Effects of the seepage capability of overlying and underlying strata of marine hydrate system on depressurization-induced hydrate production behaviors by horizontal well

Marine gas hydrates mostly occur in high-water content and non-diagenetic submarine sediments without tight stratigraphic traps. Therefore, the seepage capability of overlying and underlying strata (OUS) serves as a crucial parameter to analyze hydrate production behaviors. This study focuses on the numerical simulation study on the effects of the seepage capability of OUS on hydrate production behaviors in depressurization-induced production of three-phase hydrate deposits using a horizontal well. As shown by the simulation results, in the early stage of hydrate production, the permeability of the OUS had little effects on gas production rates. In long-term hydrate production, the hydrate deposit with lower permeability OUS has higher cumulative gas production, larger gas-water ratio and hydrate dissociation scope, and less cumulative water production. Moreover, the hydrate production efficiency first decreased rapidly and then tended to be stable as the permeability of OUS increased, and hydrate production behaviors are similar while the permeability of OUS larger than 1.0 mD. Herein, a water-resisting layers were constructed in overlying and/or underlying strata to decrease the seepage capability of OUS. Compared to the original hydrate deposits, the cumulative gas production and gas-water ratio increased by 31% and 147%, respectively but the water production was only 76% on the 5th year of the simulated production in which water-resisting layers were designed. Furthermore, the water-resisting layer in underlying strata produced more significant effects on hydrate production efficiency than the layer in overlying strata. All these indicate that the low-permeability OUS (especially in underlying strata) can restrain fluid invading into hydrate reservoirs, increase the pressure gradient of hydrate reservoirs and significantly improve the hydrate production efficiency during long-term hydrate production. Therefore, water-proof reformation in OUS may be an effective method to improve the energy utilization efficiency and promote the process of hydrate industrialization. However, further research is needed.