Investigation of Microseismicity Triggered by Raised Pore Pressure through Laboratory CO2 Injection Tests in Berea Sandstone

Abstract:

One of the critical problems for carbon dioxide capture and storage projects is the occurrence of microseismicity due to increased pore pressure during CO₂ injection. The mechanism of microseismicity can be explained by the notion that the injection-induced pore pressure increase can potentially alter the reservoir rock in the form of either creating fractures or triggering slip on pre-existing discontinuities by reducing the effective normal stress. Therefore, it is important to estimate the critical pore pressure (P_cr) to prevent excessive seismicity. The purpose of this study is to attempt to simulate the microseismicity induced from increased pore pressure by CO₂ injection into Berea sandstone. Cylindrical specimens were saw-cut at 30° from the specimen axis. Specimens were either dry or saturated by tap water. The frictional coefficients of the fractures were determined from triaxial shear tests to be 0.71 (dry) and 0.65 (water-saturated). With the frictional coefficients known, we then injected CO₂ (either gaseous of liquid state) into the specimens (either dry or water-saturated) subjected to triaxial stress conditions. Under the conditions of constant confining pressures and axial stresses, we increased pore pressure in steps by injecting CO₂ using a syringe pump. We monitored shear slip along the fracture using axial LVDTs and microseismicity using an acoustic emission sensor. The critical pore pressure that would initiate shear slip along the fracture was calculated from the Coulomb friction law. When CO₂ was injected into dry specimens, shear slip and associated microseismicity started to occur at the pore pressure levels exactly estimated from the Coulomb theory. However, when CO₂ (both gaseous and liquid states) was injected into water-saturated specimens, the same were initiated at pore pressures slightly higher (by 1.2-3.7 MPa) than that estimated from the Coulomb friction law. These results suggest that the presence of water and associated water-CO₂-rock interaction might probably strengthen friction of fractures in Berea sandstone, which needs further investigations. Nonetheless, our experimental study demonstrates that the simple Coulomb friction theory can conservatively be utilized to estimate P_cr to prevent microseismicity triggered by shear slip during CO₂ injection.