Unsaturated Soils: Advances in Geo-Engineering – Toll et al. (eds) © 2008 Taylor & Francis Group, London, ISBN 978-0-415-47692-8

ABSTRACT: During the six years long Pasachalk project devoted to the mechanical behaviour of high porosity chalks from North Sea oilfields, the constitutive model Pasachalk (Collin et al., 2002) was proposed based on the Barcelona Basic Model (BBM) (Alonso et al., 1990). The approach was based on the similarities found between the oil-water interactions (oil and water being the non wetting and wetting fluid respectively) in oil reservoir chalk and the air-water interactions in unsaturated soils. This approach appeared to be relevant to interpret the subsidence of the seafloor during waterflooding operations for enhanced oil recovery that has been observed in North Sea oilfields (e.g. Ekofisk oilfield). Another important component of subsidence was then related to the creep behaviour of the multiphase chalk (De Gennaro et al., 2003). A modified Pasachalk model was proposed to account for time effects using the framework of Perzyna’s viscoplasticity (1964) but without considering suction effects. Based on available experimental results (Priol et al., 2007), a modified version of the viscoplastic Pasachalk model including suction effects is proposed in this paper.