Analytical solution for gas production from hydrate reservoirs underlain with free gas

Abstract

Fast analytical models for predicting hydrate reservoir performance are very attractive, especially in the early phase of development of hydrate reservoirs when data are scarce and a large number of sensitivity studies are required. Recently, a number of analytical solutions have been developed that predict the rate of gas generation (and production) from hydrate-capped gas reservoirs. One of the simplifying assumptions of this research team's previous model – that the decomposition of hydrate is assumed to occur simultaneously throughout the hydrate zone – leads to optimistic results. To understand just how optimistic previous model's results may be, we present an alternative analytical solution that would lead to pessimistic results. Use of a previous optimistic model in conjunction with the current pessimistic one therefore provides bounding upper and lower limits of the rate of gas production from hydrate-capped gas reservoirs. The model presented here is developed by coupling the energy balance equation of 1D hydrate decomposition ahead of a moving interface with the material balance equation in a volumetric gas reservoir, and includes the thermodynamic relation of hydrate decomposition. Results are presented to show how this model may be used to estimate a lower bound for hydrate recovery and how these results depend on reservoir and hydrate properties.