Geomechanics and Rock Physics of Anisotropic Overburden Shales

Abstract

Shales, with their diverse composition and complex properties, present an interesting research subject and a challenging material to work with. As they constitute a significant portion of the overburden in most currently operational subsurface storage sites and reservoirs, their complicated nature poses significant engineering challenges related to subsurface stability and monitoring. This forces geoscientists and geoengineers, including those who are just starting their scientific journey and aspire to work in the field, to delve deeper into the processes taking place inside them. The low permeability of shales significantly restricts fluid movement within them, making them ideal candidates for cap rock. However, when subjected to ongoing deformation, fluid cannot enter or exit the pores instantly, causing the rock to respond to changes in external stress with alterations in pore pressure. This response further modifies the effective stress in shales, potentially pushing the rock closer to failure or increasing the risk of borehole and casing related issues. The first part of this thesis primarily examines the undrained pore pressure response. Through an in-depth introduction and an article, the author and coauthors describe development and testing of an anisotropic poroelastic model for the undrained pore pressure response in transversely isotropic shales and rocks with lower symmetry classes. Utilizing experimentally determined poroelastic pore pressure coefficients and geomechanical modeling results, the model is applied to predict induced pore pressure changes in the Valhall reservoir's overburden. This study reveals potentially significant pore pressure changes above the reservoir's top surface, indirect evidence of links between the undrained pore pressure response to known casing failures and formulates a simplified pore pressure modeling workflow for transversely isotropic overburden shales. The second article assesses the impact of the undrained pore pressure response on total stresses at shear failure for various rocks, sample orientations, and stress loading scenarios. The findings highlight the importance of considering the undrained pore pressure response in shale stability and integrity analysis. The final article in this part of the thesis examines the influence of shale plastification, expected near fault zones, on pore pressure response, revealing a gradual transition in magnitude and direction of response with accumulating plastic strains and presents a simple model accounting for plasticity induced changes in pore pressure parameters. Another ramification of shales' dominance in typical overburden is that seismic signals traversing toward a reservoir spend a considerable length of time passing through these deposits. Consequently, alterations in shales' acoustic properties could account for a significant portion of observed timelapse seismic effects utilized in monitoring injection sites or depleting reservoirs. The second part of this thesis revolves around rock physics models that connect observed changes in P- and S-wave acoustic velocities to shale deformation, enabling the connection of timelapse effects to subsurface geomechanical processes. The first paper in this part of the thesis establishes and tests a nonlinear third-order elastic strain-dependent dynamic stiffness model for shales, demonstrating a strong fit across various stress loading scenarios and propagation directions. The second paper introduces a modified inversion scheme for estimating third-order elastic coefficients from low frequency laboratory data, addressing differences in strain sensitivity between high and low frequency seismic signals. This paper highlights potential errors when inverting timelapse seismic data using ultrasonic frequency laboratory calibrations and documents low frequency strain-sensitivity factors determined from core plugs in experimental conditions consistent with field observations. The final paper expands the initial model to include nonequal horizontal strains and examines the impact of experimental errors, suggesting that shear components of the strain tensor may significantly influence time-shift and time strains recorded in overburden shales. These conclusions and observations are prefaced by a concise introduction to undrained pore pressure changes, nonlinear third-order elastic dynamic stiffness models, shale properties, and associated experimental challenges. The thesis concludes with a summary of findings and recommendations for future work.