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Cracks, broadly defined as compliant discontinuities, are a major cause of elastic anisotropy. However, few models are available for quantifying crack properties relevant to anisotropy. We developed a rock-physics model to quantify crack angular distribution and normal-to-tangential compliance ratio from pressure-dependent acoustic velocities measured in the laboratory. Our model uses a rectangular function of variable width and amplitude to extract the maximum dip angle of cracks, which is a direct quantification of crack alignment relative to the bedding plane. We tested the model on an organic-rich shale data set and confirmed that crack alignment and compliance ratio strongly impact Thomsen anisotropy parameters, thus demonstrating the model as a useful tool for better understanding how cracks affect elastic anisotropy.

From geochemical reactions to proppant emplacement, hydraulic fracturing induces various chemomechanical fracture alterations in shale reservoirs. Hydraulic fracturing through the injection of a vast amount and variety of fluids and proppants has substantial impacts on fluid flow and hydrocarbon production. There is a strong need to improve our understanding on how fracture alterations affect flow pathways within the stimulated rock volume and develop monitoring tools. We have conducted time-lapse rock-physics experiments on clay-rich (carbonate-poor) Marcellus shales to characterize the acoustic velocity and permeability responses to fracture acidizing and propping. Acoustic P- and S-wave velocities and fracture permeability were measured before and after laboratory-induced fracture alterations along with microstructural imaging through X-ray computed tomography and scanning electron microscopy. Our experiments indicate that the S-wave velocity is an important geophysical observable, particularly the S-wave polarized perpendicular to fractures because it is sensitive to fracture stiffness. The acidizing and propping of a fracture decrease its elastic stiffness. This effect is stronger for acidizing, so it is possible that proppant monitoring will be masked by chemical alteration except when propping is highly efficient (i.e., most fractures are propped). However, fracture permeability is undermined by the softening of fracture surfaces due to acidizing, while being greatly enhanced by propping. These contrasting effects on fluid flow in combination with similar seismic attributes indicate the importance of experiments to improve existing rock-physics models, which must include changes to the rock frame. Such improvements are necessary for a correct interpretation of seismic velocity monitoring of flow pathways in stimulated shales.

The transition to a low-carbon future involves every component of our productive life — from the energy we use, to the buildings we construct, to the materials we use in our daily lives. Realistically, the shift onto a low-carbon path cannot happen instantly but requires adopting short- and long-term solutions while assessing whether the solutions work in the physical world in which we live. In the short term, incremental technological improvements, such as transitioning to energy sources like natural gas, have the potential to yield immediate benefit to air quality and pollution. In the mid and long term, however, more far-reaching decarbonization technologies must be pursued to achieve game-changing outcomes. In the geosciences realm, leading technologies span from cleaner energy solutions to exploring alternative earth-inspired materials and processes. These include CO2 storage in geologic disposal sites along with its reuse for material manufacturing, the development of enhanced geothermal systems expanding the use of geothermal energy, and adapting subsurface processes to engineer greener processes and materials through geomimicry. In this landscape, experimentation and rock physics are at the crux of understanding rock-fluid processes and are the premise and foundation of decarbonizing our future. All of these applications require experimentation for wider public acceptance to avoid hasty solutions that are counterproductive. The cross-disciplinary nature of each endeavor is pivotal in assessing how processes induced by fluids, their chemistry, and thermal capacity affect the physical and mechanical properties of treated environments. This paper provides an account of the role that rock physics will play in leveraging knowledge across the nanosciences to underpin our path to a decarbonized future through chemical and thermal stimulation practices, solid-CO2 reactions, and engineering processes that manipulate geology to produce materials with functional properties.