Laboratory experiments reveal that room-dried unconsolidated Gulf of Mexico (GOM) shale from the South Eugene Island field exhibits pronounced viscous creep behavior under constant isotropic compression. The shale exhibits a lack of creep when unloaded, suggesting that the creep strain is best considered as viscoplastic deformation. Creep behavior is different above and below 30 MPa confining pressure. Above 30 MPa, the amount of creep strain is 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 simple model that combines Perzyna?s viscoplasticity and a modified Cambridge clay plastic yield theory is derived in the form of strain rate as a power law function of the ratio between the sizes of the ellipses representing dynamic and static yield surfaces in the p-q space. The model parameters determined from experimental data indicate that the yield stress of the shale increases by 6-7 % as strain rate rises by an order of magnitude. This demonstrates that the laboratory-based prediction of yield stress (as well as porosity) may significantly overestimate (underestimate for porosity) the property in situ.
Understanding of compaction in unconsolidated rocks is important in many engineering aspects such as assessment of land subsidence and estimation of rock physical/mechanical properties at depth. It is well known that compaction in such geomaterials includes timedependent deformation, i.e. creep [1-4]. The mechanism of time-dependent deformation in such materials has been attributed either to pore fluid expulsion under drained condition [1, 5, 6] or to pore pressure redistribution under undrained condition [7, 8]. Recent laboratory experiments, however, showed that timedependent deformation is also evident in dry unconsolidated sands [9-12]. The mechanism responsible for such viscous behavior was suggested to be related to load-bearing intergranular clay and mica content in the samples. In viscoplasticity theory, time-dependent deformation is closely related to rate-dependent deformation. Rock physical properties such as yield stress and porosity variation can change depending on deformation rate. This characteristic of viscous rocks may raise an issue when one attempts to estimate physical properties of rocks in situ based on relatively fast deformation rate laboratory experiments. The main objective of the present study is to investigate creep behavior in room-dried unconsolidated shale and then to develop a theoretical model that can describe creep deformation in order to understand its effects on rock physical properties.
2. TEST SPECIEMENS AND EXPERIMENTAL METHOD
The shale samples used in our experimental study were prepared from cores (102 mm diameter) extracted from a 2260 mTVD of the Pathfinder well in South Eugene Island Block 330, offshore Louisiana, Gulf of Mexico . The shale was weak and unconsolidated, with a composition of more than 50 % clay, 30 % quartz, and some minor amount of feldspar and plagioclase . The shale cores were room-dried for more than two years. Initial porosity, measured using a Boyles? law porosimeter, was 27 0B1; 3 %. Density, determined from measurements of mass and volume of prepared specimens, was 18560B1;29 kg/m3.