Abstract
This paper is the first part of a general work on the mechanical behaviour of a porous chalk and the effect of saturating fluid. It presents an experimental investigation of the behaviour of a porous chalk by taking into account the effects of saturating fluid. Two representative fluids are concerned: a wetting fluid (water) and a non-wetting fluid (oil). The laboratory-testing program includes two topics. At first, conventional hydrostatic and triaxial compression tests are carried out on samples, which are respectively saturated with water and oil. The sensitivity of chalk behaviour to saturating fluid is clearly shown. Secondly, specific water injection tests are conducted in which water is injected in chalk samples initially saturated by oil, under different hydrostatic and deviatoric stress conditions. A water induced plastic deformation is observed.
Highlights
Compaction and induced subsidence problems have been experienced in oil fields like Ekofisk, Valhall
The purpose of this paper is to provide a comprehensive database to characterise the basic behaviour of porous chalk and to quantify the additional plastic deformation due to water flooding under deviatoric stresses
The induced volumetric strain is largely compressive; – in the tests under deviatoric stress state, the water injection induced axial strain is much higher than the radial one, and this difference increases when the stress deviator is higher. This confirms the role of the deviatoric stress in enhancement of chalk pore collapse; – in the tests where water induced strains are measured at several levels, one can note that the magnitude of induced strain measured by the gauges near the sample top is higher than that measured by the gauges near the sample bottom
Summary
Compaction and induced subsidence problems have been experienced in oil fields like Ekofisk, Valhall. These North Sea reservoirs are made of water sensitive rock such as pure high-porosity chalk (Andersen, 1995). The purpose of this paper is to provide a comprehensive database to characterise the basic behaviour of porous chalk and to quantify the additional plastic deformation due to water flooding under deviatoric stresses. This will be used to define an accurate model in a following paper
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