An integrated study of fluid–rock interaction in a CO2-based enhanced geothermal system: A case study of Songliao Basin, China

• We evaluated the interactions between rock, brine, and CO2 in CO2-EGS using laboratory experiments.
• We examined changes of the dissolved ionic composition of the solution.
• Minerals dissolve (feldspar and calcite) or precipitate (secondly carbonates).
• We used numerical simulations to reproduce chemical processes of CO2-EGS.
• Numerical simulations were generally consistent with experimental results.

The reactive behavior of a mixture of supercritical CO2 and brine under physical–chemical conditions relevant to the CO2-based Enhanced Geothermal System (CO2-EGS) is largely unknown. Thus, laboratory experiments and numerical simulations were employed in this study to investigate the fluid–rock interaction occurring in the CO2-EGS. Rock samples and thermal–physical conditions specific to the Yingcheng Formation of Songliao Basin, China, an EGS research site, were used. Experiments were conducted by using of reactors at high temperature and pressure. Six batch reaction experiments injected with supercritical CO2 were designed at temperatures of 150–170 °C and a pressure of 35 MPa. Moreover, a separate experiment at the same experimental conditions without injection of CO2 was also conducted for comparison. Analyses of scanning electron microscopy (SEM) and X-ray diffraction (XRD) of the resulting solids were conducted to characterize changes in mineral phases. Numerical simulations were also performed under the same conditions as those used in the experiments. Significant mineral alterations were detected at the CO2-EGS reservoir, which may change the properties of fluid flow. The presence of supercritical CO2 led to an dissolution of primary minerals such as calcite and K-feldspar and precipitations of secondary carbonate such as calcite and ankerite. The numerical simulations were generally consistent with laboratory experiments, which provide a tool for scaling the time up for long period of reservoir simulations. The information currently available for the mineral alteration at high-temperature natural CO2 reservoirs is generally consistent with those of our lab experiments and numerical simulations.