Hydrate Decomposition and Its Material Balance in a Volumetric Tilted Hydrate-Capped Gas Reservoir by Method of Depressurization

The decline in conventional hydrocarbon resources coupled with the increase in energy demand has encouraged the development of unconventional gas resources. One specific type of unconventional gas is the natural-gas hydrate. It is expected that the substantial size of this hydrate resource will be a motivating factor in its development. In the early phases of development of hydrate reservoirs, data are limited and a large number of sensitivity studies are required. Efficient and accurate analytical models are suitable for such sensitivity studies. One such model is presented in this work. An analytical solution is developed to model the rate of gas generation and hydrate recovery when gas is produced from a tilted hydrate-capped gas reservoir in which the geothermal gradient is accounted for. As a result of the variation in temperature within the hydrate column, the size of the decomposing area increases with continued production. Development of the analytical model in this work is based on a material-balance equation that is combined with the solution for the temperature of the decomposed zone and the hydrate-equilibrium curve. The three equations are solved simultaneously for the gas-generation rate. To validate the assumptions made in the development of the analytical model, a numerical simulator was used that does not make the assumptions of the analytical model. A close agreement is shown between the two models, validating the assumptions made in the analytical solution. The effects of different reservoir parameters on the reservoir performance were investigated by performing sensitivity analyses. The sensitivity results show how a steeper reservoir that extends closer to the base of the permafrost leads to less recovery or how a thicker hydrate cap could maintain the reservoir pressure for a longer time. The model developed in this study can be used as an approximate engineering tool to evaluate the role of hydrates in improving the productivity and extending the life of tilted hydrate-capped gas reservoirs.