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.

Keywords:
production monitoring, Upstream Oil & Gas, Thickness, Reservoir Surveillance, drillstem/well testing, hydrate decomposition, production control, reservoir simulation, hydrate-capped gas reservoir, hydrate zone
Subjects:
Well & Reservoir Surveillance and Monitoring, Flow Assurance, Reservoir Characterization, Reservoir Fluid Dynamics, Reservoir Simulation, Formation Evaluation & Management, Reserves Evaluation, Production logging, Hydrates, Flow in porous media
Copyright 2006, Society of Petroleum Engineers
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