Mechanical properties of shale gas reservoir rocks, and its relation to the in-situ stress variation observed in shale gas reservoirs. [electronic resource]
Keywords: Geophysics

The main focus of this thesis is to study the basic rock mechanical properties of shale gas reservoir rocks, the in-situ state of stress in shale gas reservoirs, and their inter-relation. Laboratory studies on the elastic and deformational mechanical properties of gas shales show that these rocks exhibit wide range of mechanical properties and significant anisotropy reflecting their wide range of material composition and fabric anisotropy. The elastic properties of these shale rocks are successfully described by tracking the relative amount soft components (clay and solid organic materials) in the rock and also acknowledging the anisotropic distribution of the soft components. Gas shales were also found to possess relatively stronger degree of anisotropy compared to other organic-rich shales studied in the literature, possibly due to the fact that these rocks come from peak-maturity source rocks. The deformational properties studied by observing the ductile creep behavior and brittle strengths were also found to be influenced by the amount of soft components in the rock and exhibited mechanical anisotropy. A strong correlation between the elastic properties and the deformational properties was also found through comparison of laboratory data. The relation between the elastic modulus and magnitude of ductile creep is investigated through differential effective medium (DEM) modeling of the shale elastic properties. The partitioning of the far-field stress between the stiff and soft components was calculated in the DEM modeling to forward model the ductile creep behavior of the shales. Results showed that the correlation between the elastic properties and magnitude of ductile creep arises because they are both influenced by the stress partitioning in the rocks. Examination of a FMI image log from a vertical well in Barnett shale showed that the in-situ state of stress fluctuates rapidly within the reservoir in terms of the orientation and magnitude of the principal stress. The appearance and disappearance of drilling-induced tensile fractures roughly correlated with the variation in the clay and organic content in the formation, suggesting that there is a fluctuation in the magnitude of the horizontal stress difference, on the order of 10 MPa, that may be controlled by the mechanical heterogeneity of the formations. In order to explain the linkage between the observed stress variation and formation heterogeneity, we focused on the variation of ductile creep behavior exhibited by the gas shale rocks observed in the laboratory. The laboratory creep data was analyzed under the framework of viscoelastic theory to quantify its behavior and allow the calculation of the stress carrying capacity of the rocks. The viscoelastic behavior of the gas shales were found to be best approximated by a power-law function of time and the accumulation of differential stress over geological time in these rocks were calculated according to this constitutive description. Stress analysis assuming a simple constant strain rate tectonic loading history over 150 Ma shows that the horizontal stress difference on the order of 10 MPa observed in the Barnett shale can be explained by the variation in viscoelastic properties within the Barnett shale. Our results highlight the importance of acknowledging viscous deformation of shale gas reservoir rocks to understand the current in-situ state of stress in these reservoirs. A study of frictional properties of a saponite-rich fault gouge from a serpentinite-bearing fault in central Japan is also presented in this thesis. Field characterization of the internal structure of a fault juxtaposing serpentinites and Cretaceous sedimentary rocks show that hydrothermal metasomatic reactions took place at the fault interface which produced peculiar mineral assemblages along the fault plane. The saponite-rich fault gouge resulting from the metasomatic reaction exhibits extremely low coefficient of friction, ~0.1, at wet conditions and strong velocity-strengthening characteristics. The study highlights the importance of geochemical reactions along fault planes which may ultimately control the overall mechanical behavior of major fault zones.