Hardening in porous chalk from precompaction

Knowledge of compactant behavior and the associated subsidence for high porosity carbonate formations is crucial in applications such as enhanced oil recovery. To gain insights into inelastic compaction and failure modes of a porous chalk under different loading histories, triaxial compression experiments were conducted on a high porosity (45 %) chalk, tested using three stress paths: no precompaction (virgin rock), precompacted to 30 MPa, and precompacted to 60 MPa. For the virgin chalk with no precompaction under triaxial compression, the transition from axial splitting to shear fracturing to compaction banding was observed. By precompacting the specimens to mean stresses of 30 and 60 MPa, the brittle failure regime expanded. In 60 MPa precompacted specimens, shear fracture was observed over a larger range of mean stress. Furthermore, precompaction increased the internal friction angle due to permanent volume change, but the cohesion decreased due to damage.