Experimental investigation on the geological responses and production behaviors of natural gas hydrate-bearing sediments under various hydrate saturations and depressurization strategies

Natural gas hydrate is a potential alternative energy that has attracted attention from scientific and industrial communities. Depressurization is one of the most effective approach for extracting natural gas from hydrate reservoirs in oceans. Clarifying multi-field coupling effects between geomechanical responses and fluid production in hydrate reservoirs is very important for identifying a suitable depressurization strategy for gas production. Here, we employed a customized apparatus for modeling hydrate reservoirs and their production behaviors under the multi-field coupling effect, and a series of experiments were conducted to investigate the influence of hydrate saturations (from 11.09% to 42.14%) and depressurization strategies (single depressurization of 3, 6, 10 MPa and multiple depressurization). The results show that gas production via depressurization includes pore pressure reduction, massive hydrate dissociation and residual hydrate dissociation stages. The second stage requires more attention due to the noteworthy evolutions of permeability, deformation, and sand production of hydrate-bearing sediments. Single deep depressurization of 10 MPa in the sediments with high hydrate saturations could improve gas recovery (almost triple the gas production rate) but increase engineering risks, such as sediment subsidence (up to 501.1%) and sand production (up to 426.4%), compared to that of single depressurization of 3 MPa. A marginal effect is observed due to the limited hydrate content and boundary effect of the experimental reactor, and this difference from field production cannot be neglected. Multiple depressurizations are beneficial to reservoir stability under identical depressurization amplitude. The cumulative deformations of double and triple depressurization are 89.1% and 75.4% of that of single depressurization, respectively. Considering the observed coupling responses, a multi-objective optimization method is suggested for depressurization strategy selection.