Mudstones play an important role in hydrocarbon exploration and production, carbon capture and storage, and nuclear waste disposal. The high concentration of clay minerals contribute to the high intrinsic anisotropy (e.g., velocity, strength, permeability, and resistivity changes with direction) of mudstones. This high anisotropy complicates, among other things, seismic interpretation for hydrocarbon exploration and production, as well as predictions on the mechanical behaviour of these clayrich rocks. Mudstones are also characterized by a low-permeability matrix, which makes it difficult for fluids to flow through the rock. This impermeable character of mudstones makes them a potential natural seal for long-term CO2 storage and a potential host rock for nuclear waste disposal. For hydrocarbon production, open fractures are needed to enhance the productivity of oil and gas reservoirs, whereas the presence of such fractures can result in unwanted leakage of CO2 or nuclear waste in the subsurface. Fracture formation depends on, among other things, the mechanical properties of the mudstone. It is thus important to understand the elastic anisotropy and mechanical properties of mudstones for successful hydrocarbon exploration and production, and to safely store CO2 and radioactive waste in the subsurface. Although mudstones are important in the energy sector, the understanding of their elastic anisotropy and deformation behaviour under various physical conditions is limited, due to their complex character and the lack of laboratory experiments performed on well-preserved samples.@en

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.

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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.

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Mudstones are characterized by their tight matrix and are therefore of interest in various industries, including the petroleum and underground repository industry. These clay-rich rocks often show dynamic elastic anisotropy, which causes significant problems in geophysical interpretations. In addition, pore water has a significant effect on the bulk properties of mudstones. However, the degree of saturation is often not reported in the literature. This study investigates the impact of water saturation on the elastic anisotropy of the Whitby Mudstone. Four core plugs with different water saturations were deformed until failure and tested ultrasonically at effective confining pressure conditions of 25 MPa. P-wave and S-wave velocities were monitored along the symmetry axis, across the core diameter, and at ~49˚ to the horizontal bedding plane to calculate the full elastic tensor and subsequently the Thomsen anisotropy parameters. The degree of saturation highly affects the rock strength and static elastic properties and leads to significant changes in the elastic anisotropy parameters.@en

Mudstones are of great interest in the petroleum industry, since they can act as a source rocks, reservoirs and seals. Understanding the mechanical behaviour of these clay-rich rocks is crucial for successful hydrocarbon exploration and production. This study examines the impact of water saturation on the mechanical behaviour of the Whitby Mudstone. Triaxial compression tests are performed on mudstone samples with different water saturations at effective confining pressures of 5 MPa, 15 MPa, 25 MPa, and 40 MPa. The results show that the degree of saturation significantly affects the rock strength and static elastic properties (Young’s modulus and Poisson’s ratio). The mechanical properties obtained from partially-saturated mudstones should be treated with care when extrapolating them to in situ conditions.@en

Open natural fractures allow fluids to flow, which is necessary for the production of low-permeable geothermal and petroleum reservoirs. These reservoirs often consist of lithological layers with significant variation in rock strength, which makes it difficult to predict fracture containment within the rock succession. In this paper, fractures are classified as contained, when they do not cross the layer interface of the adjacent layer, so that formation and growth are inhibited in the adjacent layer. This study concerns the impact of the differences in rock strength (i.e., mechanical contrast) between two adjacent brittle layers on the fracture containment in finely-layered rocks. Laboratory deformation tests and 2D finite element modelling were performed on three-layered samples with varying mechanical contrasts to examine the behaviour of fractures at different stress conditions. Fractures initiate as shear fractures in weak layers, and propagate with a steeper angle (tensile fracture) through the adjacent stronger layer. The mechanical contrast within a layered rock does not always act as a containment barrier, meaning that fractures do not cross the layer interface of the adjacent layer. This is due to differences in differential stress between the weakest layer and strongest layer. The mechanical contrast combined with the magnitude of the confining pressure has a significant influence on fracture containment. An increase in mechanical contrast and confining pressure prevents fractures from propagating into stronger layers. The results contribute to predict natural fracture containment in brittle sequences at shallow depth in the subsurface.@en

Knowledge of the rock mechanical behaviour of rocks subjected to load-cycling is of importance for many engineering projects, where the material is subjected to regularly cycling stress levels. Engineering circumstances are, for example, civil construction settings of haulage roads, seasonal storage of hydrocarbons or CO2 in depleted reservoirs and in magma movement in active volcanoes. The cyclic loading could result in a gradual weakening of the material and possibly into a different type of fracture network in comparison with material that has been loaded to failure only once. The results show that cyclic loading causes material to weaken progressively up to 20% in comparison with material that has been loaded to failure in one cycle. It is also seen that the fractures that form during cyclic loading are more distributed throughout the sample. In order to increase the connectivity and distribution of the fractures, cyclic loading can be attempted. It is expected that then a better distributed network of fractures will be formed potentially increasing the permeability of this fractured material. The progressive weakening of the material due to cyclic loading can cause an earlier loss of integrity of for example seals, and needs to be taken into account in in seasonal storage of fluids in subsurface reservoirs. @en

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.

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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.@en

We studied the rock mechanical behaviour of the outcropping Whitby Mudstone Formation shales (Toarcian Age) under varying temperature and confining pressure conditions, focusing on strength and elastic moduli. We compared the rock mechanical properties of the Whitby shale to published data of time and depositional equivalent shales from Northern Europe and producing shales from the US. We performed uniaxial and triaxial tests in order to constrain the effect of temperature and pressure on the mechanical behaviour of the shale. We loaded the samples normal to the bedding plane under room humidity conditions. We performed uniaxial tests at room temperature and zero confining pressure, whereas we applied confining pressure in several steps up to 50 MPa and considered temperature values between 20-150°C during triaxial tests. Ultimate strength and elastic moduli are strongly influenced by mineralogy, temperature and pressure. An increase in temperature enhances the deformability, whereby the Young’s Modulus and ultimate strength decreases. With an increase in confining pressure, Young’s Modulus and ultimate strength increases, whereas the Poisson’s ratio decreases.@en