The Ekofisk Field, a giant North Sea oil field, has produced more than 1.2 billion barrels of oil since the start of production in 1971. Sea floor subsidence, which was discovered late in 1984, has occurred at approximately a constant rate of about 38 cm/year since then. Full field water flooding began in 1987. Over the past decade numerous, long term laboratory studies of reservoir chalks were undertaken to understand the effects of water flooding on chalk strength and to provide input data for reservoir simulators and geomechanics codes used to predict reservoir compaction and sea floor subsidence.
Recent laboratory analyses suggest that some protective mechanism, possibly oxidized, polar hydrocarbons adhering to portions of the chalk matrix, may protect the calcite matrix from intimate chemical interaction with formation brines. This observation has led to a new constitutive model of Ekofisk rock mass behavior consisting of two sets of ‘end member’ stress strain curves. The first set represents the reservoir rock prior to water flood invasion, when the rock mass behaves as an assemblage of dominantly elastic, low compressibility matrix blocks capable of moving independently of neighboring blocks. The second set represents fully water-contacted, fully water-weakened chalk that has undergone the transition from elastic to plastic behavior. In its fully water-weakened state the chalk is highly compressible and displays the high strain rates and large strains observed in water flooded intervals of the Ekofisk reservoir. The two sets of stress/strain curves are related by water saturation.