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

Abstract

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 33MPa. 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 33MPa. 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.