IS IT POSSIBLE TO SIMPLIFY THE FORMULATION OF HYDRATE DECOMPOSITION IN POROUS MEDIA?

Mehran Pooladi-Darvish and Shahab Gerami
University of Calgary, Alberta, Canada

Gas hydrates are being evaluated for their potential as a future energy source. To facilitate this evaluation, a few numerical reservoir simulators have been developed over the past decade. These models are quite comprehensive and include the important mechanisms of multi-phase flow, convection and conduction, as well as intrinsic kinetics of hydrate decomposition. Currently, three of these simulators are being used to evaluate the feasibility of suggested production methods through sensitivity studies, and to interpret the results of the production testing in Mallik field, Northwest Territories, Canada.

In this study, we investigate possibility of simplifying the formulation of gas production from hydrate reservoirs. If the formulation is sufficiently simplified, one may be able to use the commercially available reservoir simulators for study of gas hydrates. Furthermore, these simplifications could allow development of solutions to the inverse problem of determining hydrate-reservoir properties, knowing the pressure and production rate response of the reservoir. These possibilities are investigated in this work.

We use one of the comprehensive hydrate simulators, and by the way of an extensive sensitivity studiy find that kinetic parameters do not affect calculation of the gas-production rate from a hydrate reservoir that includes free gas at the bottom and a hydrate cap at top. In such a reservoir, hydrates at top contribute to the produced gas once the reservoir pressure is reduced by gas production from the free-gas zone. The results indicate that for these cases, one can make the equilibrium assumption and use many of commercial thermal simulators. These simulators include appropriate formulation of two-phase fluid flow and heat transfer, but do not include any kinetic equations.

Additional sensitivity studies are presented that suggest that the endothermic heat of hydrate decomposition is provided by the sensible heat of the formation and the heat naturally conducted into the reservoir when the hydrate cap cools. The results suggest a negligible role for convective heat transfer. This significantly reduces the coupling between the heat and fluid flow equations, and allows construction of simplified semi-analytical models.

Possibility of use of such a model as an interpretation tool for determining some of hydrate reservoir properties (i.e. the inverse problem) will be presented.