Viscous creep in room-dried unconsolidated Gulf of Mexico shale (I): Experimental results

Laboratory experiments reveal that room-dried unconsolidated Gulf of Mexico (GOM) shale from the South Eugene Island field exhibits pronounced viscous creep behavior under both hydrostatic pressure and triaxial compressive stress. In all tests, the shale exhibits a lack of creep (and nearly negligible strain recovery) when unloaded, suggesting that the creep strain is best considered as viscoplastic deformation. Following application of a step-change in pressure or stress, creep compaction is considerably more significant than that which occurs instantaneously, indicating that the process of shale compaction is largely dependent on the viscoplastic deformation of the frame. Ultrasonic velocity measurements also demonstrate that the contribution of creep to the evolution of dynamic properties is significantly larger than that associated with incremental loading. Dynamic bulk moduli are similar to static moduli especially at high pressures at which pores and cracks are closed. Interestingly, in the hydrostatic compression tests the creep behavior of the shale is different above and below ~ 30 MPa confining pressure. Above 30 MPa, the amount of creep strain that occurs in 6 h is approximately constant with equal pressurization steps, indicating a linear viscous rheology. Below 30 MPa, the amount of creep increases linearly as pressure is raised in constant incremental steps, indicating a nonlinear viscous rheology. A qualitatively analogous behavior was also observed under differential stress conditions. In a companion paper (Chang and Zoback, in prep) we examine constitutive laws that describe the viscoplastic shale deformation.