Study on P-wave velocity and mechanical response characteristic of rock in coal seam roof with supercritical CO₂ injection

Deep coal seam CO₂ geological sequestration and enhanced CH₄ recovery(CO₂-ECBM) can both increase CBM recovery and achieve carbon emission reduction, possessing dual benefits of energy and environment. The geochemical reactions between supercritical CO₂(ScCO₂), water and coal seam roof can change its physical-mechanical properties and increase the risk of CO₂ leakage. In this paper, taking the roof rock of No.3 coal seam in Hudi Mine from Qinshui Basin as the research area, the ScCO₂-water-rock geochemical reaction simulation experiment was carried out to explore the geochemical reaction process of ScCO₂-water-roof under the condition of CO₂ coal seam storage and its influence on P-wave velocity and mechanical properties of rock. Results show that the chemical dissolution reaction between ScCO₂, water and rock results in the significant decrease of Ca and Mg elements in the rock samples, which promotes the formation of isolated dissolution pores. And extensive "corrosion pits" and "corrosion fractures" are developed as the reaction time goes on. ScCO₂-water-rock reaction raises the discontinuity of internal structure of rock samples, increases the propagation path and energy loss of acoustic wave, and reduces the P-wave velocity. After ScCO₂-water-rock reaction, the peak strength and elastic modulus of rock samples decrease, while the Poisson's ratio increases. There is a logistic function relationship between peak strength change rate, elastic modulus change rate and Poisson's ratio change rate with reaction time. For this study area, the change of roof mechanical properties in the process of ScCO₂-water-rock reaction is not enough to cause cap rock fractures and CO₂ leakage, while the influence of adsorption swelling stress should also be emphasized when evaluating the security of CO₂ geological storage in a coal seam.