Experimental investigation of sandstone properties under CO2-NaCl solution-rock interactions

Carbon dioxide injection in saline aquifer disposal leads to geochemical alteration and geomechanical deformation of the host formations, both in the short- and long-terms. We performed triaxial compression and seepage-creep experiments on quartz-feldspar-detrital sandstone (as a typical storage reservoir material) as well as microscopy. Experimental results provided insight into the deformation of porous sandstone under a water-chemical environment (NaCl solution and CO2-NaCl solution), in both short- and long-terms: the long-term evolution of mechanical deformation and permeability during the migration of carbon dioxide and carbon fixation was thus analysed. Our aim was to study the influence of CO2-NaCl solution on the compressive strength and deformation of quartz-feldspar-detrital sandstone, as well as the impact of CO2-NaCl solution flow-through processes on the mechanical, hydraulic, and chemical properties of reservoir sandstone in terms of its time-dependent deformation, permeability, porosity, and mineral components. Comparison and analysis of short-term experimental results showed that the addition of CO2 enhanced the partial-mineral dissolution and the compressibility for sandstone, as well as helping the brittle-ductile transition in its stress-strain curves. The CO2 dissolved in the pore fluid reduced the compressive strength of sandstone to 7-15% of the compressive strength without CO2. For long-term mechanical and hydraulic properties of sandstone, the corresponding results showed that the threshold stress of shear dilatancy for the sandstone with the CO2-NaCl solution flow-through was reduced to about 17% of the threshold stress without CO2; its hydraulic conductivity was reduced to more than 50% at beginning and fell an order of magnitude at the axial deviatoric stress of 33 MPa. In addition, the increment of time-dependent deformation in long-term test with CO2-NaCl solution flow-through was not obvious at the beginning creep but conspicuous as the increasing stress level, which up to about 20% of the strain without CO2 at the axial deviatoric stress of 33 MPa. With the application of scanning electron microscopy and mercury intrusion porosimetry, the mechanism of the influence of CO2-NaCl solution on sandstone mechanical and hydraulic properties was further investigated at the micro-scale. Besides, the results of this microscopy described the effect of CO2-NaCl solution-sandstone interaction on the sandstone mineral components, shape, and structure; these reflect the sequestration capacity of CO2 in quartz-feldspar-detritus sandstone by carbon fixation.