ABSTRACT
ABSTRACT: In the North Sea Ekofisk oilfield, oil is contained in a 150m thick layer of porous chalk (n= 40-50%) located at a 3000 m depth. An enhanced oil recovery procedure carried out by injecting sea water in the reservoir chalk caused a significant subsidence resulting in an about 10m settlement of the seafloor. This coupled multiphase problem has been considered within a framework taken from the mechanics of unsaturated soils, by replacing the non wetting fluid (air in unsaturated soils) by oil in the reservoir chalk. This approach lead to consider the oil-water suction s= uo − uw (where uo and uw are the oil and water pressures, respectively) as a relevant independent stress variable to describe the coupled behaviour of reservoir chalks.Another important characteristic of chalk behaviour is the time dependency of its constitutive law. In an attempt to better understand the subsidence observed under waterflooding, the objective of this paper is to investigate the combined effect of suction and time on the response of the multiphase chalk under isotropic compression. The result obtained confirm that the LC concept of the Barcelona model is also relevant for reservoir chalk. The combined effect of suction and loading rate on the shape of the LC curve is evidenced, providing a continuous link between existing data on the behaviour of water saturated and oil saturated chalk samples.
