Anisotropic creep in shale gas reservoir rocks: Impacts of constitutive behavior on field-scale deformations

Abstract

Creep deformations in shale gas rocks manifest a few common characteristics: anisotropy, irreversibility, and a strong correlation to the clay and organic content. At the same time, however, the specific creep responses, such as the magnitude of creep and the degree of anisotropy, are unique to each rock. Nevertheless, it remains largely unclear how the differences in material-specific creep responses at the laboratory scale impact the time-dependent deformation of shale gas rocks at the field scale. Here, we address this question through a combination of constitutive modeling, laboratory testing, and numerical simulation. We make use of a recently developed viscoplasticity model for shale whereby anisotropic creep deformations are captured through homogenization of a bi-layer microstructure. We calibrate the constitutive model to the laboratory creep test data of samples from three different gas shale formations, namely, Haynesville, Eagle Ford, and Barnett. Using three different sets of model parameters, we conduct numerical simulations of borehole closure in shale formations to investigate the impact of material-level creep responses on time-dependent deformation processes in gas shale formations at the field scale.