GLE Faculty Candidate’s CEE/GLE Seminar
Hiroki Sone, GFZ German Research Centre for Geosciences
Monday, Nov. 18, 2013
12:30 p.m., Rm 1025 Engineering Centers Building
Viscous relaxation model for predicting stress magnitude profiles in shale gas reservoirs: Importance of transient rock behavior in geomechanical problems

ABSTRACT: We propose a new method for estimating stress magnitudes as a function of depth in sedimentary formations based on a laboratory constrained viscous rheology and steady tectonic loading. We apply this method to a well drilled in the Barnett shale in the Fort Worth Basin, Texas. Laboratory experiments show that shale gas reservoir rocks exhibit wide range of viscoplastic behavior mostly dominantly controlled by its composition. Stress relaxation in these formations is described by a simple power-law (in time) rheology. We demonstrate that a reasonable profile of the principal stress magnitudes can be obtained from geophysical logs by utilizing (1) the laboratory power-law constitutive law, (2) an estimate of the horizontal tectonic loading, and (3) the assumption that relative principal stress magnitudes ([S2-S3]/[S1-S3]) is uniform with depth. Profiles of the predicted principal stress magnitudes generated with our proposed method for a vertical well in the Barnett shale generally agrees with the occurrence of drilling-induced tensile fractures in the same well. Also, the predicted decrease in the least principal stress (fracture gradient) in the limestone formation underlying the Barnett shale appears to explain the downward propagation of the hydraulic fractures observed in this region. This stress change is not captured by the more commonly-used elastic models even when incorporating formation anisotropy. The results highlight the importance of considering time-dependent rock deformation in capturing long-term geomechanical processes. I will also reflect on the importance of long-term deformation of argillaceous rocks in future nuclear waste disposal.

GLE Faculty Candidate’s Geoscience/GLE Seminar
Hiroki Sone, GFZ German Research Centre for Geosciences
Tuesday, Nov. 19, 2013
12:30 p.m., Rm 140, Weeks Hall
Viscous stress relaxation in organic-rich shales and its possible importance in fault mechanics

ABSTRACT: Laboratory testing of shale gas reservoir rocks reveal varying amounts of viscoplastic deformation in response to applied differential stress. Although these rocks exhibit typical brittle deformational characteristics, viscoplastic deformation is also 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 viscoplastic behavior can be modeled as a power-law function of time. Its magnitude is approximately linear with the magnitude of the applied differential pressure and nearly insensitive to the confining pressure. By applying linear viscoelastic theory and using laboratory constrained constitutive parameters, we evaluated the effect of viscoplastic deformation in modifying the in-situ differential stress over time. Calculations suggest that a significant proportion of a differential stress perturbation could be relaxed over engineering and geological time-scales. This highlights the importance of the slight ductile characteristics of these seemingly brittle elastic rocks, and poses questions on whether such viscous stress relaxation is also important in other crustal tectonic settings. I will also discuss some preliminary numerical results on how viscoelastic deformation may affect the temporal and spatial evolution of stress around fault “asperities”, and reflect on its possible impact in earthquake mechanics.