Numerical modelling of the interaction between geothermal injectate-non condensable gas solutions and greywacke

Abstract

In response to global warming, it is important to explore alternative disposal technologies for greenhouse gas emissions in the geothermal power sector. One alternative which has received widespread focus is co-injection of non-condensable gases with the waste fluids from geothermal operations. Passarella et al. (2015) simulated the interaction between brine with dissolved CO2 and H2S, and a sample of greywacke in their laboratory. The present work aimed to numerically model the results from the experiment using TOUGHREACT. The goal of this study was to develop numerical simulation techniques to investigate the effects of the reinjection of brine with dissolved NCG. The resulting model provided insights into the geochemical interaction of greywacke with brine-NCG solutions under simulated reservoir conditions. The numerical simulations show that mineral dissolution occurred to a greater degree than precipitation, leading to increased permeability and porosity. It was also observed that the mineral reactive surface areas evolved as mineral dissolution progressed, through etch pit formation. Additionally, flow rate had an impact on the overall reaction rates such that a decrease in injection rate led to a corresponding decrease in reaction rates. Lastly, both experimental data and model outputs indicated that CO2 was minimally sequestered in the simulation, while H2S was clearly captured as pyrite. A similar numerical investigation was conducted on the co-injection of NCG with steam condensates subsequently. The modelling results indicate predominant mineral dissolution, CO2 is expected to be captured as magnesite within the reactor, while H2S is still captured as pyrite.