Abstract

Ekofisk, Valhall and other North Sea reservoirs are made of water sensitive rocks such as pure high porosity chalk (Andersen 1). Production of these fields may imply water- flooding which has two important consequences. On one hand water-flooding seems to fracture the reservoirs, hence increasing their gross permeability, while compaction drive becomes one of the main mechanism for production. On the other hand, compaction may endanger well completions and subsidence surface facilities.

In order to perform reliable predictions of compaction and subsidence when water-flooding such reservoirs, it is necessary to take into account not only the deformations that come from the compressibility of the reservoir rocks but also to include additional deformations induced by water-flooding.

This paper presents two constitutive models that predict such additional deformations. The first model is an elastoplastic model which depends continuously on the water saturation (WS) while the second is a particular case of the first model where the water-flow is assumed to be piston-like and water saturation jumps from Swi (not water-flooded) to Swf (water-flooded), at the waterfront. These constitutive models have not been developed taking into account any specific microscopic mechanism at waterfront. They can be used even if more than one mechanism is responsible for the water effect.

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