Simulation of Reinjection of Non-Condensable Gas-Water Mixture into Geothermal Reservoirs

摘要

Injection of non-condensable gases (NCG) into geothermal reservoirs can be used to reduce greenhouse gas emissions, providing reservoir pressure support and possibly improve reservoir permeability. First, we review existing field trials of NCG reinjection in geothermal fields. We then use numerical reservoir modelling to assess the effect of NCG reinjection on energy recovery, understand permanent trapping, and forecast potential NCG breakthrough into production wells. This modelling study was conducted to assess the feasibility of using water-NCG mixture injection into a geothermal reservoir. Although the different NCG species from geothermal systems have moderate solubility in water, formation of gas phases at lower pressures and/or the shallow subsurface requires careful consideration of the rate and composition of NCG. This is to assess the potential risk of growing into fingers that may lead to an early breakthrough or potentially leakage to the surface. In order to investigate the influence of injected NCG concentrations and determine the spatial distribution of NCG's, a four gas version of the TOUGH2 reservoir simulator was used. This version of TOUGH2 can handle non-isothermal flow of multiphase, multicomponent reacting flows of mixtures of water, H_2S, CO_2, N_2 and CH_4. Modified version of the benchmark geothermal reinjection model was constructed with initial conditions of a liquid-dominated geothermal system. The paper describes a series of numerical modelling scenarios to investigate the effects of NCG-water injection on steam production. Possible fluid paths and distribution of gas components were investigated to estimate the NCG storage capability of a reservoir and to identify potential breakthrough and leakage events. The results obtained show that the effects of injection depend on the reinjection well pattern and recharge conditions. The risk of leakage to the surface is very limited since the injected NCGs remains in the liquid phase.