Numerical simulation and optimization of injection rates and wells placement for carbon dioxide enhanced gas recovery using a genetic algorithm

The aim of CO₂ enhanced gas recovery (CO₂-EGR) is to extract more natural gas from depleted gas reservoirs and simultaneously sequestrate large amount of CO₂. To achieve these dual objectives, the optimization of CO₂ injection strategy in CO₂-EGR is proposed to achieve maximum benefit. Thus, the focus of this work is to study CO₂-EGR by numerical simulation and optimization. For this purpose, a 3D reservoir model with ‘five-spot’ well pattern (an injection well and four production wells) is established for simulations using the multiphase simulator TOUGH2, and the main focus is on optimization of CO₂ injection rate and well placements by coupling the genetic algorithm (GA) with TOUGH2. Simulations are performed to determine the optimal injection rate for both two types of injection wells placements, one vertical injection well with number of perforations at various depth and orthogonal horizontal wells on top of the reservoir. The results show that multipoint perforations in the vertical injection well and the horizontal injection wells with appropriate length show improvement in natural gas recovery and CO₂ storage compared to the injection location at the reservoir bottom. More importantly, the optimized injection rate determined by GA-TOUGH2 can substantially improve the natural gas recovery factor for both vertical and horizontal well injection. After computing a number of optimized cases, it's shown that the case employing the horizontal injection wells with optimized injection rate of 0.0778 kg/s achieves the maximum recovery of 67.21% and maximum CO₂ storage efficiency of 69.54%, equal to 23.88 × 10⁶ kg of natural gas exploited from the depleted reservoir and 74.52 × 10⁶ kg of CO₂ stored in pore volume of 1.81 × 10⁶ m³ of the reservoir. This case also has minimum injection pressure jump of 0.08 MPa thereby reducing the risk of formation fracture and CO₂ leakage. However, by conducting a simple cost/benefit analysis, it is determined that this optimally performing case is not the best economically because of higher drilling cost of horizontal well compared to a vertical well. It is found that a vertical well with two suitably placed perforations with optimized injection rate of 0.1025 kg/s is economically superior since the previously used production wells can be reused for CO₂ injection, which can therefore reduce both the equipment and the drilling costs. It is hoped that the findings of this work should provide some insights into the optimization of CO₂-EGR with maximum economic benefit for implementation in industrial practice.