CO2 Storage and Enhanced Heat/Gas Recovery in Fractured Deep High-Temperature Tight Gas Reservoirs: A Simulation Study

Deep reservoirs (5km-8km), with their high-temperature geological environments and sealed structural advantages, are emerging as a global research focus and potential application area for low-carbon energy transition. This paper proposes a novel approach for developing CO2-enhanced geothermal systems (CO2-EGS) in deep gas reservoirs. By leveraging the adsorption displacement characteristics of supercritical CO2 to displace and enhance natural gas recovery, in situ adsorption sequestration achieves carbon geological storage. Combining the low viscosity and high diffusivity of supercritical CO2 enhances deep geothermal energy extraction. Based on thermo-fluid dynamics and CO2 isothermal adsorption theory, this study establishes a model for gas recovery and heat extraction through CO2 injection and production via long horizontal wells with fracture networks. Using the Finite Element Method (FEM), the coupling of temperature-pressure equations in CO2 horizontal wellbores and fractures with reservoir temperature fields and flow fields is solved. The impacts of wellhead injection-production parameters, fracture dimensions, and reservoir properties on geothermal energy extraction efficiency and CO2 sequestration efficiency are analyzed. The results from the base case confirm the system's technical feasibility, demonstrating that CO2 injection can effectively create a second gas recovery peak while establishing a stable geothermal production plateau lasting for over 15 years. A systematic sensitivity analysis reveals the key trade-offs that govern system performance. A higher CO2 injection rate accelerates the initial recovery of gas and heat but leads to premature thermal and CO2 breakthrough, which in turn reduces the ultimate recovery factor and shortens the system's geothermal production lifespan. A lower production bottom-hole pressure (BHP) maximizes gas recovery during the primary depletion phase but diminishes the effectiveness of the subsequent CO2 displacement and heat extraction. Conversely, a higher BHP, while boosting the heat extraction rate, shortens the geothermal production period by increasing the system-wide pressure differential, which accelerates the advection of the cold front. Similarly, while denser fracture spacing improves the gas recovery factor by increasing the contact area, it leads to thermal short- circuiting, shortening the heat production period. Higher reservoir permeability enhances initial productivity but compromises long- term sweep efficiency and net CO2 storage. This research provides a theoretical foundation for the design and optimization of CO2-enhanced geothermal systems in deep high-temperature tight gas reservoirs and offers critical insights into integrating CCUS (carbon capture, utilization, and storage) technology with geothermal energy.