Osmotic generation of ‘anomalous’ fluid pressures in geological environments

Abstract

Osmotic pressures are generated by differences in chemical potential of a solution across a membrane. But whether osmosis can have a significant effect on the pressure of fluids in geological environments has been controversial, because the membrane properties of geological media are poorly understood. 'Anomalous' pressures--large departures from hydrostatic pressure that are not explicable in terms of topographic or fluid-density effects--are widely found in geological settings, and are commonly considered to result from processes that alter the pore or fluid volume, which in turn implies crustal changes happening at a rate too slow to observe directly. Yet if osmosis can explain some anomalies, there is no need to invoke such dynamic geological processes in those cases. Here I report results of a nine-year in situ measurement of fluid pressures and solute concentrations in shale that are consistent with the generation of large (up to 20 MPa) osmotic-pressure anomalies which could persist for tens of millions of years. Osmotic pressures of this magnitude and duration can explain many of the pressure anomalies observed in geological settings. They require, however, small shale porosity and large contrasts in the amount of dissolved solids in the pore waters--criteria that may help to distinguish between osmotic and crustal-dynamic origins of anomalous pressures.

A slug test to assess the osmotic and hydraulic properties of argillaceous formations

“…For instance, an osmotic flow, i.e., a flow of water from high-salinity to low-salinity zones, is considered a potentially significant process in clay formations [see, e.g., Neuzil, 2000;Gonçalvès et al, 2004]. The work by Neuzil [2000] has clearly established the possibility of osmotic behavior in clay rocks at the field scale. This result, which emphasizes the poor accuracy of the classical Darcy law in such clay-rich media, has attracted the attention of the scientific community involved in research programs concerning nuclear waste confinement in shales.…”

“…In order to develop an analytical solution, constant parameters are assumed in the following development while, for instance, the diffusion coefficient and e vary with the concentration [see, e.g., Neuzil, 2000;Revil et al, 2005;Gonçalvès et al, 2007]. Consequently, the pressure diffusion equation which has to be solved for an argillaceous porous medium is ), r is the density of the fluid (kg m…”

Modeling Coupled Processes in Clay Formations for Radioactive Waste Disposal

“…Mechanisms that could lead to reactivation include sedimentation and glaciation (Mazurek et al 2003, pp. 341;Caillet 1993), changes in tectonic stress (Darby et al 2001), seismic or igneous activity, the accumulation of hydrocarbon or nonhydrocarbon gas (e.g, CO 2 , and He, Wiprut et al 2000), and osmosis driven by changes in recharge and natural contrasts in solute concentrations of pore waters (Neuzil 2000). Fracture and fault behavior for a clay or shale host rock is sensitive to the diagenetic history.…”

What is the significance of pore pressure in a saturated shale layer?

“…Note that the reference to this hypothetic initial thermodynamic equilibrium is secondary since constitutive transport equations in deforming porous media can be derived on the basis of nonequilibrium thermodynamics [see, e.g., Revil, 2007]. These fluxes are calculated with the well-known coupled flow expressions using pressure and concentration gradients [see, e.g., Neuzil, 2000;Gonçalvès et al, 2004;Revil and Linde, 2006;Garavito et al, 2006;Gueutin et al, 2007]. The electrical properties of clay surfaces explain the presence of additional driving forces for fluid flow besides the pressure gradient and the gravity, namely the chemical potential gradient of the salt ▿m f and the electrical potential gradient ▿y.…”

“…The equilibrium concentration can be obtained by squeezing experiments or by modeling the geochemical equilibrium between the minerals and the pore fluid. The mean pore size (2b) can be identified by the following simple mass balance equation assuming a parallel plane geometry [Neuzil, 2000]:…”