Time and Temperature Dependent Creep in Tournemire Shale

Abstract

We conducted a series of triaxial creep experiments on shale specimens coming from Tournemire, France, using the stress‐stepping method up to failure, at a confining pressure of 80 MPa, on two orientations (parallel and perpendicular to bedding), and at temperatures of 26 and 75 °C. In these week‐long experiments, stress, strains, and P wave ultrasonic velocities were recorded (quasi‐) continuously. The strength at creep failure of Tournemire shale was ~70% higher than the peak strength measured during constant strain rate (~10−7/s) experiments, and failure was reached at larger strains. An overall transition from P wave velocity increase at moderate differential stress to P wave velocity decrease closer to brittle failure was also observed. At a smaller timescale, P wave velocities initially decreased and then increased gradually during each step of creep deformation. The magnitude of these variations showed important (i) stress, (ii) orientation, and (iii) temperature dependences: larger increase was observed for P wave propagating along the main compressive stress orientation, larger decrease for P wave propagating perpendicular to it, and a changing behavior enhanced at a higher temperature. Scanning electron microscopy performed postmortem revealed evidence of time‐dependent pressure solution, localized compaction, crack growth, and sealing/healing. Our data reveal that shale deformation is highly stress sensitive only in a narrow stress domain where stress corrosion cracking‐induced brittle dilatant creep deformation is dominant. At stresses below, pressure solution compaction creep dominates the deformation and shales compact, consolidate, and heal. This has important implications for the mechanics of shallow fault zones and accretionary prisms.

Creep Deformation in Vaca Muerta Shale From Nanoindentation to Triaxial Experiments

“…Several physical deformation processes might be active during creep, either singly or concurrently. These processes might include intragranular (Bernabe et al, 1994;Den Brok & Spiers, 1991) or intergrain (Baud et al, 2000) cracking and slip, which may activate via subcritical crack growth (Brantut et al, 2012(Brantut et al, , 2013(Brantut et al, , 2014Chester et al, 2007;Heap, Baud, & Meredith, 2009;Heap et al, 2015;Kemeny, 1991) or pressure solution at intergranular contacts (Geng et al, 2018;Rutter, 1983;Spiers et al, 2003). The clayey nature of shale may also house active creep processes, such as granular and particle sliding (Kuhn & Mitchell, 1993;Kwok & Bolton, 2010), overcoming interparticle forces (Santamarina, 2003), and particle rearrangement (Hartley & Behringer, 2003;Vandamme & Ulm, 2009).…”

“…Thus, all three regimes may not be observed in a given set of tests. A range of mechanisms may drive creep in rocks such as subcritical crack growth (Brantut et al, 2012;Brantut et al, 2013;Brantut et al, 2014;Chester et al, 2007;Heap et al, 2015;Kemeny, 1991), pressure solution (Brantut et al, 2012;Geng et al, 2018), and particle rearrangement (Hartley & Behringer, 2003;Vandamme & Ulm, 2009).…”

“…Unlike stiff, well-indurated rocks, shales show measurable creep strains, even at low fractions of the FS (Parsons & Hedley, 1966). Among the parameters influencing creep response in shales are confining pressure (Rybacki et al, 2017), differential stress (Ghassemi & Suarez-Rivera, 2012;Li & Ghassemi, 2012;Sone & Zoback, 2013b), temperature (Geng et al, 2017(Geng et al, , 2018, rock anisotropy (Sone & Zoback, 2013b), and shale composition (Sone & Zoback, 2013b). Transient creep under triaxial loading may involve compaction (Villamor Lora & Ghazanfari, 2015), but, in some cases, high differential stresses can also activate dilational processes (Heap et al, 2015), which may lead to tertiary creep (Geng et al, 2017;Ghassemi & Suarez-Rivera, 2012;Nicolas et al, 2017;Rybacki et al, 2017).…”

“…Creep strain in rocks, when measured under a constant load, has often been described as consisting of three sequential regimes: transient (decelerating rate), secondary (constant rate), and tertiary (accelerating rate, leading to failure; Scholz, ). This description is a generalization that depends on the active mechanisms and their kinetics, which vary for different rock types and for different conditions of differential stress (Geng et al, ; Kemeny, ), pore fluid chemistry (Chester et al, ), confining stresses, temperature (Brantut et al, ; Geng et al, ), and test duration. Thus, all three regimes may not be observed in a given set of tests.…”

Experimental and Theoretical Study on Dynamic Hydraulic Fracture

“…Rocks are well known to have cracks and defects at all scales [42][43][44][45][46][47][48]. In some case rocks are even elastic-anisotropic [49,50]. Another problem regarding performing the actual field-scale experiment is that the pore pressure was not considered in this study.…”