Studies of smectite membrane behavior Electrokinetic, osmotic, and isotopic fractionation processes at elevated pressures

Abstract

A 1.087 m NaCl brine was forced through a day membrane 0.32 cm thick at 294 ± 1°K. The clay membrane was made by compacting the 0.5 to 2.0 micron size fraction of Cheto montmorillonite from Chambers, Arizona. A confining pressure of 34.5 MPa (5000 psi) and a mean hydraulic pressure of 15.9 MPa (2300 psi) were chosen to simulate a depth of about 1.5 km (5000 ft) in sedimentary basins. Differential hydraulic pressure across the day was 13.8 MPa (2000 psi). This differential hydraulic pressure setting was disturbed for only brief intervals during the experimental run. Hydraulic flow rate, electrical conductance, electrical potential across the clay, electroosmotic flow rate, and the chemical composition of the brine were determined periodically until constancy of both effluent chemical composition and electrical potential across the clay was achieved. Then, osmotic flow rate and stable oxygen isotopic compositions of both input and effluent reservoirs were determined before the run was terminated. The experimental results conform to the predictions of nonequilibrium thermodynamics under these simulated subsurface conditions. The electroviscous effect caused a significant reduction in the hydraulic flow rate but did not disturb the linearity between the flow rate and differential hydraulic pressure. Osmotic and electroosmotic effects may be responsible for driving large amounts of cross-formatkmal subsurface fluid flow in nature. A measured 13O fractionation of 0.25%. compares well with published field and laboratory data.