Compaction of granular quartz under hydrothermal conditions: Controlling mechanisms and grain boundary processes

Abstract

[1] We report isostatic compaction experiments performed on granular quartz under hydrothermal conditions (3-129 mm of initial grain size, 300-600°C, 200 MPa of fluid pressure, and 25-100 MPa of effective pressure). From microstructural evidence, it was determined that, whereas microcracking controlled precompaction at room temperature, pressure solution was the main mechanism during hydrothermal compaction, although a role of microcracking could not be excluded entirely. Our mechanical data, together with theoretical pressure solution rate models, further indicated that pressure solution was controlled by interface kinetics, dissolution being the most likely rate-controlling mechanism. An empirical relation of the form _ e = 10 À7.8 (f/f 0 ) 10.0 s 3.4 d À1 exp (À105,000/RT) was fitted to our data to describe experimental compaction rates. Electron backscatter diffraction (EBSD) analysis performed on one sample showed limited evidence for plastic deformation (Dauphiné twinning and lattice bending) at grain contact points under high stress. Contact microstructures formed during compaction were studied using quartz single crystal discs or a mica plate as reference surfaces. This showed that, in all contacts, microstructures were rough, with a micrometer scale roughness. Many contacts also showed internal microcracking, and it is inferred that microcracking is likely an important mechanism for creating and maintaining rough contacts during quartz pressure solution, at least under the present experimental conditions. Extrapolation of our empirical equation for compaction rates to natural conditions is consistent with previous observations that, during burial and diagenesis of a sandstone, pressure solution starts to operate at depths of about $1.5-2 km, becoming the dominant compaction mechanism at depths greater than $2.5-3 km. The extrapolation gives good agreement with porosity-depth trends reported for natural, arenitic sandstones.

The Role of Pressure Solution Creep in the Ductility of the Earth’s Upper Crust

“…Note, however, that empirical laws have also been experimentally measured and showed another category of strain-stress relationship where 1) a dependence of the strain rate on the total strain was observed van Noort et al, 2008a); 2) the strain rate displays a power law dependence on time during deformation along a single contact Dysthe et al, 2003) or during compaction of aggregates (Chester et al, 2007;Croize et al, 2010b;Renard et al, 2001). Such power law behaviour in time was related either to some dynamics of grain boundary roughness, or the presence of another mechanism of deformation, for example subcritical crack growth or fracturing processes (Gratier, 2011b;Gratier et al, 1999), that acted concomitantly with pressure solution creep.…”

“…In such system, the diffusive transport of solutes along the grain-grain boundary can become the limiting step for deformation and the diffusion distance can be either the grain contact radius (Robin, 1978;Weyl, 1959) or the size of the actual contacts (den Brok, 1998;Gratz, 1991) in the island-and-channel model of grain boundary. Such an approach of dynamic grain contact was used to model pressure solution creep rate for complex contacts van Noort et al, 2008a).…”

Creep of simulated reservoir sands and coupled chemical‐mechanical effects of CO2 injection

“…Niemeijer et al, 2002;van Noort et al, 2008]. As discussed in Appendix A, strain rates resulting from dissolution or pressure solution processes alone can easily be estimated for the experimental conditions employed (room temperature, pH 3-11), using the dissolution rate data for quartz given by Brady and Walther [1990] and the pressure solution model of Spiers et al [2004], assuming a simple cubic pack of spherical grains.…”

Reaction-driven casing expansion: potential for wellbore leakage mitigation

“…[22,105]). This may, for instance, be in the form of thin films or microscale channels, observed to occur during stress-driven dissolution or pressure solution [28,29,120,121]. The initial solid (CaO powder in our experiments) will be considered to consist of a single, pure phase.…”