Mécanismes physiques de l'évolution de la perméabilité d'un grès sous chargements simulant la déplétion d'un gisement

Abstract

During the early years of a reservoir life, production is done by depletion (decrease of pore pressure) which results in an increase of the effective stresses in the reservoir. In situ measurements show that the stress increase seems to follow two loading pathways, oedometric or proportional, depending on parameters such as the rock petrophysical characteristics, the reservoir shape, boundary conditions, etc. All these changes induce variations of petrophysical characteristics of in situ rocks and particularly permeability variations, which depend on pore geometry, mineral composition of the rock and the loading type conditions. The modelling of permeability evolution during the primary production is then a complex problem which can be translated into the following question: Should we link permeability variations to the change of stresses or to strains? At first, a strain-permeability relationship seems to be more logical because permeability is a geometrical value. However, the analysis of the physical phenomenon (either mechanical or hydraulical) induced by the increase of the effective stress shows that a similar approach cannot be applied systematically to all rocks. That is what we want to illustrate, by an experimental work, on Vosges sandstone of good petrophysical characteristics (average porosity Φavr = 20% and average permeability kavr ≈ 500 mD) where simultaneous measurements of strains and monophasic permeabilities were conducted.