Abstract
Osmosis has been cited as a mechanism for explaining anomalously high fluid pressures in the subsurface. Clays and shales act as membranes, and osmotic flux across these units may result in pressures sufficiently high to explain these anomalies. The theoretical osmotic pressures as calculated solely from solution properties can be quite large; however, it is not yet resolved whether these geologic membranes are sufficiently ideal to generate such pressures. Osmotic efficiencies of a Na-bentonite membrane were measured by a series of hyperfiltration experiments using various molarities of NaCl at two different porosities. The highest osmotic efficiency (0.8912) occurred at the lower porosity and the lowest NaCl input solution. The lowest measured osmotic efficiency (0.0423) occurred at the high porosity and the highest NaCl input concentration. The osmotic efficiencies obtained from the hyperfiltration experiments correlate very favorably with the Fritz-Marine Membrane Model. This model predicts that the maximum osmotically-induced hydraulic pressures in the subsurface should occur across shales having low porosities and high cation exchange capacities in which the unit separates solutions of brackish waters.
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