Smectite-Brine-CO2 Interactions: Implications on CO2 Sequestration

In this thesis, the interaction of smectite, brine and CO2 under varying temperature (T), pressure (P), and brine concentration and composition was investigated using volume experiments and geochemical modeling. Volume experiments were performed in a high-pressure environmental pressure chamber (HPEC) using X-ray diffraction to measure the molar volume [d(001) value] of smectite. In addition, geochemical modeling was used to calculate the extent of dissolution of smectite. In Chapter 2, the molar volume of a Na-rich montmorillonite, Na-SWy-2, under various NaCl brine concentration, P(CO2) and T conditions, were examined. The experiments revealed that the d(001) values of Na-SWy-2 decrease significantly when NaCl brine concentration and T were increased, whereas little to no change was observed with P(CO2). A series of experiments were also performed using helium (He), an inert gas, to pressurize the HPEC. Using the same P, T, and brine concentration, the d(001) values of Na-SWy-2 under P(He) and P(CO2) were similar, which indicate that CO2 does not enter the interlayer under various brine concentrations. In Chapter 3, the findings from the previous chapter were expanded by exchanging the interlayer cation of SWy-2 with Ca2+ and Mg2+ (hereafter CaSWy2 and MgSWy2). The experiments were perfomed using the same brine chemistry as the interlayer cation, e.g., CaSWy2 in CaCl2 brines. The trends obtained from CaSWy2-CaCl2 and MgSWy2-MgCl2 systems when P(CO2), T, and brine concentrations were varied are similar to Na-SWy-2-NaCl. However, in contrast to Na-SWy-2, both CaSWy2 and MgSWy2 only exhibit crytalline swelling and not osmotic swelling in all experiments. Using geochemical modeling, the experimental conditions were determined to be acidic from the dissolution of CO2 into the brine. However, despite acidic conditions extent of dissolution of SWy-2 is low, which is attributed to a rate limiting step during the dissolution process. In Chapter 4, the layer charge density and charge location within smectite were examined. The molar volume of smectite under varying P(CO2), T, and brine concentration was affected. A high-charge Ca2+-exchanged montmorillonite (hereafter CaSAz1) and Ca2+-exchanged smectite (hereafter CaSapCa1) with charge located in the tetrahedral sheet were used. Volume experiments of CaSAz1 and CaSapCa1 showed similar trends with CaSWy2 when P(CO2), T, and brine concentraiton are varied. However, the d(001) values of CaSAz1 are higher than CaSWy2 which is attributed to the higher charge density of CaSAz1. In contrast, CaSapCa1 has d(001) values lower than CaSWy2, which is attributed to the charge being located in the tetrahedral sheet. Geochemical modeling of CaSAz1 and CaSapCa1 revealed that composition is a major factor in the extent of dissolution of smectite. CaSAz1 showed similar calculated pH and extent of smectite dissolution with CaSWy2 because their similar structure and composition. In contrast, CaSapCa1 has a greater extent of dissolution that is 35 times higher than CaSAz1 and CaSWy2, which is attributed to its octahedral sheet that is primarily composed of Mg2+. The dissolution of CaSapCa1 results in a brine pH of 1.5 units higher than CaSAz1 and CaSWy2, which is a result of forming Ca and Mg carbonate phases. The results of this thesis may be used to evaluate storage sites for CO2 and high-level nuclear wastes, where changes in pressure, temperature, and brine concentration may occur and affect the integrity of the storage site.