A simultaneous gas and water reinjection system for a high temperature geothermal project has successfully been modeled, which provides critical insight into how to better design and operate a gas and water injection system under various operating conditions. A model of the system was developed using the OLGA™ dynamic multiphase flow simulator software, including two-phase buoyancy and thermal effects, and has been able to accurately model the downhole injection of non-condensable gases (NCGs) (consisting of primarily CO2) into a water injection stream. The overall results of the modeling analysis have demonstrated that the single-well injection system is a feasible approach to reinject both NCGs and water.

The modeling feasibility study was performed using the OLGA simulator, which is a general transient multi-phase simulator for flow in pipes and wellbores used extensively within the oil industry to model multi-phase flow. The OLGA simulator simulation model developed was able to account for the individual properties and solubility of all seven gas components found in the geothermal fluid (CO2, H2S, NH3, Ar, N2, CH4, and H2). The model was then used to perform sensitivities to investigate the impact mixing depth, injectivity index, and gas mixture solubility would have upon the required gas injection pressure.

The system was modeled through complete operating conditions including start-up, steady-state operation, and shut down, in order to best understand how the system will perform under both static and dynamic operating conditions.

The results indicate that the optimum gas reinjection depth, which would minimize cost and the injection pressure, is between 300 m and 500 m depth for the system modeled. The model indicates that the NCG-water mixture can be injected under all conditions examined, provided enough injection pressure on the brine and NCG injection line. In addition to minimizing injection power requirements, the OLGA simulator model has also benefited the project by demonstrating that by reducing the gas injection depth the difficulty of shutdown and startup operations for the injection well can be reduced by reducing the gas injection pressure.

Overall, the modeling feasibility study has confirmed the concept of using a single well for concurrent NCG and water reinjection, the next step will be to design the well completion to implement this injection system.