A model for predicting gas well performance of free gas zone beneath hydrate layer

Free gas beneath hydrate is an important natural gas resource and may be more feasible to exploit than dispersed hydrate of low hydrate saturation. Here we construct a single well production model of sub-hydrate free gas in which pressure and pressure gradient at any node of a finite element are expressed by gas density and density increment at the point. The model is used to predict the reservoir pressure and cumulative production by depressurization from a single vertical well. Gas production capacity correlates with well fluid pressure, gas saturation, permeability, and of course exploitation time. The model is tested by using the production data from the Dako trial reservoir tests. The simulation shows that gas production rate is almost constant over the course of production (e.g., there is very little decline) in the case of a 100% gas reservoir. The numerical computation also indicates that the free gas reservoir beneath the hydrates at the Blake Ridge may be of economic value for industry production. For a 69 m thick gas reservoir with 20% gas saturation, 8.65 × 108 m3 (∼ 618 thousand tons) of gas can be extracted within 10 years for a permeability of 10 mD, and the mass fraction of the gas in the fluids produced at the well during production period is 46%.