Effects of drying, CO₂, H₂O, and CO₂-H₂O treatment on the acoustic characteristics of marine shales

The marine shale gas reservoirs are viable geological target formations for CO2 storage to mitigate greenhouse gas emissions in South China. CO2 would be injected into the in-situ or the depleted shale gas reservoir, but the effects of CO2 on the pore-fracture structure of shales are different for these two types of shale gas reservoirs, which have different moisture contents. The acoustic characteristics of shale can reflect the pore-fracture structure and mechanical strength of shale effectively, so it is crucial to obtain the acoustic features of CO2 injected shale. This paper presents a set of measurements on drying, CO2, H2O, and CO2-H2O treatment on the acoustic characteristics of marine shales. The variations of acoustic parameters for drying, CO2, H2O, and CO2-H2O treated shales were contrastively analyzed. The results indicate that ultrasonic waveform can be characterized to three stages, including main wave, attenuation wave, and coda wave. Compared with dry shales, the CO2-treated, H2O-saturated, and H2O-CO2 treated shales P-and S-wave velocity and amplitude are decreased, but the attenuation coefficient increased. Variations of acoustic parameters for H2O-CO2 treated shales are most significant, and their macro-behaviors of mechanical softened are obvious. The effect mechanisms on the acoustic characteristics of CO2-treated, H2O-saturated, and H O-CO treated shales are different. The pressure and swelling 2 2 are factors that affect acoustic characteristics of CO2-treated shales. The hydration is the factor that affects acoustic characteristics of H2O-saturated shales. However, the integrated effects produced by pressure, swelling, hydration, and acidification that affect acoustic characteristics of H2O-CO2 treated shales.