Effect of CO2 pressure, temperature, and brine composition on the interlayer spacing of Na-rich and K-exchanged montmorillonite

Abstract One of the more important strategies to curb the increase of anthropogenic carbon dioxide (CO2) in the atmosphere is to inject large amounts of supercritical CO2 (scCO2) into the sandstones of deep saline sandstone-shale sequences, where the shale layers act as impermeable seals. An important clay mineral constituent in these sequences is smectite, which exhibits significant volume changes in response to the activity of H2O [a(H2O)]. The present study investigates the effect of a(H2O) on the molar volume of smectite in a brine by varying temperature (T), CO2 pressure [P(CO2)], and brine concentration. In addition, because inert gases do not interact with smectite, a series of experiments using He as a pressure medium at similar P-T conditions were made for comparison. A high-pressure environmental chamber (HPEC) was used to investigate the effects of T, P(CO2), P(He), and brine composition on the d(001) of montmorillonite (Clay Minerals Society Source Clay SWy2) using X-ray diffraction. Measurements were performed using a Na-rich montmorillonite (Na-SWy2) and a K-exchanged montmorillonite (K-exchanged SWy2) at P(CO2) and P(He) in intervals of 100 bars to 500 bars and T in intervals of 25 °C to 150 °C. Eight NaCl brines with concentrations ranging from 0.17 M to saturation and a select series of KCl brines were used. Increasing the NaCl brine concentrations from 0.34 M to saturation at P(CO2) = 1 bar and T ~ 33 °C resulted in a substantial decrease of d(001) of Na-SWy2 from 20.1 to 15.7 A (25%). In contrast, increasing P(CO2) from 55 to 500 bars at T ~ 33 °C and a brine concentration of 0.17 M, resulted in a decrease in d(001) from 20.4 to 19.6 A (4%). Using a saturated brine (5.99 M), increasing P(CO2) from 1 bar to 500 bars did not cause a significant decrease in d(001). Similar trends were observed when T was increased from ~33 to 150 °C. At 0.17 M brine concentration, d(001) decreased from 19.6 to 19.0 A (3%). At 1.71 M, d(001) decreased from 18.6 to 15.7 A (17%) with T = 50 to 150 °C. Experiments using P(He) as the pressure medium showed identical results with the experiments with CO2, indicating that at the P-T range of these experiments and with excess water, CO2 does not enter the interlayer of these clay minerals. In the experiments with K-exchanged SWy2, d(001) measurements were not possible because K-exchanged SWy2 lacks periodicity and different hydration states are randomly distributed in the structure. The results show that both T and brine concentration have a major effect on d(001) because of the loss or addition of interlayer H2O, i.e. change in molar volume in Na-SWy2, whereas P(CO2) is less important. Smectite in reservoir sandstones may be present as a coating of mineral grains. An increase in a(H2O) can, therefore, decrease the permeability of the reservoir sandstone because of the expansion of smectite. Similarly, an increase in a(H2O) can enhance the sealing property of the confining shales, because these can contain a significant amount of smectite. In contrast, contraction of Na-SWy2 may occur when a(H2O) decreases. The contraction can cause shrinkage of the shale and produce secondary fractures that may form conduits for CO2 to escape.