Effect of the Codetermined Seepage Ability by Varying Hydrate Saturation and Rock-Matrix Permeability on Depressurized Gas Coproduction in Hydrate Reservoir with an Underlying Free-Gas Layer

Within hydrate reservoir, depressurization in the underlying free-gas layer could facilitate efficient gas coproduction, provided that its overall seepage ability is conducive to pressure propagation. Such overall seepage ability is codetermined by hydrate saturation (<i>S</i> _H) and rock-matrix permeability (<i>k</i>), and varies across different hydrate reservoirs and even at different locations of the same reservoir. In this study, we designed an experiment/simulation integrated approach to investigate the effect of the codetermined overall seepage ability by varying hydrate saturation and rock-matrix permeability on depressurized gas coproduction. The experiment reveals that under the conditions of moderate saturation (<i>S</i> _H = 0.19) and low rock-matrix permeability (<i>k</i> = 1.5 mD), gas coproduction rate initially increased quickly, followed by a gradual decrease until eventual stabilization. The prediction results of a coupled Thermo-Hydro-Chemical model with carefully specified parameters can match well with the experimental measurements. The simulated spatiotemporal evolution of pressure and <i>S</i> _H in the whole reactor demonstrates that pressure propagates preferentially in the free-gas layer, and then gradually propagates vertically upward into the overlying hydrate-bearing layer, causing slow hydrate dissociation. Thus, the initial rapid gas production is mainly dominated by free gas production, while the low-rate gas production stage is controlled by hydrate dissociation. Raising the initial <i>S</i> _H to a high level (0.40) only further suppresses the pressure propagation within the hydrate-bearing layer, without significantly reducing the peak gas production rate. Comparatively, increasing the <i>k</i> to a moderate level (25 mD) significantly facilitates pressure throughout the reservoir, causing a several-fold increase in the peak gas production rate. However, in this case, the rapid hydrate dissociation results in excessive consumption of the reservoir's latent heat, thereby reducing the driving force for sustained dissociation and ultimately constraining the increase in the gas production rate during the subsequent low-rate gas production stage.