Even though gas hydrate are known to occur in numerous marine and Arctic settings, little is known about the technology necessary to produce gas hydrate. Most of the existing gas hydrate "resource" assessments do not address the problem of gas hydrate recoverability. Proposed methods of gas recovery from hydrate usually deal with dissociating or "melting" in-situ gas hydrate by (1) heating the reservoir beyond hydrate formation temperatures, (2) decreasing the reservoir pressure below hydrate equilibrium, or (3) injecting an inhibitor, such as methanol or glycol, into the reservoir to decrease hydrate stability conditions. Gas recovery from hydrate is hindered because the gas is in a solid form and because hydrate is usually widely dispersed in hostile Arctic and deep marine environments. First order thermal stimulation computer models (incorporating heat and mass balance) have been developed to evaluate hydrate gas production from hot water and steam floods, which have shown that gas can be produced from hydrate at sufficient rates to make gas hydrate a technically recoverable resource (Sloan, 1998). However, the economic cost associated with these types of enhanced gas recovery techniques would be prohibitive. Similarly, the use of gas hydrate inhibitors in the production of gas from hydrate has been shown to be technically feasible (Sloan, 1998), however, the use of large volumes of chemicals such as methanol comes with a high economic and environmental cost. Among the various techniques for production of natural gas from in-situ gas hydrate, the most economically promising method is considered to be the depressurization technique. However, the extraction of gas from a gas hydrate accumulation by depressurization may be hampered by the formation of ice and/or the reformation of gas hydrate due to the endothermic nature of gas hydrate dissociation.
The Messoyakha gas field in the northern part of the West Siberian Basin is often used as an example of a hydrocarbon accumulation from which gas has been produced from in-situ natural gas hydrate. Production data and other pertinent geologic information have been used to document the presence of gas hydrate within the upper part of the Messoyakha field (Makogon, 1981). It has also been suggested that the production history of the Messoyakha field demonstrates that gas hydrate are an immediate producible source of natural gas, and that production can be started and maintained by conventional methods. Long-term production from the gashydrate part of the Messoyakha field is presumed to have been achieved by the simple depressurization scheme. As production began from the lower free-gas portion of the Messoyakha field in 1969, the measured reservoir-pressures followed predicted decline relations; however, by 1971 the reservoir pressures began to deviate from expected values. This deviation has been attributed to the liberation of free-gas from dissociating gas hydrate. Throughout the production history of the Messoyakha field, it is estimated that about 36% (about 5 billion cubic meters) of the gas withdrawn from the field has come from the gas hydrate (Makogon, 1981). Recently, however, several studies suggest that gas hydrate may not be significantly contributing to gas production in the Messoyakha field (reviewed by Collett and Ginsburg, 1998).
It should be noted, that our current assessment of proposed methods for gas hydrate production do not consider some of the more recently developed advanced oil and gas production schemes. For example, the usefulness of downhole heating methods such as in-situ combustion, electromagnetic heating, or downhole electrical heating have not been evaluated. In addition, advanced drilling techniques and complex downhole completions, including horizontal wells and multiple laterals, have not been considered in any comprehensive gas hydrate production scheme. Gas hydrate provinces with existing conventional oil and gas production may also provide us with the opportunity to test relatively more advanced gas hydrate production methods. For example, in northern Alaska existing "watered-out" production wells are being evaluated as potential sources for hot geopressured brines that will be used to thermally stimulate gas hydrate production.
As previously noted, the low concentration of hydrate in most of the world's marine gas hydrate occurrences raises a concern over the production technology required to produce gas from highly disseminated gas hydrate accumulations. In addition, the host-sediments also represent a significant technical challenge to potential gas hydrate production. In most cases, marine gas hydrate has been found in clay-rich unconsolidated sedimentary sections that exhibit little or no permeability. Most of the existing gas hydrate production models require the establishment of reliable flow paths within the formation to allow the movement of produced gas to the wellbore and injected fluids into the gas-hydrate-bearing sediments. It is unlikely, however, that most marine sediments possess the mechanical strength to allow the generation of significant flow paths. It is possible that in basins with significant input of coarse-grained clastic sediments, such as the Gulf of Mexico or along the eastern margin of India, gas hydrate may be reservoired at high concentrations in more conventional clastic reservoirs; which is more analogous to the nature of gas hydrate occurrences in onshore permafrost environments (Collett, 1993; Dallimore et al., 1999).
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