Non-linear stress-strain behaviour of reservoir rock under brine saturation: An experimental study

CO₂ geo-sequestration process in deep saline aquifers has recently attracted attention as it addresses one of the current global issues of climate change. Suitable saline aquifers for CO₂ storage are generally located very deep underground, where the aquifer s pore fluid is highly saline. The strength of the reservoir rock mass is important to ensure safe sequestration, it is essential to check the aquifer rock mass strength in this highly saline environment. Although the Mohr-Coulomb failure criterion is widely used for many geotechnical engineering applications, its applicability to brine-saturated sedimentary rocks or saline aquifers has not been confirmed to date. This study therefore identifies the brine saturation effect on the stress-strain behaviour of saline aquifer reservoir rock (sandstone) under in-situ stress conditions, to check and if necessary modify the conventional Mohr-Coulomb failure criterion to capture the brine saturation effect on reservoir s rock strength parameters. A series of tri-axial experiments was conducted on Hawkesbury sandstone samples, obtained from the Sydney basin, for a range of confining pressures (5-20 MPa) and brine saturation conditions (0% , 10% , 20% and 30% NaCl concentrations) at 25 ºC constant temperature. According to the experimental results, the presence of NaCl in pore fluid causes the reservoir rock strength and shear parameters (friction angle and cohesion) to be increased in deep saline aquifers, and this effect increases with increasing brine concentration in the pore fluid. In addition, the presence of brine causes the dilatancy strength of saline aquifer rocks to be enhanced and the post-peak dilation (at which macroscopic shear faults may occur in the rock mass) of saline aquifer rock mass to be reduced. These enhance the micro-fracturing resistivity of the rock mass and increase the rock mass stability, both of which are favourable for the long-term integrity of the CO₂ sequestration process in deep saline aquifers. In addition, the conventional linear Mohr-Coulomb failure criterion fails to simulate the measured stress-strain data of brine-saturated Hawkesbury sandstone. This can be precisely corrected using the modified failure criterion proposed in this study.