Frequency-dependent seismic attenuation in shales: experimental results and theoretical analysis

Samples of shales from the Ordovician Bongabinni and Goldwyer source rock formations were recovered from the Canning Basin (Western Australia). Attenuation was experimentally measured on preserved plugs from these formations in the frequency range between 10−2 and 102 Hz. Samples cored with different orientations with respect to the sedimentary bedding were prepared and tested in their native saturated state and after drying in the oven at 105 °C for 24 hr to assess the effect of fluids and of the sediment anisotropy on attenuation. To aid the interpretation of the experimental results, the clay-rich samples were characterized in terms of mineralogy, water content, porosity, permeability and microstructure. The two shales have significantly different quality factors; and this is seen to be dependent on both the saturation state of the samples and the propagation direction of the oscillatory signal. The attenuation coefficient for compression/extension parallel to bedding is less than that vertical to bedding in both the preserved and partially dehydrated situations. No frequency dependency is observed in the preserved samples within the range of frequencies explored in this study. On the other hand partially saturated samples show peaks in attenuation at around 40 Hz when the stress perturbation is transmitted normal to the macroscopic bedding. The interpretation of the attenuation measurements in terms of well-established theoretical models is discussed in view of the physical characteristics and microstructure of the tested rock

Induced-seismicity geomechanics for controlled CO2 storage in the North Sea (IGCCS)

The aim of the current study, IGCCS (2017–2020), is to evaluate the feasibility of micro-seismic (MS) monitoring of CO2 injection into representative storage candidates in the North Sea, based on broad and quantitative characterization of relevant subsurface behavior with respect to geology, geomechanics and seismicity. For this purpose, we first group potential CO2 storage sites in the North Sea into three different depths. Then, advanced triaxial rock mechanical tests are performed together with acoustic emission (AE) acquisition under representative loading for CO2 storage sites in the North Sea and for formations of each depth group, covering shale, mudstone and sandstone cores. Our work focuses particularly on quantifying the effects of injected fluid type and temperature on mechanical behavior and associated MS response of subsurface sediments. The experiment results show that each depth group may behave differently in responses to CO2 injection. Particularly, the occurrence of detectable MS events is expected to increase with depth, as the combined effects of rock stiffness and temperature contrast between the host rock and injected CO2 are increasing. In addition, lithology plays an important role in terms of the MS response, i.e. high AE event rate is observed in sandstones, while aseismicity in shale and mudstone. The test results are then scaled up and applied to advanced coupled flow-geomechanics simulations and a synthetic field-scale MS data study to understand micro-seismicity at fracture, reservoir and regional scales. The numerical simulation of scCO2 injection scenario shows quite different stress-strain changes compared to brine injection, resulting mainly from the thermally-induced behavior. Furthermore, the numerical simulation study via so-called Cohesion Zone Modeling (CZM) approach shows strong potential to improve our understanding of the multiphase-flow-driven fracture propagation. Our synthetic MS data study, focused on slow-earthquake scenario, also suggests that sensors with high sensitivity at low frequency might be necessary for better signal detection and characterization during CO2 injection. This manuscript covers the main findings and insights obtained during the whole study of IGCCS, and refers to relevant publications for more details

Geomechanical appraisal and prospectivity analysis of the Goldwyer shale accounting for stress variation and formation anisotropy

Profitable exploitation of unconventional shale gas reservoirs relies on the success of hydraulic fracturing stimulation. This is more likely in brittle rock formations because natural and hydraulic fractures remain open after stimulation, allowing for more hydrocarbon production. Identification of the most favourable depth intervals relies on the robust analysis of available well-logs, and on laboratory-derived mechanical and elastic data obtained under controlled stresses replicating the actual conditions at depth. Beyond their use for predictive geomechanical modelling such laboratory data can act as calibration points for existing well-logs. Well-logs can also be used to guide the selection of the rock samples to be characterised and tested in the laboratory, ensuring that they are representative of the rock formation. Here we apply the above principles and demonstrate how this improves the geomechanical appraisal of the Goldwyer formation and assesses its prospectivity. This workflow integrates Rock-eval geochemical analyses, elastic properties, anisotropy, in-situ stress state and pore pressure, mechanical brittleness and fracturing indices derived from petrophysical and sonic logs in the Theia-1 and Pictor East-1 wells. We estimated an average total organic carbon of 2 w. t.% (maximum 5 w. t.%), a moderate to high dynamic Young's modulus (14–52 GPa), a low Poisson's ratio (0.24–0.27), and an average elasticity-based brittleness index B1 of 41% in the deeper G-III unit. This unit also exhibits a low differential horizontal stress ratio and a high fracture index. Such attributes suggest a good prospectivity of the G-III unit, not only in terms of potential resources but as importantly in terms of fracability

Triaxial Deformation of the Goldwyer Gas Shale at In Situ Stress Conditions—Part I: Anisotropy of Elastic and Mechanical Properties

The evolution of shale’s mechanical properties with confining pressure, temperature, and mineral composition directly influences fracture closure besides the effect of in situ stress variation across lithologies. We are the first to perform experimental study to characterize the mechanical properties of the Goldwyer gas shale formation located in the Canning Basin, Western Australia. We have performed constant strain rate multistage triaxial tests at in situ stress condition (confining pressure ≤ 22 MPa) on 15 samples of the Goldwyer gas shales with variable minerology, organic content, and heterogeneity. Deformation tests were conducted at room temperature and in drained conditions on cylindrical samples cored parallel (horizontal) and perpendicular (vertical) to the bedding plane. Both triaxial compressive strength (σTCS) and static young’s modulus E show a strong sensitivity to confining pressure and mineralogy, while only E shows a directional dependency, i.e., Eh > Ev. The internal friction coefficient µi in a plane parallel to the bedding is 0.72 ± 0.12, while it is only 0.58 ± 0.17 in the orthogonal direction. Both σTCS and E are significantly lower when larger fractions of weak mineral constituents are present (clays or organic matter). We observe that the Young’s modulus of most vertical samples is best approximated by Reuss’s bound, whereas that of horizontal samples is best approximated by Hill’s average of Voigt and Reuss bounds. The most prospective G-III unit of the Goldwyer shale formation (depth > 1510 m) is semi-brittle to brittle, making it suitable for future development