Determination of effective stress parameters for effective CO2 permeability in deep saline aquifers: An experimental study

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Global warming has been a major threat to the world for many decades, and CO2 geo-sequestration in deep saline aquifers has recently been identified as an effective solution due to its ability to greatly mitigate anthropogenic CO2 emissions to the atmosphere. However, CO2 sequestration-induced chemical and mineralogical reactions affect the hydro-mechanical characteristics of natural formations, resulting in limited injectability to aquifers. A detailed knowledge of the hydro-mechanical behaviour of natural formations is therefore important to enhance the safety and effectiveness of the CO2 storage process. Such understanding can only be gained on the basis of in-depth knowledge of the applied effective stresses on the formations. The aim of this study was therefore to understand the effect of reservoir salinity level on the effective stress parameters of deep saline aquifer rock under various in-situ conditions, including salinity levels ranging from 0 to 30% (NaCl concentration by weight) and confining pressures ranging 20–35 MPa. Tri-axial permeability tests were conducted for a range of injection pressures (1–12 MPa) under different confining pressures (20, 25, 30 and 35 MPa) at 35 °C constant temperature. Comprehensive SEM (scanning electron microscopy) and acoustic emission analyses were also conducted to clarify the observed results.According to the results, the effective stress coefficient (α) for CO2 permeability decreases with increasing aquifer salinity level, and increasing salinity level from 0 to 30% causes the effective stress coefficient to be reduced by 31%. Moreover, the Skempton coefficient (B) increases with increasing salinity level from 0 to 30% and the increment is about 18%. Interestingly, the poro-elastic coupling parameter (αB) decreases from 0.89 to 0.72 as the salinity level increases from 0 to 30% and the reduction is about 19%. The SEM analysis conducted on tested samples confirmed the deposition of NaCl crystals in rock pore space during the saturation period of one year, and these observed variations in effective stress parameters are probably due to the NaCl crystal deposition in the rock pore space. This significantly alters the rock porosity and pore geometry, causing the simple effective stress law for CO2 permeability to be inapplicable to saline aquifers.