Numerical studies on CO₂ injection–brine extraction process in a low-medium temperature reservoir system

CO₂ sequestration in deep saline formations has been proved to be an effective method for reducing greenhouse emissions into the atmosphere. However, pure sequestration of CO₂ will add to the costs incurred by both industries and governments. A win–win method of CO₂ injection and hot brine (water) extraction can become attractive to the investors, as it will not only increase the storage capacity of the injected CO₂, but also offset the costs by selling and using the produced hot water for industrial, agricultural or household purposes. For instance, water from very hot geothermal reservoirs (T ≥ 150 °C) can be used for electricity generation in power plants and water from low-medium temperature reservoirs, the most predominant in natural systems, are more popular for direct use, e.g., in heating systems, household hot water, baths, aquaculture, etc. In this paper, low-medium geothermal reservoirs widely distributed in China, especially those in the Ordos Basin, were selected for the numerical case studies using TOUGH2MP with the ECO2N module for the simulations. Generally, simulation parameters were taken from the Ordos Basin, where the first full-integration CO₂ sequestration project had been operated since 2010. The simulations in the base case study lasted 35 years, based on the lifespan of a normal geothermal project. Shallow re-injection systems were also considered to investigate the influence of thermal breakthrough, pressure perturbation, etc. Results show that injection of cold CO₂ causes sharp decrease in temperature in the reservoir region near the injection well, which is enlarged with continuous injection. The region near the production well is dominated by different fluid phases during the CO₂ driven process, including a single water phase, a two-phase fluid (water and CO₂) and a phase of almost pure CO₂. Results also show that the CO₂ breakthrough lags far behind the pressure response in the geothermal production system. Before breakthrough, the injected CO₂ pressurizes the reservoir, improving the overall performance of the geothermal reservoir. Furthermore, the heat extraction efficiency of CO₂-based system is obviously higher than H₂O-based system.