Integration of Rock Physics and Geomechanics to Improve 4D Overburden Monitoring

Abstract

This PhD thesis focuses on integrating rock physics and geomechanics to improve the interpretation of 4D seismic data for monitoring stress and pore pressure changes in the overburden above depleting hydrocarbon reservoirs. These changes can have operational impacts, e.g., affecting infill drilling. The research is also relevant for monitoring caprock integrity in Carbon Capture and Storage (CCS) projects. The first part of this thesis links numerical geomechanical and rock physics modeling with overburden 4D seismic responses. The 4D seismic response in the overburden depends on the stress changes and strains induced by reservoir depletion or injection. This study quantifies stress changes, undrained pore pressure changes, and strains around a depleting reservoir through numerical geomechanical modeling, using anisotropic static elastic moduli and anisotropic Skempton parameters from laboratory data for two different field shales to simulate the surroundings. The impact of these overburden changes on 4D seismic strain sensitivities (as R-factors) and time shifts is quantified by an empirical rock physical model calibrated through laboratory tests. Key findings of this work highlight the importance of material anisotropy in modeling stress paths and the role of elastic stiffness contrast between the reservoir and surrounding anisotropic rock. This work also demonstrates how overburden anisotropy and stiffness contrast impact stress path alterations and undrained overburden pore pressure changes after reservoir depletion, thereby impacting 4D seismic attributes such as seismic velocities and time shifts. The second part of this thesis aims to understand the factors influencing stress paths in the overburden of depleting reservoirs by conducting a parametric study using numerical simulations. The geometry of the reservoir, particularly its aspect ratio, significantly affects stress changes in the reservoir surroundings, especially in regions near the reservoir. While reservoir depth has a minor impact on stress paths, it becomes more important for conditions with shallow and wide reservoirs as affected by the free surface. Additionally, stiffness contrast between the reservoir and its surroundings, both isotropic and anisotropic, significantly influences stress arching. Numerical simulations for linear elastic scenarios show empirical relations between elastic stiffness and stress paths. Using these relations, an analytical approximation is applied to predict stress paths when considering nonlinearity. Insights from this study extend beyond existing analytical models (e.g., the commonly recognized Geertsma model). The third part of this thesis aims to enhance the ability of analytical modeling to accommodate more realistic scenarios by considering topographic changes in the subsurface. It is based on a semi-analytical solution that extends the Geertsma solution to accommodate static VTI properties with five independent elastic stiffnesses in the surroundings of a reservoir. This extension improves the ability of the model to represent the vertical stiffness variations characteristic of realistic subsurface conditions. The extended Geertsma solution is applied through linear superposition to adapt the model for practical use in scenarios with inhomogeneous pore pressure distributions and three dimensional variations in rock stiffness. Deformation results from the linear superposition approach are validated by comparing them with those from a finite element numerical method under several scenarios focusing on complex topography and non-uniform pore pressures. After validation, the approach is applied to the Valhall field, which is known for its significant subsidence and complex topography. The practical application highlights the capability of the approach to accurately model realistic subsidence changes, demonstrating its utility for complex geomechanical assessments and the potential to be linked with geophysical monitoring. Together, these studies provide a foundation for subsurface monitoring related to reservoir depletion or injection and emphasize the importance of further research into static anisotropy, anisotropic wave propagation, the frequency dependence of stress sensitivity, and the broader application of numerical and analytical geomechanical modeling in geophysics for subsurface monitoring.