Water/Chalk (or Collapsible Soil) Interaction: Part II. Results of Tests Performed In Laboratory On Lixhe Chalk to Calibrate Water/Chalk Models

Abstract Ekofisk, Valhall and other North Sea reservoirs are made of water sensitive rocks such as pure high porosity chalk. Production of these fields implying water-flooding leads in several cases to compaction and subsidence. 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 depletion in the compressible reservoir rocks but also to include additional deformations induced by water saturation (WS) increase. These additional deformations may eventually be determined by performing water-injection (WI) tests in laboratory. This paper presents additional tests that have been performed in laboratory in order to understand the mechanical behavior of chalk at the water-front. It shows the existence of a degree of WS above which chalks mechanically behaves as if it is fully saturated, while chalk with a lower WS looks stronger than if the degree is higher. Greater the WS is, smaller the consolidation pressure Pc0. Moreover, the deformations induced by water-flooding happen in two steps: – the first one happens as soon as chalk get into contact with water and is instantaneous. – the second is delayed and looks more like creep as it happens after chalk gets in contact with water. Observations and tests results are interpreted thanks to Piau's model. P. 505