A Theoretical Framework for Integrating Fracture Mechanics and Reservoir Geomechanics to Predict Induced Seismicity Risk in Wastewater Disposal Operations in Nigerian Sedimentary Basins

Induced seismicity from subsurface fluid injection has emerged as a significant operational and regulatory risk in petroleum engineering globally, and the theoretical underpinning for its prediction in the specific geological context of Nigerian sedimentary basins is absent from the published literature. This study develops an original theoretical framework integrating fracture mechanics, reservoir geomechanics, and poroelastic stress coupling to predict induced seismicity risk from wastewater disposal and produced water reinjection operations. The framework draws on Mohr-Coulomb effective stress theory, critically stressed fault analysis from Anderson's tectonic stress regimes, and coupled pore pressure-stress diffusion equations derived from Biot's poroelastic theory. An original dimensionless Seismicity Risk Index is derived, expressing the probability of fault reactivation as a function of injection volume, injection rate, formation permeability, distance to critically oriented faults, and ambient tectonic stress state. The theoretical framework is calibrated against induced seismicity datasets from analogous injection operations in the Permian Basin and the Midcontinent United States, and applied prospectively to the Niger Delta tectono-stratigraphic setting using in-situ stress orientation data from borehole breakout analyses and published stress field characterisations. Application to three Niger Delta disposal well scenarios quantifies the relative seismicity risk gradient between shallow aquifer disposal, deep saline formation disposal, and sub-Akata Formation disposal targets. The framework provides a scientifically grounded screening tool for regulators and operators in Nigeria. Keywords: induced seismicity, geomechanics, wastewater disposal, Niger Delta, fracture mechanics.