Hydration states of smectite in NaCl brines at elevated pressures and temperatures

A high-pressure, high-temperature cell was used to monitor the basal X-ray powder diffraction spacing of Na-saturated Cheto montmorillonite in contact with NaCl solutions at temperatures as high as 200 degrees C and hydraulic pressures as high as 6700 psi (456 bar). The 003 and 005 reflections were used to determine d(001) of the smectite. The montmorillonite, in 1 molal NaCl, exhibited a d(001) of 15.4 Aa at room temperature and pressure and a d(001) of 15.6-15.7 Aa under 500-2200 psi hydraulic pressure. The basal spacing of the clay in 5 molal NaCl was 15.2 Aa and 15.33-15.45 Aa at 1 bar and 750-6700 psi (53-456 bar), respectively. Because no changes in the basal spacing with increasing temperature to 200 degrees C were detected in any of the experiments, this Na-smectite probably exists as a two-water-layer complex under diagenetic conditions.