Geomechanical characterization and in-situ stresses analysis for predicting CO₂ storage potential: A case study of Toba Field, Niger Delta.

This study addresses a critical gap in geological carbon storage (GCS) assessment by explicitly analyzing the brittleness-ductility characteristics and failure mechanisms of caprock-reservoir systems under CO₂ injection-induced stress an area often overlooked in prior research. The mechanical response of subsurface formations whether brittle (leading to fracturing) or ductile (resulting in plastic deformation) directly affects containment integrity, leakage risk, and long-term storage performance. To fill this gap, we conducted a quantitative geomechanical characterization of six wells in the TOBA field, Niger Delta. Results from 1D Mechanical Earth Models (MEMs) show that mechanical strength increases with depth: Young’s Modulus rises from 4.95 GPa (shallow caprock) to 9.27 GPa (A6000 reservoir), and UCS increases from 2,427 psi to 4,037 psi. Skerlec’s density-based classification identifies hydrocarbon caprocks (2.42 g/cm³) as 37% more brittle than saline equivalents (2.28 g/cm³). Brittleness Index (BI) further confirms this trend, ranging from 0.27 in ductile saline units to 0.39 in the A6000 reservoir. Stress analysis using Mohr-Coulomb criteria shows a transitional failure mode from shear to tensile and that the A6000 hydrocarbon reservoir tolerates the highest pore pressure increment (ΔPp = 21.1 MPa), while saline reservoirs have the lowest failure threshold (ΔPp = 3.80 MPa). Stress regime analysis (Sv > σH > σh) indicates dominant normal faulting, consistent with Niger Delta growth fault systems. These findings provide critical insights for predicting rock failure behaviour, optimizing injection strategies, and ensuring long-term storage integrity in deltaic settings.