INTRODUCTION
Since the late 1960's, when Phillips Petroleum Company discovered the giant Ekofisk Field, the North Sea has become a major petroleum producing province. A large number of reservoirs, many of which are hosted in Cretaceousand Paleocene-age chalk, have since been discovered and are currently in production. High-porosity, mechanically-weak chalks in these reservoirs typically undergo substantial vertical compaction as formation pressures decline during production. Depending upon the depth of the reservoir and its vertical and lateral dimensions, reservoir compaction can result in significant subsidence of the overlying seafloor (Geertsma, 1973; Boade, Chin and Siemers, 1989) which may necessitate modifications of platforms and other structures, as has been the case at Ekofisk.
Waterflooding chalk reservoirs with seawater is an attractive option that has the dual advantages of substantially increasing the amount of petroleum ultimately recovered from the reservoir while maintaining reservoir pressure, thereby mitigating reservoir compaction and reducing the potential for seafloor subsidence. Before waterflooding the reservoirs it is essential to determine what effect seawater contact has on the compaction behavior of the chalk.
A number of studies have addressed the relationship between water saturation and chalk strength. Meigh and Early (1957) and Carter and Mallard (1974) found dry chalk to be substantially stronger than water-wetted chalk. Blanton (1981) measured similar volume strains in hydrostatic tests of dry and oil-saturated North Sea chalks. Marek (1976) described some highly compressible chalk-like limestones that were at or near yield stress under initial reservoir stress conditions when saturated with simulated formation brine. Boozer, Hiller and Serdengecti (1963) reported on the reduction in yield strength of Indiana limestone saturated with calcium carbonate-saturated water, and argue that the weakening of the limestone is due to the strong adsorption of water on the calcite matrix. However, Newman (1983) found that North Sea chalks remain strong when injected with oil or formation water, but are much weakened when injected with seawater or a 30,000-ppm NaC1 brine.
