Influences of Layered Heterogeneity on Poroelastic Behavior of Geological Reservoirs

Ganiyat Shodunke, Junle Jiang, & Segun S. Bodunde

Submitted February 27, 2025, SCEC Contribution #12629

Fluid-rock interactions in geological reservoirs can alter pore pressure and cause ground deformation at rates reaching tens of centimeters annually. Elastic deformation models often simplify structural heterogeneity that controls pore pressure distribution, potentially biasing inference of the subsurface from surface data. Here, we investigate the impacts of depth variations of rock physical properties (porosity, permeability, confining rock thickness, and compaction) and elastic properties (Young’s modulus and Poisson’s ratio) on the magnitude, rate, and spatiotemporal pattern of poroelastic deformation and pore pressure in geological reservoirs. Motivated by structures of the Salton Sea geothermal field, we design finite-element models to explore a single-well fluid extraction scenario with a constant rate in homogeneous half-space or multi-layered geologic settings. Our cases of layered heterogeneity involve (1) caprock-reservoir systems with varying permeability and caprock thickness, (2) compaction-induced porosity variations following Athy’s law, and (3) depth-dependent profiles for Young’s modulus. We investigate key parameters over geologically realistic ranges, including porosity of 10–40%, reservoir permeability of 10E-15–10E-13 m^2 , caprock and basement permeability of 10-21 m^2 , and Young’s modulus of 10–50 GPa. We assess the simulated reservoir behaviors at the early transient and later steady-state stages over a two-week operational period. For homogeneous structures, lower porosity or permeability increases displacement rates and negative pore pressure primarily at the early or later stages, respectively, whereas reduced elastic moduli amplify surface displacement with minimally altered pore pressure. For heterogeneous structures, the presence of a less permeable caprock and its increasing thickness result in decreased vertical surface displacements, while pressure changes vary from negative at depth to positive near the surface. Depth variations in elastic moduli in alternating or linear profiles generally produce larger vertical displacements and non-monotonic displacement rate histories, indicating cross-layer fluid migration. Focusing on the spatial characteristics of surface deformation, we find that lower porosity, higher permeability, or thicker caprock facilitates faster quasi-linear radial expansion of deformation signals. In contrast, only elastic property variations can change the initial crossover distance and peak-value ratio between the vertical and horizontal radial surface displacements, suggesting different impacts on surface deformation. Our findings can improve the prediction and inference of coupled fluid-rock processes in complex geological reservoir structures.

Key Words
Creep and deformation; Elasticity and anelasticity; Permeability and porosity; Numerical modeling; Geomechanics; Hydrothermal systems

Citation
Shodunke, G., Jiang, J., & Bodunde, S. S. (2025). Influences of Layered Heterogeneity on Poroelastic Behavior of Geological Reservoirs. Geophysical Journal International, (submitted).

Related Projects & Working Groups
Stress and Deformation Over Time (SDOT)