number of sites in water depths up to 7,000 meters, the average being 3,500 meters. The ODP program concentrated in shallower waters, but took more cores, and thanks to a safer drillship, penetrated the BSR.

Together, as much as 250 km of cores were recovered with an average recovery of about 60%. Accordingly, it can be said that the first few hundred meters of the seabed in oceanic areas, covering some 360 million sq km have been thoroughly explored.

Uncertain resource size

The cynic might say that the study of hydrates is tailor made for academic research insofar as it can continue for a very long time without providing conclusive evidence. It is furthermore a wide subject covering such matters 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 when brought to the surface. So, it is difficult to study them in their original state. The research goes on, but never seems to deliver a concrete answer.

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

The US 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 15 offshore hydrate basins, development of production models for de-pressuring and thermal extraction, and development of a test lab instrument.

Core evidence

Many thousands of boreholes that have been drilled and cored in the oceanic seabed should have encountered ample indications of hydrates. But in fact, there are only three major known occurrences of massive hydrates:

• 14 cm at Site 498 (Leg 67 Guatemala)
• 105 cm were recovered at Site 570 (Leg 84 Guatemala not far from an exploratory well), but hydrate was not expected as there was no BSR. On logs, the body is 3-4 meters thick with an unknown extent as there is no corresponding seismic reflection.
• 5, 7, and 14 cm at Site 997A (Leg 164 Blake Ridge), but no hydrates were found at same depth 20 meters away at Site 997B.

Massive hydrates are connected with faults and the presence of a deep methane source. At many hydrate sites, the hydrates consist of a few dispersed grains or thin laminae. These are interpreted as biogenic, resulting from microbial methane production. Ginsburg has widely sampled the seabed off Russia, finding numerous hydrate occurrences in mm to cm thick layers. Thus, there is no direct evidence from all the worldwide research and extensive coring for any massive hydrate deposits.

Researchers find hydrate in abundance from a great variety of indirect indicators, including:

• Gas analysis - the gas from the pressure core sampler is assumed to come from hydrates
• Water analysis - interstitial water chloride being assumed to correlate with hydrates
• Seismic analysis - the BSR is assumed to come from hydrates, and/or free gas
• Log analysis - resistivity and density are assumed to infer hydrates

Bottom simulating reflector

The BSR seismic event typically occurs at a depth of between 200 - 600 meters 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 deepens with water depth. The BSR 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.

Velocities of Leg 164 are a good example of poor quality and disagreements between vertical seismic profiling (VSP), sonic, and seismic surveys. Site 994 was drilled at a location lacking BSR, but close to Sites 995 and 997 with strong BSR. The three holes are similar. There are BSR without hydrates and hydrates without BSR.

Previously, it was assumed that 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 sediments is assumed now to be about 1% of the porosity. In fact, the presence of only a few percent of natural 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. There are other possible explanations for the BSR reflector:

• An artifact of the recording equipment related to automatic gain control
• A formation boundary, with the crossing events being artifacts caused by diffraction from faults or the seabed
• Diagenetic contrasts or compaction
• Opalization
• Thermocline multiples (sharp contrast of temperatures inside the oceans), or Sofar Channel (minimum of sound velocity in waters around 1,000 m).

Methane solubility

Minshull (1989) wrote, "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." Many graphs have been published on the stability of hydrates versus pressure and temperature, but a complete graph of the solubility of methane in deepwater is lacking, save one graph of solubility of methane in water sediments in Bonham (1978). It shows a drastic increase of solubility of methane with presssure explaining the lack of giant gasfields in HPHT conditions.

Du Rouchet (1980) mentioned that Makogon drew a graph in 1971 where the solubility of methane of 2 liters of gas/liter of water for 70 bars decreases sharply to 0.4 liter/liter (l/l) when converting into hydrate. This theory of discontinuity of solubility was not kept in recent articles.

Louisiana State University, in an article on leak detection in deepwater pipelines, reports that the solubility of methane can increase up to 150 times, from 0.68 cu-ft/bw (0.12 l/l) in shallow water to 24-100 cu-ft/bw under deepwater pressures and temperatures. These values are in agreement with Bonham which gives 50 cu-ft/bw (8 l/l) for 5,000 meters. It is in agreement with the values given on geopressured gas brines of the US Gulf Coast which have produced around 30 cu-ft/bw (5 l/l). So, there is a drastic discrepancy (over 1 to 10) between the solubility of methane in seawater given by hydrates experts and the solubility given by gas experts.

Hydrates experts say that this value determines the hydrates stability, while Bonham and the gas pipeline experts say that 0.5 tcf can be dissolved in a cubic mile of deepwater. Site 994 found a concentration of 5:1 liters of methane to water at 569 meters depth. It was reported as too high to be true, but corresponded to the solubility of Bonham in these conditions.