Numerical Simulation of Mixed Brine-CO2/H2S-Rock Interaction During the Reinjection of Non-condensable Gases

Abstract

There is an increased interest in the reinjection of non-condensable gases (NCG) from geothermal power stations into geothermal reservoirs. NCG reinjection can serve as a means of reducing greenhouse gas emissions into the atmosphere, provide reservoir pressure support and possibly can be considered a stimulation method to improve reservoir permeability. Reservoir modeling studies are essential in understanding the behavior of injected gases in the reservoir and forecast possible NCG breakthrough to production wells. The NCG can be injected either as dissolved gas in water or as super critical fluid. This paper aims to determine how a geothermal reservoir rock will behave during injection of a representative mixture of CO2/H2S-water injection. A laboratory scale flow-through experiment was conducted to investigate the complex interaction between CO2/H2S-brine and greywacke rock sample. By introducing detailed reactive chemistry into the injected liquid mixture and mineral composition, this study simulates the conditions of the experiment, and attempts to quantify the fluid-rock interactions for a better understanding of the impact of NCG reinjection on the reservoir porosity and permeability. This is to assess the potential use of long and short term NCG reinjection in conventional geothermal systems. Reasonable match was obtained between experimental data and geochemical modeling using the TOUGHREACT simulator. The simulation study showed that all the reactions are taking place very fast at reservoir temperature, indicating thermodynamically controlled (equilibrium) reactions.