Experimental Study on the Depressurization of Methane Hydrate in the Clayey Silt Sediments via Hydraulic Fracturing

Poor seepage capacity always occurs in the clayey silt hydrate reservoir, making it more difficult to achieve depressurization. Hydraulic fracturing is an important means of enhancing reservoir seepage and increasing gas production rates. In this work, a series of depressurization experiments were performed in a triaxial hydraulic fracturing apparatus. The samples were prepared according to the physical parameters of samples obtained from the South China Sea. The fracturing behavior and gas production characteristics of methane hydrate-bearing sediments were investigated. The effects of hydrate saturation, fracturing fluid properties, and in-situ stress on fracturing behavior were analyzed. It was found that it was feasible to be fractured for sediments with low hydrate saturation (S-h < 0.30), and the fracture initiation pressure was positively correlated with the hydrate saturation. The results showed that medium-viscosity fracturing fluid had excellent flow and filtration loss reduction, which were beneficial to the maintenance of fractures inside sediments. The in-situ stress difference determined the direction of fracture propagation, which was always along the vertical direction of the minimum principal stress. Besides, the gas production enhancement and permeability improvement by hydraulic fracturing were evaluated. The average gas production rate and peak rate in the fractured run were 3.01 times and 5.84 times of those in the unfractured run, respectively. To further enhance the seepage capacity, the inorganic salt-based clay stabilizer solution was added to the fracturing fluid. The results showed that the permeability of the sediments fracturing with NH4Cl was 3.07 times of that in the unfractured run due to the inhibition of the hydration swelling of clay particles. The results are valuable to provide a basic understanding and suggestions for the application of hydraulic fracturing in gas hydrate reservoirs.