Numerical Investigation of the Gas Production Efficiency and Induced Geomechanical Responses in Marine Methane Hydrate-Bearing Sediments Exploited by Depressurization through Hydraulic Fractures

Methane hydrates are a promising source of natural gas. Several field tests have validated the feasibility of gas production from marine methane hydrate-bearing sediments. However, sustained and commercial production has not been obtained due to limited gas production efficiency and production-induced damage to the mechanical properties in the sediments. This study investigates the gas production efficiency and the geomechanical responses in hydrate-bearing sediments developed by depressurization in hydraulic fractures. A numerical model for the simulation of coupled thermal-hydraulic-mechanical-chemical behaviors is described. The model considers three phases of water, gas, and hydrate in the sediments. Heat transport and the kinetics for hydrate dissociation are also taken into account. A Mohr-Coulomb criterion for marine methane hydrate-bearing reservoirs is employed to describe the shear failure and damaged rock strength caused by depressurization in hydraulic fractures. Then, a base case for a 750-day depressurization in a hydraulic fracture is presented. The spatial and temporal evolutions of the pore pressure, temperature, hydrate dissociation, principal stress, and shear failure are described. Parametric studies for reservoir permeability, depressurization pressure, and fracture number are also presented. The results indicate that early-stage gas rates are high, while they drop significantly with time. At late stages, shear failure is highly correlated with the reduced rock strength, and stress concentrations are obtained near fracture tips. The evolution of strength is correlated with hydrate dissociation as dissociated hydrates weaken the mechanical properties in sediments, which is also time dependent. Local stress concentrations are observed around fracture tips as well. Higher reservoir permeabilities lead to greater early-stage production and lower late-stage production, indicating that the use of hydraulic fractures is preferable in low-permeability reservoirs. Also, increasing the hydraulic fracture number can improve the overall gas production efficiency, while the average gas production efficiency from individual fractures is decreased.