Permeability Evolution in Hydrate-Bearing Sediments during Hydrate Recovery by Depressurization Combined with Replacement

Permeability is a key parameter that affects hydrate recovery. However, the evolution law of reservoir permeability in depressurization combined with the replacement process is still unclear. In this study, depressurization combined with replacement behavior of an argillaceous silty hydrate reservoir in a reactor used for depressurization-assisted replacement was experimentally studied to understand the permeability evolution of an argillaceous silty hydrate reservoir. In the first replacement stage, a slight increase of 0.77–5.62% in the mixed hydrate saturation (SH-mixed) led to a small decrease of 0.1–4.13 mD in permeability. In the hydrate dissociation stage, the pore pressure decreased to 2 MP, causing a significant decrease in sample permeability, up to 39.31%. When CO2 was reinjected into the sample, the recovery of reservoir permeability varied from 1.69 to 6.45%. Due to the sample permeability damage happening in the depressurization phase, reservoir permeability damage rate (PDR) in the process of second replacement varied from 1.50 to 7.64%. When the depressurization pressure of hydrate samples was increased, the reservoir PDR increased during second replacement, with a variation range of 3.85–13.01%. The permeability change rate of the sample reached 39.31–67.06% in the depressurization stage. Reasonable control of reservoir damage caused by depressurization can effectively protect the permeability of the reservoir.