Effect of pore-bearing fluids on the mechanical behavior of siliceous cemented sandstones containing different clay contents and types

Abstract

The clay minerals contained in the reservoir rock of CO2 geological storage formations can adsorb fluids and swell, which weakens the rock and promotes deformation. This phenomenon has not been widely investigated due to the complexity of the mineral components of the rock. To study the relationship between fluids in rock pores and the mechanical behavior of clay-bearing reservoir rocks, three types of siliceous cemented sandstones mainly composed of clay minerals and quartz, both dry and saturated with various fluids (i.e. N2, scCO2, H2O), were subjected to conventional triaxial compression tests. The results show that the fluids weakened the mechanical strength of the sandstones, and the degree of weakening depended on the clay composition and the type of fluid, with no significant relationship to the clay content. For the sandstone containing montmorillonite, compared to the dry condition, scCO2 and H2O reduced the peak strength by up to 7.16% and 15.33%, respectively, while for the sandstone containing only kaolinite as clay component, scCO2 and H2O reduced the peak strength by 3.34% and 3.18%, respectively. As a control, N2 weakened the strength of the three types of sandstones between 1.46% and 4.04%. By considering the failure characteristics of sandstone at the macro- and microscales, the failure mechanism of the sandstones was explained from two perspectives: the intercrack surface energy reduction and the swelling of clay. This study offers new perspectives for assessing the mechanical stability of fluid-bearing reservoirs and clarifies the role of clay in the connection between fluids in rock pores and the mechanical behavior of rocks.