Experimental investigation on changes in mechanical properties of volcanic sandstones under CO₂ geological storage conditions

In CO₂ geological storage, CO₂/brine/rock chemical interactions may lead to changes in mechanical properties of rocks. These changes can have impacts on performance and integrity in storage depending on their intensities. In case of sandstones consisting mainly of quartz and feldspar grains, there may be little changes in the properties because of low reactivities of the minerals. On the other hand, significant changes may occur in case of volcanic sandstones because they contain high-reactivity volcanic glasses. However, influences of the chemical interactions on the properties of volcanic sandstones have not been investigated so far. Thus, we have conducted triaxial compression experiments on two cylindrical volcanic sandstone samples consisting mainly of andesite/basalt and scoria grains (porosity: ca. 33%) at a confining/axial pressure of 30 MPa, pore pressure of 15 MPa, CO₂ saturation of 50% or 77% and 60^oC for several weeks. Bulk modulus, Young’s modulus and Poisson’s ratio were measured intermittently. Before and after the experiment, porosity and permeability were also measured on each sample, and initial and final brine chemistries were analyzed. Additionally, X-ray CT was conducted on the samples before and after the experiment. Changes in bulk modulus and Young’s modulus were qualitatively similar whereas Poisson’s ratio was almost constant. In case of one of the sample that contained relatively large scoria grains of high porosities, bulk modulus first decreased, then recovered partially, and finally became constant. In case of the other sample, bulk modulus first increased and then became constant. The decrease and increase in bulk modulus may have been caused respectively by dissolution-induced collapse of the relatively large scoria grains and by precipitations of some minerals such as silica minerals. Permeability of one of the samples increased while permeability of the other sample decreased, although porosity decreased for both samples.