The Key Factors of Low-Frequency Electric Heating Assisted Depressurization Method in the Exploiting of Methane Hydrate Sediments

Abstract

Natural gas hydrate is widely distributed in the permafrost and marine deposits, and is regarded as an energy resource with great potential. The low-frequency electric heating assisted depressurization (LF-EHAD) has been proven to be an efficient method for exploiting hydrate sediments, which involves complex multi-physics processes, i.e. current conduction, multiphase flow, chemical reaction and heat transfer. The physical properties vary greatly in different hydrate sediments, which may profoundly affect the hydrate decomposition in the LF-EHAD process. In order to evaluate the influence of hydrate-bearing sediment properties on the gas production behavior and energy utilization efficiency of the LF-EHAD method, a geological model was first established based on the data of hydrate sediments in the Shenhu Area. Then, the influence of permeability, porosity, thermal conductivity, specific heat capacity, hydrate saturation and hydrate-bearing layer (HBL) thickness on gas production behavior is comprehensively analyzed by numerical simulation method. Finally, the energy efficiency ratio under different sediment properties is compared. Results indicate that higher gas production is obtained in the high-permeability hydrate sediments during depressurization. However, after the electric heating is implemented, the gas production first increases and then tends to be insensitive as the permeability decreases. With the increasing of porosity, the gas production during depressurization decreases due to the low effective permeability; while in the electric heating stage, this effect is reversed. High thermal conductivity is beneficial to enhance the heat conduction, thus promoting the hydrate decomposition. During depressurization, the gas production is enhanced with the increase of specific heat capacity. However, more heat is consumed to increase the reservoir temperature during electric heating, thereby reducing the gas production. High hydrate saturation is not conducive to depressurization because of the low effective permeability. After electric heating, the gas production increases significantly. High HBL thickness results in a higher gas production during depressurization, while in the electric heating stage, the gas production first increases and then remains unchanged with the increase of thickness, due to the limited heat supply. The comparison results of energy efficiency suggest that electric heating is more advantageous for hydrate sediments with low permeability, high porosity, high thermal conductivity, low specific heat capacity, high hydrate saturation and high HBL thickness. The findings in this work can provide a useful reference for evaluating the application of the LF-EHAD method in gas hydrate sediments.