Coupling between pressure solution creep and diffusive mass transport in porous rocks

Abstract

[1] Pressure solution is widely regarded as a mechanism of ductile deformation in the upper crust. It is driven by stress differences and its rate is affected by temperature, grain size, and fluid chemistry. Pressure solution involves dissolution at grain contacts under high stress and precipitation at grain contacts on pore surfaces under low stress, leading to porosity reduction by precipitation in the pore space or by grain indentation. For a system closed at a grain scale, pressure solution is traditionally described by a mechanism involving three steps: (1) dissolution at intergranular interfaces, (2) diffusion of solutes inside the contact between two grains, and (3) precipitation on the surface of the grains in contact with the pore fluid. In this paper we propose a model where we have added a fourth step to this process, diffusive transport to other open pores, to account for the macroscopic diffusion of solutes in pore fluids, such that the deformation is not closed at the grain scale. In this model, differences in mineral solubility due to variations in stress and grain size produce concentration gradients which drive diffusive mass transport. The interaction between pressure solution at a grain scale and transport over distances of several grains can lead to the amplification of initial porosity heterogeneities and subsequent localization of deformation. Regions of intense dissolution compact and form ''bands'' in close proximity to regions where the porosity reduction is mainly due to cementation. Pressure solution augmented by large-scale diffusional transport will cause mass transport from fine-grained to coarse-grained rock volumes. We show that such processes are important during both diagenesis of sediments and compaction of fault gouge.

Numerical Modeling of the Effect of Carbon Dioxide Sequestration on the Rate of Pressure Solution Creep in Limestone: Preliminary Results

“…This equation is solved using a Galerkin finite element method, with linear basis functions on the spatial domain and a Crank-Nicolson scheme in the time domain. All the numerical schemes were programmed in C++ and make use of the numerical library Diffpack (Langtangen, 1999;Gundersen et al, 2002). The program was extensively tested for numerical stability.…”

“…The conservation relationships are derived by a global mass balance of the solute phase in the pore volume and a local mass balance at each grain contact (Gundersen et al, 2002). These relationships are then coupled to equations which express the deformation of the grains and the evolution of the rock texture.…”

“…The model treats the coupling between chemical reactions of calcite dissolution and precipitation, diffusion, and PSC deformation in two steps: -textural equations at the grain scale allow for grain deformation due to chemical reactions occurring at contact and pore surfaces (including diffusion in the contact fluid); -a mass conservation relationship at a global scale which takes into account the transfer of dissolved material by diffusion in the interconnected pore spaces. Therefore, microscopic grain scale processes and macroscopic mass transport within the model volume are fully coupled (for a complete review, see Gundersen et al, 2002).…”

“…Weyl, 1959;Rutter, 1976;Gratier and Guiguet, 1986;Dewers and Ortoleva, 1990;Spiers and Brzesowsky, 1993;Gundersen et al, 2002;Yasuhara et al, 2003). This ductile deformation mechanism occurs in the upper crust and plays an important role in the compaction of sedimentary rocks during diagenesis (Ortoleva, 1994;Tuncay et al, 2000).…”

“…PSC is driven by differences in chemical potential induced by differential stress along grain surfaces in the rock matrix. PSC can be modeled as a serial process involving four successive steps: -stress-enhanced dissolution at grain-grain interfaces subject to elevated normal stress; -diffusion of dissolved material (solutes) through intergranular fluid films; -precipitation of dissolved material in adjacent pore spaces (grain surfaces in contact with pore fluid); -transport of dissolved material to distant pores, which can induce local mass transfer (Gundersen et al, 2002).…”