Recent experimental data may point to a greater role for osmotic pressures in the subsurface

Abstract

[1] Uncertainty about the origin of anomalous fluid pressures in certain geologic settings has caused researchers to take a second look at osmosis, or flow driven by chemical potential differences, as a pressure-generating process in the subsurface. Interest in geological osmosis has also increased because of an in situ experiment by Neuzil (2000) suggesting that Pierre Shale could generate large osmotic pressures when highly compacted. In the last few years, additional laboratory and in situ experiments have greatly increased the number of data on osmotic properties of argillaceous formations, but they have not been systematically examined. In this paper we compile these data and explore their implications for osmotic pressure generation in subsurface systems. Rather than base our analysis on osmotic efficiencies, which depend strongly on concentration, we calculated values of a quantity we term osmotic specific surface area (A so ) that, in principle, is a property of the porous medium only. The A so values are consistent with a surprisingly broad spectrum of osmotic behavior in argillaceous formations, and all the formations tested exhibited at least a modest ability to generate osmotic pressure. It appears possible that under appropriate conditions some formations can be highly effective osmotic membranes able to generate osmotic pressures exceeding 30 MPa (3 km of head) at porosities as high as $0.1 and pressures exceeding 10 MPa at porosities as high as $0.2. These findings are difficult to reconcile with the lack of compelling field evidence for osmotic pressures, and we propose three explanations for the disparity:(1) Our analysis is flawed and argillaceous formations are less effective osmotic membranes than it suggests; (2) the necessary subsurface conditions, significant salinity differences within intact argillaceous formations, are rare; or (3) osmotic pressures are unlikely to be detected and are not recognized when encountered. The last possibility, that osmotic pressures routinely escape detection or are attributed to other mechanisms, has important implications for understanding subsurface flow regimes.Citation: Neuzil, C. E., and A. M. Provost (2009), Recent experimental data may point to a greater role for osmotic pressures in the subsurface, Water Resour. Res., 45, W03410,

Critical review of coupled flux formulations for clay membranes based on nonequilibrium thermodynamics

“…The previously noted dependency of ω on salt concentration complicates the use of ω in practical applications, because chemico-osmosis occurs only when there is a concentration gradient in or across a membrane, such that ω generally also will vary spatially within the membrane (see Neuzil and Provost, 2009). As a result, values of ω used in Eq.…”

Semipermeable Membrane Properties and Chemomechanical Coupling in Clay Barriers

“…Theoretical expressions of the osmotic efficiency show a strong dependence of this coefficient on the half-pore size b and on the salinity c f (see Section 8.3.1 and discussion by Neuzil and Provost (2009)). The effective pore size, i.e., the average value representative of macroscopic volume of porous medium, was estimated by using the simple mass balance equation given by Neuzil (2000) valid in a plane-parallel conceptual model for the porous media and which is written as (Takeda et al, 2013), Bearpaw formation (4) (Cey et al, 2001), and Bulldog, North sea, Pierre II, Muderong shales (@) (Bunger et al, 2014).…”

“…The variable c tot contains all the control parameters for the coupling coefficients: geometry (u), surface charge (CEC), and salinity (c f ) and was previously used to describe thermoosmotic data . To translate Bresler's curve into the (3 c , c tot ) space, b ffiffiffiffi c f p values were converted into c tot using a clear correlation between these two variables obtained by Monte Carlo simulations based on plausible values for u, CEC, c f , and a relation between A S and CEC (Neuzil and Provost, 2009). …”

“…Based on a set of experimental data on chemical osmosis collected by Neuzil and Provost (2009) for different clay-rocks, large osmotically induced overpressures of up to tens of MPa were predicted by these authors. However, according to them, such large overpressures have not been found in sedimentary basins.…”

Saltstone Osmotic Pressure

“…Darcy's law for fluid flow can be written in a generalized form to include fluid flow due to osmosis as follows: (5) where = is Darcy flux of pore fluid, is permeability, is dynamic viscosity, is gravitational constant, is osmotic efficiency is ideal osmotic pressure (Neuzil and Provost 2009). The first term accounts for flow due to hydraulic head gradient and the second term accounts for osmosis.…”