Mechanical properties of shale-gas reservoir rocks - Part 2: Ductile creep, brittle strength, and their relation to the elastic modulus

We studied the elastic moduli, ductile creep behavior, and brittle strength of shale-gas reservoir rocks from Barnett, Haynesville, Eagle Ford, and Fort St. John shale in a series of triaxial laboratory experiments. We found a strong correlation between the shale compositions, in particular, the volume of clay plus kerogen and intact rock strength, frictional strength, and viscoplastic creep. Viscoplastic creep strain was approximately linear with the applied differential stress. The reduction in sample volume during creep suggested that the creep was accommodated by slight pore compaction. In a manner similar to instantaneous strain, there was more viscoplastic creep in samples deformed perpendicular to the bedding than parallel to the bedding. The tendency to creep also correlated well with the static Young's modulus. We explained this apparent correlation between creep behavior and elastic modulus by appealing to the stress partitioning that occurs between the soft components of the shales (clay and kerogen) and the stiff components (quartz, feldspar, pyrite, and carbonates). Through a simple 1D analysis, we found that a unique relation between the creep compliance and elastic modulus, independent of composition and orientation, can be established by considering the individual creep behavior of the soft and stiff components that arises from the stress partitioning within the rock. This appears to provide a mechanical explanation for why long-term ductile deformational properties can appear to correlate with short-term elastic properties in shale-gas reservoir rocks.