Energy Exploration & Exploitation

Gas hydrates have been touted as the next generation of energy resources exploitable for commercial gain and anthropogenic use. It would then seem relevant to estimate the potential resources available in order that one has an appreciation of availability. Several such estimates have been made over the years for both onshore and offshore gas hydrates. This paper examines the relative percentages and the ranges of uncertainty for both the onshore and offshore estimates. In addition, the estimated resources are plotted versus the year in which the estimate was made to see if any convergence of results is being achieved with time as more data become available.

The main conclusions are that there seems not to be any systematic pattern of convergence of resource estimates with time, nor does there seem to be any narrowing of the uncertainty of the estimates for either onshore or offshore resource estimates as more data have become available over the last twenty years. It would seem that a concerted effort is needed to improve estimates if there is to be any hope of assessing the commercial worth of hydrate resources and of deciding whether hydrates really do represent a significant exploitable energy resource on a worldwide basis.

Naturally occurring gas hydrates forming in the ocean floor sediments have, ever since the 1970s, been heralded as the next century's viable energy source. Is this really so? What has the Ocean Drilling Program (ODP) found out about the nature of the Bottom Simulating Reflector (BSR) as an indicator of large volumes of gas hydrates and free natural gas? The findings so far from ODP's legs 146 and 164 have been rather discouraging with respect to the in situ amounts of both gas hydrates and free gas. In the sediments drilled on Leg 146, on the Cascadia accretionary wedge off Oregon, USA, the average amount of gas hydrates above a prominent BSR was estimated at 1 – 2 percent by sediment volume. The free gas amount below the BSR was 3–4 percent by sediment volume. On Leg 164 in the Western Atlantic Ocean (Blake Plateau) some higher amounts, averaging up to 4 – 6 percent and 5 – 7 percent by volume of sediment, respectively, were found. However, on the Hydrate Ridge, off Oregon (Cascadia, just south of Leg 146), there has recently been discovered apparently large amounts of sediment-mixed gas hydrates, which may be exploitable.

Although the general percentages found so far are much too low for commercial extraction (even a high-grade copper-ore on land must average more than 6 percent by volume), there may exist some high-grade regions worth exploring. But, the question remains: Where shall we seek the high-grade gas hydrate deposits in ocean sediments? Besides the obvious deep-water high porosity conventional type of reservoirs, and judging from recent findings in the Gulf of Mexico and the Caspian Sea, perhaps the most promising locations are those where mud volcanoes occur on the ocean floor.

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.

Two multi-channel seismic reflection profiles in the deepwater of the South Caspian Sea, offshore Azerbaijan, document one of the first examples of buried gas hydrates. Based on their geophysical signature, these clathrates are characterized by (1) a depth-restricted, lenticular body well beneath the seafloor, (2) the apparent accumulation of free gas within the underlying sediment, and (3) evidence of associated recent slope failure in the overlying strata. The interpreted thickness and depth of gas hydrates in the South Caspian Basin fall within the hydrate stability field predicted from the rmobaric modelling of gas compositions identified from coring at the seafloor. Predicted minimum water depths (∼150 m) and maximum thicknesses (1,300 m) for hydrate stability are much shallower and considerably thicker in the South Caspian Sea than for other known hydrate occurrences. Accumulation of these hydrates near the base of the continental rise appears to control a large region (> 200 km²) of shallow deformation, here named the Absheron allochthon. Such attributes make these gas hydrates important, and perhaps previously underestimated, geo-hazards of the South Caspian region. Primary among these hazards are (1) uncontrolled release of free gas trapped beneath the hydrate seal, (2) disruption of the gas hydrate stability field leading to either explosive dissociation of the gas hydrate, or (3) reduction in sediment strength, slope instability, and mass sediment transport. Documentation of the presence and distribution of gas hydrates, especially when concealed at depth in the subsurface, is a clear pre-requisite for exploration activities in the deepwater region of the South Caspian Sea.

A one-dimensional numerical simulation model has been developed to forecast the distribution of methane hydrate in the subsurface. The model includes the generation and migration of methane, and the process of methane-hydrate formation after sedimentation. The following processes are consideredfor each cell within the model;

1)

Calculation of methane generated within the sediment

The model will predict the distribution of methane hydrate after applying the above processes for all the sediments.

We use this model to simulate the process of methane hydrate formation at ODP Leg 164 Site 997 at Blake Ridge, offshore Atlantic coast of the USA. At this location, the methane hydrate zone indicated by the bottom simulating reflector (BSR) is about 100 m shallower than the base of gas (methane) hydrate stability zone (BGHS) which is calculated from the model using measured temperatures and pressures. To explain this difference, the following hypotheses are proposed;

1)

The BSR is a remnant of the past the BGHS that was created when temperature was higher or sea level was lower.

The model simulation indicates that analysis, based not only on present physical conditions but also on the geohistrical time scale, is necessary to explain the present day distribution of methane hydrate.

This paper examines the maximal extent of shallow thermal gradient distortions brought about by hydrate rich regions of finite lateral dimensions in sediments. In general, the anomalous gradient depends on both the thickness of the hydrate layer, the fraction of hydrate in the pore space and the lateral dimensions of the hydrate-bearing body. It is shown that it is difficult to obtain more than about a 5 to 10 percent change in the regional temperature gradient for hydrate fractions of up to 35%-near the maximum of observed values. The identification of hydrate bodies by use of shallow thermal gradient probes would, therefore, seem to be difficult but not impossible.

The northern Gulf of Mexico's passive margin is interrelated and synergistic with both regional and local inputs. Regional inputs may be thought of as deterministic and may include eustatic sea-level oscillations, sediment deposition from erosion of uplifted areas, climate above and below sea level, compression and dewatering of sediments once deposited, and related tectonics. The synergistic sum of these deterministic processes impacts the occurrence of natural gas and their hydrate phases.

Along the foot of the continental slope, compression is transmitted to the sediment beneath and down-dip. Hydrates occur when suitable cold temperatures and high pressures are present. In water depths of ∼4 km, the hydrate stability zone (HSZ) may be up to 1 km thick. A prograding slope imparts regional pressures. Local pressures may be sufficient to catalyze the gas-to-hydrate phase change. Further compression may cause the zone to heat and/or to generate fractures, promoting hydrate disassociation and/or escape.

The impact of a salt/shale lateral wedge or diapir (or both) entering the zone can be observed in several ways. Increased salinity raises the gaseous phase, thereby reducing hydrate existence. Increased heat also augments gaseousness. Note that shales dissipate heat and that the top of a salt diapir can be cool, even though overall salt is a heat conductor relative to sediments. Migrating salt/shale can promote fracturing, where the size and rate of migration determine the dimensions and frequency of fractures. As gases enterfractions, gas may expand and possibly block further gas entrance. Local sediment shattering and thermal anomalies could obviate the gas expansion effect.

As the Louann Salt, which is located over the slope-sediment wedge, advances basinward, the amount of compression rises. Thus, temperatures and pressures change and the pattern of gases/hydrates are dynamic. Specific local predictions are subtle and may be regarded as probabilistic in model development.

Equal subtlety is experienced along the ocean/land contact. Hydrate existence is determined by gas chemistry, whether methane or ethane is present. As the sea level oscillates, temperatures and pressures also oscillate. Landward, the HSZ limit during sea-level highs is disassociated, and a new, more basinward HSZ is established. Specific landward HSZ limit is determined by the gas chemistry.