Chalk model for pore collapse and permeability decline

ABSTRACT: A constitutive model for chalk is proposed that addresses several hydromechanical behaviors of chalk. These behaviors include high porosity, pore collapse and water sensitivity of strength and stiffness. This mechanical behavior is coupled with low permeability which suffers a significant decline at pore collapse. The model is a single yield surface model that accounts for both shear and compaction failure. It combines a Drucker-Prager yield surface for shear with a pressure cap for compaction failure. It is non-associative since it needs to account for compaction failure during shearing at high confining stresses. The model accommodates water weakening using water saturation as an internal variable. The mechanical model is coupled with a stress dependent permeability model that models the observed rapid decline of permeability during pore collapse. The model is calibrated on experimental results from various chalk tests along different stress paths and different pore fluid saturations. The model can be applied in various industrial application such water injection operations, chalk influx control, and borehole stability.