Small-Scale Mechanical Behavior of Evaporite Caprocks and Implications for CO2 Sealing Capacity

Abstract:

The mechanical deformation of an evaporite caprock with field-mapped meter-scale heterogeneities is modeled numerically. Evaporite caprocks can provide important barriers to CO2 migration. However, lithologic variability could alter caprock quality, reducing CO2 storage efficiency. Caprock deformation is driven by stresses induced by reservoir depletion and subsequent re-inflation. A method to consistently upscale the elastic properties of the caprock from a given two-dimensional geological map of the caprock is proposed. To assess the uncertainties associated with this homogenization approach, we present stress/strain patterns of finite-element models at the meter-scale, based on detailed high-resolution maps of exposures of evaporite successions at a subsurface outcrop. A case study of evaporite caprock is considered; specifically, a sequence of anhydrite in the Upper Jurassic Purbeck Beds, Weald basin, UK. Mechanical perturbations induced by CO2 injection are quantified using a mapped geometric model of the evaporite caprock containing meter-scale sedimentological heterogeneities, represented down to a cut-off size of five centimeters in the direction of maximum extension. The stratigraphic interval in the Purbeck Beds comprises a basal unit of gypsum and three rhythmic carbonate-anhydrite couplets. The mapped model is mirrored to the left and right to form a 15mx2.3m detailed geometric model of the sequence, subjected to tensile deformation. The thirteen mechanical layers include nodular anhydrite, algal limestone, shale, mudstone, satin spar veins, carbonate mud with anhydrite, and nodular gypsum. The central fine-grained carbonate mud layer is populated by hard anhydrite nodules, implicitly modeled as variations of the local Young’s modulus at a sub-mesh level, assumed to vary in density as a function of height. When applying tension to the detailed model, stress concentrations develop at material boundaries, and localize in the shale layers. Stress concentrations also emerge in the nodular anhydrite layer as a result of the contrast in elastic moduli. The modeled stresses do not exceed the locally assumed tensile strength. Upscaled anisotropic Young’s moduli and Poisson’s ratios, suitable for large-scale (≥100 m) CO2 injection models, are proposed for the studied caprock.