Natural gas hydrate have been estimated to make up the largest component of fossil fuel resource worldwide. Hydrate reservoirs may be found in different geologic settings including deep ocean sediments and arctic areas. Some reservoirs include a free-gas zone beneath the hydrate, and such a situation is referred to as a hydrate-capped gas reservoir. Gas production from such a reservoir could result in pressure reduction in the hydrate cap and endothermic decomposition of hydrates. Well testing in a hydrate-capped gas reservoir is a developing art; development of a new model, incorporating the dynamic effects of gas hydrate decomposition, is necessary for realistic and accurate predictions. This paper presents a two-dimensional (r,z) mathematical model for a constant rate drawdown test performed in a well completed in the free gas zone of a hydrate-capped gas reservoir during the early-time period of production. Using energy and material balance equations, the effect of endothermic hydrate decomposition appears as an increased compressibility in the resulting governing equation. The Laplace-domain solution for the dimensionless wellbore pressure is derived using Laplace and finite Fourier cosine transforms. The solution to the analytical model was compared with a numerical hydrate reservoir simulator over some range of hydrate reservoir parameters. The results show that the effect of decomposition on wellbore pressure is substantial. The analytical model can be used for analysis of drawdown tests in hydrate-capped gas reservoirs. Furthermore, it may be used to quantify the contribution of hydrate decomposition on production performance.
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