Material Balance and Boundary-Dominated Flow Models for Hydrate-Capped Gas Reservoirs

Abstract

Abstract Gas hydrates are being considered as an alternative energy resource of the future, considering the enormous quantities existing in permafrost and offshore environments. Some of the hydrate reservoirs discovered (e.g., in Alaska and Siberia) are overlying a free-gas layer. These reservoirs are thought to be the easiest and probably the first type of hydrate reservoirs to be produced1. This paper presents the first-ever developed material balance model for such a reservoir (which we shall call a hydrate-capped gas reservoir). The technique presented herein differs from the traditional approach of applying material balance methods to conventional gas reservoirs because it includes the effects of gas generated from hydrate decomposition and its associated cooling effect. The material balance equation is developed by analytically and simultaneously solving the mass and energy balance equations. The solution yields the average reservoir pressure and the gas generated from hydrate decomposition as a function of cumulative gas produced, for a reservoir that is produced at a constant rate. In the second portion of the paper, we develop a flowing material balance equation by first writing the inflow performance equation, relating the wellbore pressure to the average reservoir pressure and then combining it with the material balance equation. This yields an estimate of initial gas-in-place from production data. Using a recently developed hydrate reservoir simulator, it is shown that this model is valid over a wide range of reservoir parameters. The success of this model relies on coupling of the energy and mass balance equations, where the energy equation accounts for the endothermic nature of hydrate decomposition. In its "forward solution" mode, the model developed here is used as an engineering tool for evaluating the role of hydrates in improving the productivity and extending life of hydrate-capped gas reservoirs. In addition, in its "backward solution" or inverse approach mode, the application of this new model is providing an estimate of initial free gas-in-place from production data. Introduction Gas hydrates are ice-like crystalline structures of water that form "cages" in which low molecular weight gas molecules, especially methane, are trapped. Under favorable conditions of low temperature and high pressure, hydrocarbon gases will form hydrates. Methane hydrates have been located in vast quantities around the world, which tend to form in two geologic settings2:on land in permafrost regions, andin the ocean sediments of continental margins. Bearing in mind that 1 m3 of hydrate can yield about 181 m3 of gas2 (at standard conditions), substantial amounts of gas could be produced from these hydrate deposits. According to Kvenvolden3, the world resources of carbon trapped in hydrates have been estimated to be twice the amount of carbon in known fossil fuel deposits. Therefore, developing methods for production of natural gas from hydrates and the study of their production behavior are attracting considerable attention. A number of recovery processes have been suggested for producing gas from hydrates in sediments. Sloan4 has presented an extensive review of the suggested methods including depressurization, thermal stimulation, and inhibitor injection. The simplest form of the depressurization technique envisions drilling through the hydrate layer in hydrate reservoirs that have an underlying free-gas zone, and completing the well in the free-gas zone. By simple production from the underlying gas, the hydrates at top would naturally decompose over time and contribute to the produced gas.