The objective of this study is to develop an analytical model to calculate hydrate recovery and rate of gas generation when a hydrate-capped gas reservoir is produced at a constant bottomhole pressure. A similar model has been developed in the past for constant rate production. Such models are useful, when large number of sensitivity studies are necessary to evaluate the uncertainty in reservoir parameters and only limited specific properties of a hydrate reservoir are available.
In this work, a tank-type material balance equation is considered. This is combined with the solution for the temperature of the hydrate cap to obtain the rate of gas generation during the period of constant bottomhole pressure production from a hydrate-capped gas reservoir. A numerical solution was used to validate the assumptions made to develop the analytical model; this numerical model relaxes some of these assumptions.
The role of hydrate in improving the productivity and extending the life of hydrate-capped gas reservoir is demonstrated. The results show that the overlying hydrates can have a significant effect on improving the productivity of the underlying gas reservoir and increasing the reserve.
In this study we develop an analytical model to calculate hydrate recovery and rate of gas generation when gas is produced at a constant bottomhole pressure and decomposition occurs throughout the hydrate cap. For this purpose, a volumetric material balance equation is developed. The solution for the temperature of the hydrate cap is combined with the tank-type material balance equation and is solved for the rate of gas generation during the period of constant bottomhole pressure production from a hydrate-capped gas reservoir. In the following, after presenting the analytical modeling of the process, we perform a sensitivity study to investigate the effect of various reservoir parameters on reservoir performance, including the importance of porosity, thickness of hydrate zone, thermal conductivity and well bottomhole pressure.
In order to obtain an analytical model to predict the recoverability of hydrate during constant bottomhole pressure production from a hydrate-capped gas reservoir, we assumed decomposition of hydrate occurs simultaneously throughout the hydrate layer.
Four equations are used to develop the analytical model for predicting the behavior of the hydrate-capped gas reservoir during the period of constant bottomhole pressure production:
Material balance equation
Energy balance equation
Hydrate decomposition model
Inflow performance equation
In the following, first the above mentioned equations are explained, and then the mathematical modeling of the process leading to the analytical model is presented.
It is assumed that the pressure and temperature within the reservoir are instantaneously uniform; a depletion type material balance model can be used to predict the average reservoir pressure of the hydrate-capped gas reservoir. The simplified form of this equation for a volumetric hydrate-capped gas reservoir can be written as (Gerami et al., 2006):
Equation (1) (Available in full paper)
In the above equation, Gp and Gg are cumulative gas production and generation, respectively, and Gf is the initial free gas-inplace.
