Evaluating the mechanical properties of carbonate and evaporite caprocks in the Sichuan Basin

Abstract

Caprocks are nearly impermeable rocks that inhibit the migration of hydrocarbons. Caprock efficiency at sealing hydrocarbon reservoirs is contingent upon maintaining its integrity, or the absence of features that enhance fluid flow such as fractures/faults. Caprock lithology is an important factor in sealing efficiency, as it controls caprock mechanical behavior and brittleness. Evaluating caprock failure due to chemo-thermo- mechanical forces requires an understanding of the deformation and failure properties consistent with effective caprocks. Core samples of caprocks from the northeast Sichuan Basin – primarily evaporites, limestones and dolostones – were tested under laboratory conditions to determine the relationships between lithology and geomechanical properties of different caprocks. The results of hardness, ultrasonic velocity, and axial deformation tests of the caprock samples indicate that caprock brittleness is strongly correlated with high values of hardness, peak and yield strength, Young’s modulus and coefficient of internal friction. Lithologic heterogeneity in caprocks is found to enhance sample failure in most cases, though the inclusion of minor phyllosilicate and evaporite minerals may conversely decrease caprock brittleness. Evaporites and dolostones consistently exhibit the least and most brittle characteristics, respectively, though individual mechanical properties can indicate contradictory caprock behavior. These results suggest evaluation of the geomechanical-risk to caprocks can thus be enhanced by combining mechanical properties to map trends in caprock effectiveness. When combined with accurate in- situ conditions, the mechanical properties of a caprock allow for a quantitative interpretation of caprock integrity.