Time-dependent behaviour of oil reservoir chalk: A multiphase approach

In the North Sea Ekofisk oilfield, oil is located in a 300 m thick layer of porous chalk (n = 40-50%) at a 3000 m depth. After the initial phase of depletion an enhanced oil recovery procedure was carried out by injecting sea water (waterflooding). An unexpected consequence of this waterflooding has been the occurrence of a seafloor subsidence, corresponding up to now to a decrease of the seafloor level of approximately 10 m. It is now well recognised that hydro-mechanical coupling involving multiphase fluid interactions (oil and water) is determinant for the interpretation of the phenomenological aspects associated with the chalk compaction and the related subsidence observed in the North Sea oilfields (Ekofisk reservoir) when water flooded. The subsidence due to waterflooding is interpreted as a collapse phenomenon due to suction decrease, typical of loose and low plasticity unsaturated soils when wetted under load. On the other hand, time-dependent stress-strain behaviour of geomaterials is one of the major concerns in soil mechanics and, in effect, subsidence includes creep effects. A multiphase approach, including creep effects under controlled suction levels, is proposed in this paper; the preliminary results of this study are presented and discussed. Attention will be focused at first on the theoretical approach, supplying the essential elements for the work, and on the interpretation of the experimental results. This will provide secondly the base for formulation and validation of the constitutive law proposed for the description of the time-dependent mechanical behaviour of the chalk.