Geomechanical Characterization and In-Situ Stresses Analysis for Predicting CO₂ Storage Potential: A Case Study of Toba Field, Niger Delta

Abstract

This study evaluates the geomechanical and petrophysical properties of the TOBA field to assess its suitability for CO₂ sequestration. Correlation of gamma-ray and resistivity logs across multiple wells indicates continuous rock units, except in TOBA-06, where sand A5000 is absent due to fault displacement. The lithostratigraphic analysis reveals the presence of the Benin, Agbada, and Akata formations, with the Agbada formation serving as the primary reservoir unit. Geomechanical properties such as Young’s modulus, Poisson’s ratio, bulk modulus, shear modulus, and unconfined compressive strength (UCS) were analyzed to evaluate rock stiffness, brittleness, and compaction behavior. The results indicate increasing rock stiffness with depth, with sandstone reservoirs exhibiting higher UCS and frictional angles than the caprock shale. Mechanical stratigraphy classification reveals that the saline aquifer (A3000) is a clean shale while the hydrocarbon reservoir (A6000) is a shaley sandstone reservoir, which exhibits greater heterogeneity due to shale intercalation. Stress profile from 1D MEM confirms σv > σH > σh for both well TOBA-05 and TOBA-06, characteristic of normal faulting environments. Stress and failure analysis using 3D Mohr-Coulomb criteria reveal that pre-existing faults, governed by low cohesion and shear stress, reactivate at pore pressure increases of 552–1206 psi, significantly lower than intact rock failure thresholds (1292–3060 psi). Shale caprocks exhibit transitional ductile-to-brittle behavior (2.2–2.5 g/cm³), validated by Skerlec’s model and brittleness indices (0.27–0.35). Slip tendency (Ts) analysis confirms fault instability at increased pore pressure conditions, with slip tendency values rising from 0.38–0.49 under stable conditions to 0.65–0.66 at failure and while dilation tendency at failure (Td = 0.64–0.76) highlights fracture risks. While reservoirs demonstrate high mechanical strength (UCS: 3000–7000 psi; frictional angles: 25°–30°), the normal faulting regime dominates geomechanical risk, necessitating pore pressure management below 552 psi to prevent fault reactivation. The study demonstrates that while the TOBA field is mechanically viable for CO₂ storage, fault reactivation poses the primary geomechanical risk, necessitating careful pressure management during injection and establishing operational thresholds for safe CO₂ storage in extensional basins.