The interrelation of hydraulic and electrical conductivities, streaming potential, and salt filtration during the flow of chloride brines through a smectite layer at elevated pressures

Abstract

Solutions of NaCl and NaCl-CaCl 2 were forced through a clay plug made by compacting the 0.5–2.0 μm size fraction of Cheto montmorillonite. The initial concentration of NaCl solution was 1.10 molal ( m), and that of NaCl-CaCl 2 solution was 0.92 m in NaCl and 0.075 m in CaCl 2. The thickness of the clay plug was 0.5 cm. Two experiments were carried out, one with the NaCl solution and one with the NaCl-CaCl 2 solution. It took six to seven weeks to achieve constancy of effluent chemical composition and of streaming potential. Compaction pressure ( P c) in these room temperature experiments was 34.5 MPa (5000 psi), differential hydraulic pressure across the clay was 13.8 MPa (2000 psi), and mean hydraulic pressure ( P mh) was 15.9 MPa (2300 psi). These values of P c and P mh are approximate for a depth of 1525 m in sedimentary basins. Hydraulic flow rate, streaming potential, and brine chemical composition were measured periodically until steady state. Electrical conductance of the clay plug was measured in the beginning and at the end of each experimental run. The presence of Ca 2+ in the brine resulted in lower hydraulic conductivity and streaming potential but slightly higher electrical conductivity. The electroviscous effect was shown to cause a significant reduction in the flow rate. The experimental results conform to the predictions of nonequilibrium thermodynamics, for both Na-Cl and Na-Ca-Cl systems, under these simulated subsurface conditions. Salt filtration efficiencies of 48% were measured. In the Na-Ca-Cl system, Na + was preferentially transported through the clay relative to Ca 2+.