Integrated petrophysical, rock physics, and geomechanical modelling of reservoir response to increasing gas saturation for enhanced recovery in the Niger Delta

Abstract

This study presents an integrated petrophysical-rock physics-geomechanical workflow that combines petrophysical analysis, Gassmann fluid-substitution modelling, elastic-property evaluation, and sand-production assessment to investigate reservoir responses to increasing gas saturation (Sg) during gas-based enhanced recovery in the Niger Delta. Three representative reservoirs (Reservoir C-A1, Reservoir B-A2, and Reservoir C-A3) were analysed using well-log data to evaluate the coupled elastic and geomechanical effects of increasing Sg. Fluid-substitution modelling was performed by increasing Sg from 0 to 70% in Reservoirs C-A1 and C-A3 and 0–50% in Reservoir B-A2 while maintaining constant rock-frame properties. The simulations show that P-wave velocity (Vp) decreases initially (3598.29–3520.35 m/s) before stabilising, whereas S-wave velocity (Vs) increases consistently (2035.68–2074.90 m/s). Bulk density decreases from approximately 2.52 to 2.42 g/cm3, producing up to ~7% reduction in P-wave impedance (Zp), whereas S-wave impedance (Zs) remains relatively stable. Vp/Vs and Poisson’s ratio (ν) decrease, indicating reduced bulk modulus (K) and increased compressibility, while shear modulus (G) remains essentially stable. Advanced rock-physics cross-plots confirm that fluid substitution predominantly influences compressional properties, whereas shear behaviour remains controlled by the rock framework. The integrated geomechanical assessment indicates low-to-moderate sanding susceptibility, although increased compressibility may promote localized fines migration under significant pressure depletion. Reservoir B-A2 exhibits the strongest elastic response to gas substitution, whereas Reservoirs C-A1 and C-A3 display more gradual responses. The proposed workflow provides a practical framework for reservoir characterisation, production management, seismic monitoring, and evaluation of geomechanical behaviour during enhanced recovery in clastic reservoirs.