Fracture Permeability in Seals and Reservoirs - New Insights on Joint Structure and Formation Mechanisms from Laboratory

Abstract

Joints are pervasive fractures strongly affecting the reservoir permeability and seals integrity. They are currently interpreted as mode I cracks forming normal to the minimal tensile effective stress S3. This view is challenged by the results of both axisymmetric and polyaxial (true 3-D) extension experiments carried on rock analogue materials where joint like S3-normal fracture sets were produced. They are of two types defined by the effective mean stress S. When S is very small, the fractures are mode I cracks with smooth fracture surfaces. At higher S, these surfaces have plumose morphology. In the latter case, both SEM observations and mechanical measurements show that joints are initiated as dilatancy deformation localization bands. They form under slightly tensile or even compressive normal stress and therefore cannot be mode I fractures. The similarity between the plumose fractography and the internal structure of experimental and natural joints observed in dolomicrite strongly suggests a similarity in the formation mechanism. It is therefore proposed that most of natural joints could form as dilatancy (dilation) bands at higher pressure (depth) than expected for the mode I mechanism. These “dilatancy joints” ca be seen as constitutive instabilities and can be modelled numerically.