Using active ultrasonic source survey data, coda wave decorrelation (CWD) time-lapse imaging during the triaxial compression of Whitby Mudstone cores provides a 3-D description of the evolution and redistribution of inelastic strain concentrations. Acoustic emissions (AEs) monitoring is also performed between any two consecutive surveys. From these data, we investigate the impact of initial water saturation S_w on the onset, growth, and reactivation of inelastic deformation, compared to the postdeformation fracture network extracted from X-ray tomography scans. Our results indicate for the applied strain rate and degree of initial water saturation, and within the frequency range of our ultrasonic transducers (0.1 to 1 MHz), that inelastic strain localization and propagation in the Whitby Mudstone does not radiate AEs of sufficient magnitude to be detected above the average noise level. This is true for both the initial onset of inelasticity (strain localization) and during macroscopic failure. In contrast, the CWD results indicate the onset of what is interpreted as localized regions of inelastic strain at less than 50% of the peak differential stress the Whitby Mudstone can sustain. The seemingly aseismic nature of these clay-rich rocks suggests the gradual development of inelastic strain, from the microscopic diffuse damage, up until the macroscopic shear failure.

In layered materials, the deformation style, orientation, confinement, and 3D connectivity of natural fractures is generally impacted by changes in sedimentary facies and alternations in mechanical properties. In this study we address this effect and perform a numerical sensitivity analysis. Mechanical properties, confining pressures, and interfacial frictions are varied for a three-layered model, to investigate and quantify the relation between contrasting material properties, the principal horizontal stresses and fracture behaviour (i.e. deformation style and orientation). Firstly, the results show that tensile stresses develop in the stiffer layers due to the contrasting elastic parameters. The magnitude of these stresses is dependent on the ratio between the elastic parameters of stiffer and softer layers (i.e. E_stiff/E_soft and ν_stiff/ν_soft). There are no horizontal tensile stresses, when applying a compressive horizontal confining pressure (approx. 1/5 of the applied vertical stress). Implementing an interfacial friction lower than 0.2 will result in decoupling of the layers, resulting in slip on the layer boundaries and no tensile stresses within the stiffer layers. Further, the acquired numerical results are in good agreement with previously conducted laboratory work. Finally, we discuss whether the presented results can be used for better relating contrasting mechanical properties to potential fracture deformation styles and orientations in layered outcrops or subsurface reservoirs.

Finding generic trends in mechanical and physical rock properties will help to make predictions of the rock-mechanical behaviour of shales. Understanding the rock-mechanical behaviour of shales is important for the successful development of unconventional hydrocarbon reservoirs. This paper presents the effect of heterogeneities in mineralogy and petrophysical properties on the validity of generic trends on multiple scales in rock-mechanical and rock-physical properties of the Whitby Mudstone. Rock-mechanical laboratory experiments have been performed on Whitby Mudstone samples from multiple outcrops within five kilometres laterally in order to investigate the heterogeneity and possible trends on an outcrop scale. Unconfined compression tests and acoustic measurements have been conducted to obtain the rock-mechanical properties, including rock strength, Young's modulus, Poisson's ratio, and velocity anisotropy. The rock-physical properties, including mineralogy, porosity, and matrix density, were measured using X-ray fluorescence and helium pycnometry. Various methodologies have been applied to the resultant data in order to derive different brittleness indices. Significant heterogeneity in rock-mechanical and rock-physical properties is present on an outcrop scale. There is no obvious correlation between mineral content and rock-mechanical properties on an outcrop scale in the Whitby Mudstone. Comparison with shales from different basins show, however, correlations between composition and elastic properties. The presence of significant heterogeneities on an outcrop scale and between shales from different basins make it difficult to find generic trends in rock-physical and rock-mechanical properties.

Heterogeneities in finely-layered reservoirs complicate predictions on the fracture behavior. Understanding the controls on fracture behavior in these unconventional reservoirs is important for their successful production. This research examines the influence of rock-mechanical properties on fracture characteristics in finely-layered reservoirs. Rock-mechanical laboratory experiments have been performed on layered shale-sandstone samples and layered granite-sandstone samples, in order to investigate the rock-mechanical parameters and fracture geometry. Fracture characterization, including fracture initiation, propagation, orientation, and interaction between the multiple layers was performed using X-ray micro-computed tomography scans. Predictions of the rock-mechanical properties of a layered material can be made using the Reuss-bound averaging each individual layer. The strength, however, cannot be averaged. Fracture initiation is highly dependent on rock strength: Fractures initiate at the failure point of the weakest layer, after which they do propagate at low average stress levels through stronger layers due to local stress amplification at the fracture tip. The fractures change their orientation through layers with different mechanical properties. The results show that the variability in rock-mechanical properties influences the fracture behavior in a finely-layered reservoir.