Effect of Temperature, CO2 Pressure, and Brine Composition on the Layer Spacing of Montmorillonite

CO2 sequestration is one proposed method to help combat the increasing levels of anthropogenic CO2 in the atmosphere. It involves the capture and pressurization of CO2 to form supercritical CO2 (scCO2), which is then pumped deep underground into saline aquifers or depleted oil reservoirs, where it is hydrodynamically trapped below an impermeable shale caprock. Smectite clays, which are an integral component in the sealing capabilities of shales, are “swelling clays” that can exhibit large changes in molar volume in response to changes in the activity of H2O (aH2O), which can fluctuate based on brine composition, temperature, and pressure. These changes in molar volume of smectite clay can greatly affect the sealing capabilities of the caprock shales. The present study investigates the effect of CO2 on a Na-rich montmorillonite smectite (Clay Minerals Society Source Clay, SWy-2) as a function of temperature (T), pressure using CO2 gas [P(CO2)], and NaCl brine composition. For comparison, the clay mineral was investigated at the same conditions, but in the presence of He [P(He)]. The molar volume of SWy-2, as determined by the direct measurement of d(001), was determined using X-ray diffraction techniques and a specially constructed environmental chamber. Measurements were made at P(CO2) and P(He) to ~1000 bars and temperatures to 150°C, with eight NaCl brine compositions from 0.17M to NaCl saturation. At near ambient pressure and temperature conditions, a change in brine composition from 0.17M to 5.99M resulted in d(001) decreasing from 22.6 Å to 16.7 Å (26%). Increasing P(CO2) from 1 to 500 bars at T ~31°C resulted in minimal to no change in d(001). Increasing T from ~31 to 150°C at an initial P(CO2) = 500 bars resulted in a large decrease in d(001) of ~10 to 17% for clay in the 1.37 M and 1.71 M brines. This large reduction in molar volume observed could lead to dehydration cracking in the overlying shale, compromising the sealing capabilities of the caprock. Results using P(He) were identical, within error, suggesting that at these experimental conditions, CO2 does enter the interlayer of Na-SWy-2.