Simulation and optimization of heat transfer in underground in-situ shale upgrading

Abstract

In-situ heating and upgrading technology is a key method to improve the recovery efficiency of shale oil and gas with medium-low maturity. It aims to simulate and optimize the heat transfer effects of underground shales during the in-situ heating and upgrading process to explore the feasibility of in-situ upgrading to accelerate shale ripening and improve the recovery efficiency of shale oil and gas. Therefore， the physical and chemical characteristics of shale samples were tested to determine the basic phase composition， thermal response characteristics， porosity， and other key parameters of characteristic shales. A three-dimensional fracture-network heat transfer model was established. Combined with the finite element fluid simulation technology， the heat transfer law of the fracture-network was systematically simulated and analyzed， and the influence law of fracturing， gas injection volume， gas injection temperature， rock layer heating methods， and insulation on the heating effect of in-situ upgrading was explored. The results show that the thermal loss of the heating gas before entering the fractures can be significantly reduced by improving the insulation effect of the wellbore， thereby increasing the heating efficiency. Compared with unfractured wellbores， the presence of fractures significantly increases the heating efficiency by up to 14 times and nearly quadruples the area of the heat-affected zone. The research also reveals that increasing the gas injection volume and temperature could make a gradient decrease in the heating efficiency of the fractures at the front end of the wellbore， which helps the heating gas spread to more distant areas. This strategy is important for expanding the heat-affected zone. Furthermore， the use of intermittent heating and soaking well strategies can save 53% of the heating gas input compared with continuous heating while keeping the temperature distribution within 5% of the heat-affected zone. By optimizing the heating strategy and improving the insulation effect， the recovery efficiency of shale oil and gas can be increased， which is expected to promote the application of in-situ upgrading technology in the recovery of unconventional oil and gas.

Keywords

• underground shale
• in-situ upgrading
• heat transfer simulation
• optimization strategy
• shale oil and gas development