Numerical simulation and optimization of CO₂-enhanced water recovery by employing a genetic algorithm

Carbon dioxide geological sequestration simultaneously combined with water production from deep saline aquifer can effectively address the challenge faced by modern energy systems of reducing carbon dioxide emissions and water intensity while providing reliable, affordable, and secure energy. However, little attention has been paid to date in the literature on determining the carbon dioxide injection strategy for achieving both the optimal water production and the optimal carbon dioxide storage capacity while maintaining operational safety. This paper first establishes three injection–extraction scenarios based on the typical geological parameters of the Junggar Basin in China to analyze the effect of carbon dioxide injection on water extraction and the effect of water extraction on the carbon dioxide storage. The three injection scenarios considered are sole carbon dioxide injection, sole water production and combined carbon dioxide enhanced water recovery. The carbon dioxide enhanced water recovery scenario is then optimized using a genetic algorithm to determine the trade-off between the maximum possible water recovery and the safe carbon dioxide storage for both the constant mass injection and the variable time-dependent mass injection (for achieving constant pressure injection). The influence of number of pumping wells is also analyzed. It can be concluded from this work that the carbon dioxide enhanced water recovery technology can effectively manage the pressure perturbation caused by the carbon dioxide injection as well as the water production while significantly enhancing the carbon dioxide storage capacity, security and water production efficiency. Injection strategy is essential to the efficiency of carbon dioxide enhanced water recovery; the genetic algorithm based optimizer combined with the well-known multi-phase flow solver TOUGH2, designated as GA-TOUGH2 is used for the optimization of injection scenarios. It is found that in the allowable range of pressure perturbations, a higher carbon dioxide injection rate is more beneficial for carbon dioxide storage as well as for water production. Constant pressure injection operation is superior to the constant rate injection and leads to higher carbon dioxide storage capacity and stability, and water production efficiency. Adding pumping wells can somewhat enhance the carbon dioxide storage capacity especially for the constant pressure injection operation compared to the constant rate injection operation as well as the water production efficiency; however, the enhancement in water production is very small compared to the single well and cannot be regarded as economical considering the additional capital cost of well drilling. This paper addresses the method of combined numerical simulation and optimization for carbon dioxide enhanced water recovery from saline aquifers that can be employed in the implementation of enhanced water recovery at industrial scale.