Deformation Experiments on Bowland and Posidonia Shale—Part II: Creep Behavior at In Situ pc–T Conditions

Abstract

To unravel their long-term creep properties at simulated reservoir conditions, we conducted constant stress deformation experiments at elevated confining pressures, p c = 50-115 MPa, and temperatures, T = 75-150 °C, on Posidonia (GER) and Bowland (UK) shale, which exhibit varying petrophysical and mechanical properties. Depending on applied p c-T conditions and sample composition, recorded creep curves exhibit either only a primary (decelerating) or additionally a secondary (quasi-steady state) and a tertiary (accelerating) creep phase during deformation. At high temperature and axial differential stress and low confining pressure, creep strain is enhanced and a transition from primary towards secondary and tertiary creep behavior is observable. Creep strain of Posidonia shale, which is rich in weak constituents (clay, mica, and organic content), is enhanced when compared to creep strain recorded during deformation of either carbonate-or quartz-rich Bowland shale. Electron microscopy observations revealed that creep strain is mainly accommodated by the deformation of weak minerals and local pore space reduction. In addition, microcrack growth occurred during secondary creep. An empirical correlation between creep strain and time based on a power law was used to describe the decelerating creep phase, also accounting for the influence of confining pressure, temperature, and axial differential stress. The results suggest that the primary creep strain can be correlated with mechanical properties determined from short-term constant strain rate experiments such as static Young's modulus and triaxial compressive strength.

Effect of Diagenesis on Geomechanical Properties of Organic‐Rich Calcareous Shale: A Multiscale Investigation

“…All rights reserved. shown that (primary) creep is mainly accommodated by pore volume compaction and bending of clay minerals (Herrmann et al, 2020). Sone and Zoback (2013b) note that the correlation is generally evident in samples from a single shale unit but cannot be extrapolated to all shales, as other factors also influence the propensity for a shale to creep.…”

“…Q. Zhang et al (2014) state that in ideal linear viscoelastic materials that creep deviatorically with no volume change, the relationship between the triaxial and indentation creep modulus (Mighani et al, 2019) and triaxial testing (Herrmann et al, 2020).…”

An anisotropic viscoplasticity model for shale based on layered microstructure homogenization

“…This importance has motivated a number of experimental investigations into creep of shale at multiple scales (eg, Refs. 9–20).…”

“…This bilayer representation is a simple but promising approach. It is well justified by the fact that shale creep is mainly attributed to the clay and organic constituents rather than the hard minerals, 9,11,13,16,17,19 and it naturally captures the transverse isotropy resulting from the depositional process.…”

Long‐Term Settlement of High Concrete‐Face Rockfill Dam by Field Monitoring and Numerical Simulation

“…e FEM can also provide useful guidance for the design and construction of new high CFRDs by optimizing dam body structures, construction procedures, and dam construction materials [8]. In order to obtain the creep model of rockfill for FEM simulations, researchers have conducted large-scale triaxial tests for investigating the time-dependent behavior of coarse granular materials under different stress paths, confining pressures, and moisture contents [28][29][30][31][32][33][34][35][36][37]. e analyses reveal that creep deformation of the rockfill is caused by stress redistribution in the dam body associated with a process of abrasion of sharp corners of stones, particle shape, and particle breakage [38][39][40][41][42][43][44].…”