Effect of Hydrates on Sustaining Reservoir Pressure in a Hydrate-Capped Gas Reservoir

Abstract

Abstract A hydrate-capped gas reservoir is defined here as a reservoir that consists of a hydrate-bearing layer underlain by a two-phase zone involving mobile gas. In such a reservoir, hydrates at the top contribute to the produced gas stream once the reservoir pressure is reduced by gas production from the free-gas zone. Large gas reservoirs of this type are known to exist in Alaska and Siberia and are expected to exist in the Mackenzie Delta of the Northwest Territories in Canada. Gas production from a hydrate-capped gas reservoir is a process governed by a combination of mechanisms of heat transfer, fluid flow, thermodynamics and kinetics of hydrate decomposition. Using a comprehensive numerical simulator, an extensive simulation study indicates that some of the non-linear processes involved in gas production from hydrate reservoirs (i.e. the convective heat transfer and the kinetics of hydrate decomposition) have a negligible effect on the overall physics of the process. This significantly reduces the complexity of the heat and fluid flow equations and legitimizes the construction and use of simplified models. In this work, we invoke the above approximations and develop a generalized gas material balance equation. This equation has two significant differences from the material-balance equation for conventional gas reservoirs, including the incorporation of:the effect of cooling due to endothermic decomposition of the hydrate; andthe effect of generated gas and water from the hydrate decomposition. In this model, it is assumed that a mobile phase exists in the hydrate zone; thus, no sharp hydrate dissociation interface is assumed. Considering the sensible heat of the hydrate zone and heat transfer from cap and base rocks, the gas and water generation rates are determined on the basis of the equilibrium rate of the decomposition process. Verification of the solution is obtained by comparing results with those of a comprehensive hydrate reservoir numerical simulator. The model developed here can be used as an approximate engineering tool for evaluating the role of hydrates in improving the productivity and extending the life of hydrate-capped gas reservoirs. Introduction Natural gas hydrates are solid molecular compounds of water with natural gas that are formed under certain thermodynamic conditions. There is evidence that enormous amounts of natural gas exist in the form of hydrate deposits in many regions of the world(1). These deposits occur in sub-oceanic sediments as well as in arctic regions. Every unit volume of gas hydrate has the potential to contain 170 to 180 volumes of gas at standard conditions, making the energy content of one cubic metre of a hydrate reservoir more than other types of unconventional gas reservoirs(2). In view of the large untapped resources of natural gas hydrates, extensive research and development work is underway to determine what fraction of this resource is recoverable. A number of recovery processes have been suggested for producing gas from hydrates in sediments. Sloan(3) and Makogan(4) have presented an extensive review of the suggested methods including depressurization, thermal stimulation and inhibitor injection.