Investigation of the Mineral Content and Geomechanical Behaviors of Shale Gas Reservoirs after CO ₂ Injection Using Rock Physics Modeling

Abstract

Gas-bearing shale is a desirable option for geological CO2 sequestration while fracturing and enhancing recovery using supercritical CO2. However, owing to the geomechanical nature of gas shale, there is a chance that leakage will occur during large CO2 injections. It is therefore necessary to assess the geomechanical risks associated with the geological sequestration of CO2 in gas shale. In this study, we measure by modeling the chemical and physical impacts of supercritical (Sc)CO2 injection on the mineral content and elastic properties of organic-rich shale and examine changes in the mechanical characteristics of the Longmaxi upper members (LUM) and Longmaxi lower member (LLM) after flooding with gas for 30 days. Several rock physics models, both before and after ScCO2 injection, are used to accomplish this objective. The LUM and LLM specimens of Well A in the Sichuan Basin are sampled twice. Two specimens are subjected to X-ray diffraction analysis both before and after CO2 saturation. The findings show that CO2-induced invasion would most likely result in a high percentage of quartz and a low content of carbonate and clay minerals. Compared with clay, carbonate reacts more readily to CO2. The elastic properties and densities of both lithologies decrease at varying rates as a result of the post-CO2 injections. The effective modulus of the organic-rich shale is somewhat degraded by CO2 saturation. Also, CO2 injection affects the mechanical properties of the LLM specimen much more than the LUM sample. This is due in particular to the high proportion of clay mineral contained in LUM. ScCO2 has less of an impact on S-wave velocity than on P-wave velocity. A range of shale formations should be studied in order to ascertain how the mineralogy of various shales may affect long-term ability to store CO2, given that organic-rich shales are heterogeneous.