Simulation Study on the Heat Transfer Characteristics of Oil Shale under Different In Situ Pyrolysis Methods Based on CT Digital Rock Cores

Abstract

To analyze the heat transfer characteristics of oil shale under different in situ pyrolysis methods from a microscopic perspective, a combination of experimental and simulation approaches was employed. Initially, high-temperature in situ pyrolysis experiments on single-fracture oil shale were conducted using high-temperature steam and electrical methods. Subsequently, micro-CT scanning technology was utilized to obtain digital rock cores under different in situ pyrolysis conditions. Finally, these digital rock cores were seamlessly integrated with COMSOL 6.0 to achieve numerical simulations of high-temperature steam convective heating and electrical conductive heating in the in situ state. The relevant conclusions are as follows: Firstly, during the in situ pyrolysis of oil shale with high-temperature steam convective heating, the overall temperature increase is uniform and orderly. Heat is conducted gradually from the pores and fractures to the matrix. The uneven distribution of pores and fractures causes an uneven temperature field, but no localized overheating occurs, which can effectively enhance the pyrolysis efficiency. Secondly, the heat transfer direction in electrical conductive heating is primarily inward along the normal direction of the heat source end face. The closer the section is to the heat source end face, the higher the rate of temperature increase. Within 1 s, the temperature rise at 100 μm (near the heat source end face) is 2.27 times that at 500 μm (near the farthest cross-section from the heat source end face). The heat transfer effect of high-temperature steam convective heating consistently surpasses that of electrical conductive heating. The Tc value initially increases and then decreases as pyrolysis progresses, reaching a maximum of 1.61331 at 0.4 s, but Tc remains greater than 1 throughout. Finally, in the initial stages of pyrolysis, the high-temperature region formed by conductive heating is superior to that of convective heating. However, once the heat carrier fluid flow stabilizes, the volume of the high-temperature region formed by convective heating grows rapidly compared to that of conductive heating. At 1 s, the volume of the high-temperature region formed by convective heating reaches 5.22 times that of the high-temperature region formed by conductive heating.