Creep Deformation in Vaca Muerta Shale From Nanoindentation to Triaxial Experiments

We tested samples cored from the Vaca Muerta shale reservoir using nanoindentation (2 min) and triaxial (12 hr) creep experiments in which confining pressure and differential stresses were <40 MPa. In all cases, we observed transient creep wherein strain increased as the logarithm of time. Creep was always compactional, led to increased moduli, was triggered by changes in either hydrostatic or deviatoric stress, and occurred under loads well below the failure stress. Our results are consistent with yield cap models proposed to describe shear-enhanced compaction of sandstones and carbonates, assuming that the yield surface depends on strain rate. We compared our results to earlier studies that observed the transition from transient creep to approximately constant strain rate behavior. If short-term creep can be quantified by a creep modulus, C, and long-term creep, after a transition time, t(c), by a linear viscosity, eta, then eta = Ct(c). Owing to heterogeneity, the local values of Young's and creep moduli, E and C, measured during nanoindentation, varied by several orders of magnitude. Simple averaging of the indentation results overestimated E and C, as compared to their triaxial counterparts. The kinetics of log time creep, which is seen in various materials and loading circumstances, has been represented by a simple conceptual model incorporating the interplay of viscoelastic elements with widely distributed characteristic times. Thus, we argue that log time creep is an emerging phenomenon, independent of the identities of the underlying physical mechanisms.