Viscoplastic Behavior of Fine-Grained Geological Units and its Applicability to Compressed Gas Caverns

Time-dependent deformation is an important, but often neglected, aspect in evaluating the geomechanical behavior of fine-grained geological units. Triaxial compression creep tests were performed on samples of laminated siltstone and shale from the Kanawha Formation. Creep tests were conducted at a temperature of 25°C and a confining pressure of 20 MPa. Specimens were tested at differential stresses of 7, 13, and 18 MPa, and these conditions were maintained for over 30 days while the axial strain of the specimen was monitored. Strain-time curves for these specimens indicate time-dependent behavior comparable to halite rocks at similar stress differences. A power law function was used to fit the creep test data. After the creep tests were completed, triaxial compression strength tests were performed on the specimens and a strength criterion was developed for plastic failure. The viscoplastic behavior was incorporated into a numerical model to evaluate the suitability of a mined storage cavern. The numerical modeling indicated that creep will allow redistribution of stress over time. This redistribution of stress results in an increase in factors of safety against plastic failure in areas. The implication is that compressed gas storage caverns in shale may be plausible at depths greater than previously considered.