Permeability Evolution During Non-linear Viscous Creep of Calcite Rocks

Abstract

Permeability, storage capacity and volumetric strain were measured in situ during deformation of hot-pressed calcite aggregates containing 10, 20, and 30 wt% quartz. Both isostatic and conventional triaxial loading conditions were used. The tests were performed at confining pressure of 300 MPa, pore pressures between 50 to 290 MPa, temperatures from 673 to 873 K and strain rates of 3 · 10 )5 s )1 . Argon gas was used as the pore fluid. The initial porosities of the starting samples varied from 5% to 9%, with higher porosity correlated to higher quartz content. Microstructural observations after the experiment indicate two kinds of pores are present: 1) Angular, crack-like pores along boundaries between quartz grains or between quartz and calcite grains and 2) equant and tubular voids within the calcite matrix. Under isostatic loading conditions, the compaction rate covaries with porosity and increases with increasing effective pressure. Most of the permeability reduction induced during compaction is irreversible and probably owes to plastic processes. As has been found in previous studies on hot-pressed calcite aggregates, permeability, k, is nonlinearly related to porosity, /. Over small changes in porosity, the two parameters are approximately related as k / / n . The exponent n strongly increases as porosity decreases to a finite value (from about 4 to 6% depending on quartz content), suggesting a porosity percolation threshold. When subjected to triaxial deformation, the calcite-quartz aggregates exhibit shearenhanced compaction, but permeability does not decrease as rapidly as it does under isostatic conditions. During triaxial compaction the exponent n only varies between 2 and 3. Non-isostatic deformation seems to reduce the percolation threshold, and, in fact, enhances the permeability relative to that at the same porosity during isostatic compaction. Our data provide constraints on the governing parameters of the compaction theory which describes fluid flow through a viscous matrix, and may have important implications for expulsion of sedimentary fluids, for fluid flow during deformation and metamorphism, and melt extraction from partially molten rocks.

Mechanical behavior and microstructure of simulated calcite fault gouge sheared at 20–600°C: Implications for natural faults in limestones

“…An additional process influencing the behavior of our samples at 500–600°C, where GSI and GSS creep mechanisms are expected to play a significant role, is the possibility that efficient compaction by these mechanisms during shear leads to porosity reduction and disconnection to the extent that the samples become (near) impermeable. Porosity occlusion/permeability reduction of this type has been previously reported by Xiao and Evans [] in triaxial compression tests on wet calcite/quartz mixtures at similar temperatures and pressure conditions [see also Xiao et al ., ] and could potentially lead to shear flow resembling that seen in rock systems containing a few percent of partial melt trapped in intergranular pores [ Holtzmann et al ., ; Holtzmann and Kohlstedt , ] at near‐zero effective stress. In our experiments, however, this seems unlikely as the porosity was 10% to 15% (i.e., above typical percolation threshold values), and the deformation rates and shear strains applied were much higher than in the experiments reported by Xiao and Evans [].…”

“…To test the shear strength dependence upon increasing effective normal stress, we also conducted experiments using stepped values of , in the range 30 to 100 MPa, employing 12 MPa ≤ P f ≤ 60 MPa. Given the conditions employed in our experiments, the temperature increase from 20°C to 600°C can be expected to span the transition from (semi)brittle/frictional to plastic deformation behavior [e.g., Heard , ; Rutter , ; Schmid et al ., ; Fredrich et al ., , ; Xiao and Evans , ; Xiao et al ., ; Schubnel et al ., ]. Specifically, intracrystalline plasticity of calcite grains in the bulk gouge layer can be expected to be important at temperatures as high as 500°C to 600°C [ De Bresser and Spiers , ; Barnhoorn et al ., ].…”

Plastic flow of two‐phase marbles

“…Four parameters seem most important in determining the strength of the aggregates: matrix grain size, total quartz content, the mean spacing of the quartz particles, and porosity. For a discussion of the effect of porosity on the mechanical and transport properties of calcite aggregates, see work by Xiao and coworkers [ Xiao and Evans , 2003; Xiao et al , 2006]. In this discussion we focus on the effect of the strong second phase and consider only samples with 20% quartz or less, which do not compact significantly during the deformation step.…”

A Hydromechanical Model for Lower Crustal Fluid Flow

“…Although the hydraulic permeability of rocks is extraordinarily variable, it is well established from both theoretical studies and empirical observation (Wark and Watson 1998;Xiao et al 2006) that the permeability of a given rock will vary as a strong function of its connected porosity. Typically, a power-law relationship is assumed such that if the permeability is k 0 at porosity f 0 , then…”