Time-dependent deformation of shale gas reservoir rocks and its long-term effect on the in situ state of stress

•Organic-rich shales exhibit varying degree of time-dependent behavior.•Creep strain is linearly dependent on differential stress at studied condition.•Elastic and creep deformation behavior is described as a power-law function of time.•Significant fraction of elastic stress is relaxed via viscous deformation over time.

Laboratory testing of shale gas reservoir rocks reveal varying amounts of time-dependent viscous deformation in response to applied differential stress. The time-dependent deformation is an inherent property of the dry rock as it occurs in the absence of pore fluid. The contribution of the time-dependent deformation is generally larger for rocks with more clay and organic content. The time-dependent behavior can be modeled as a power-law function of time. Its magnitude is approximately linear with the magnitude of the applied differential stress and nearly insensitive to the confining pressure. By applying linear viscoelastic theory and using laboratory constrained constitutive parameters, we evaluated the effect of the time-dependent deformation in modifying the in situ differential stress over time. Modeling suggests that a significant proportion of a differential stress change would be relaxed over time-scales on the order of days. Because of this short time scale, the composition of the rock (as it influences the time-dependent behavior) may influence the in situ differential stress magnitudes stronger than the formation׳s geological loading history for these reservoirs.