Fracture potential of evaporite seals during CO2 injection operations a numerical modelling & field study approach

Abstract

This project, focuses on the potential for evaporite cap-rocks to undergo fluid induced failure as a result of CO2 injection in underlying carbonate reservoirs. The specific rock properties that control fracture propagation through layered evaporites and the features that might influence fluid flow in these low permeability units were studied using a multidisciplinary approach. Field work was carried out on a local scale studying a gypsum mine, drill cores and several outcrops in order to determine the internal structures within seals that might influence fracture formation. On a regional scale studies were carried out to determine the stress history of the seal in Qatar and the likely fractures it might contain as a result of burial, diagenesis and tectonism. A detailed study of the theory of brittle failure (in particular of fluid induced failure) was also conducted in order to lay down the principles that control stress distribution in the crust. This information allowed to define the boundary conditions for the Finite Discrete Element Method models used to look at fracture propagation in layered materials. It was concluded that evaporite sequences are vertically and horizontally heterogeneous with both primary and secondary layers and interfaces, some of which are linked to internal hydraulic fracturing of the rock. The field studies together with the numerical models show that low friction along an interface contributes to fracture arrest; that the thickness of an incompetent layer is not a decisive factor in determining whether or not a fracture propagates through that weak layer and that a weak layer (low Young’s Modulus) is able to impede a fracture that formed in a strong layer from propagating into it. It is also suggested that trough-going fractures are not always formed by the propagation of a fracture through successive adjacent layers but that they occur by the linking of individual extensional fractures to form a through-going fracture.