Subcritical crack growth in a chemically reactive environment-implications for caprock integrity for CO2 storage

Abstract:

Seal integrity of cap-rock is a critical constraint on the long term performance of CO₂ containment site. During fluid migration, the coupled geochemical reaction of minerals and geomechanical deformation of rock matrix may affect the seal integrity. The potential leakage of injected CO2 from cap-rock through preexisting fractures/faults represents a major concern associated with geological storage of CO₂. To address the fundamental question of CO₂ leakage through subcritical growth of fractures driven by chemically reactive fluid across caprocks, we build a Dugdale cohesive model. Ahead of the physical crack tip, a narrow band of cohesive zone is assumed to exist with the upper and lower cohesive surfaces held by the cohesive traction. In the vicinity of the crack tip, minerals dissolve due to the acidic environment and migrate from the physical crack tip into the cohesive zone causing damage of rock matrix in the form of a reduction of cohesive traction.Focusing on the dissolution of calcite and following the stress corrosion theory, we assume the degradation of cohesive traction is linearly proportional to the concentration of Ca²⁺whose evolution follows the reactive diffusion equation. Using a critical crack opening displacement criterion, the subcritical propagation behavior of crack due to stress corrosion is captured and the rate-limiting effects including the chemical reactions to produce the Ca²⁺ and the transport of minerals along the newly generated fracture cohesive zone are incorporated. Subcritical crack growth rate under different chemical environment conditions is examined and compared with the experimental fracture mechanics testing.