Supercritical CO2 injection and residence time in fluid-saturated rocks: Evidence for calcite dissolution and effects on rock integrity

Abstract

CO2 injection and storage in geological reservoirs is an attractive prospect for mitigating the anthropogenic production of greenhouse gases and global warming. The technology could lead to mineral precipitation and therefore stable storage over geological time scales. This contribution investigates the evolution of three calcite-rich reservoir rock analogues during injection of and exposure to supercritical CO2 (scCO2), i.e., two limestones (Tuffeau and Savonnieres) and a synthetic calcite-cemented sandstone (CIPS). Three types of exposure protocols have been conducted: (i) scCO2 injection and a four-hour residence time in an initially dry rock; (ii) scCO2 injection and a two-hour residence time in an initially brine-saturated rock; and (iii) scCO2 injection and a four-hour residence time in an initially brine-saturated rock. Two aspects are monitored during these experiments: (i) the evolution of the pore fluid chemical composition; and (ii) the evolution of the rocks’ physical properties (i.e. porosity, permeability, P-wave velocity and electrical resistivity). Additionally, some scCO2 injection and exposure experiments in the brine-saturated rocks have been conducted using X-ray tomographic monitoring.X-ray tomographic monitoring suggests that scCO2 first displaces the water, leading to an average water saturation of about 70–90%. Then, scCO2 dissolves in the pore brine, leading to a homogeneous decrease by about 3% in water saturation of the sample. As a result, the pore brine acidifies even after 2h of exposure only, which leads to calcite dissolution and a significant increase in the brine’s concentration in calcium cations. For the samples and most exposure experiments, evidence of calcite dissolution is inferred from the measured physical properties. For the brine-saturated Tuffeau limestone and CIPS sandstone, calcite dissolution leads to significant mechanical weakening. For the brine-saturated Savonnieres limestone, the sample subject to two-hour residence time shows evidence of calcite dissolution, whereas the sample after four-hour residence time does not. Calcite re-precipitation could be the cause of this unexpected response after four hours.