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Fracture Potential of Evaporite Seals

Abstract

Cap-rocks or seals are fine grained, low capillary-entry pressure units that serve as aquitards and prevent the upward migration of fluids (Gluyas and Swarbrick, 2003). Evaporite rocks such as anhydrite are generally seen as efficient cap-rocks, as a result of their low permeability and because they are thought to deform in a ductile manner. However, examples from mudrock such as the Mercia Mudstone, where the preservation within evaporite-rich horizons of generally transient fracture networks by the injection of sandstone dykes or the precipitation of satin spar veins, demonstrate that these networks of fluid induced fractures form in low permeability rocks (Cosgrove, 2001) allowing the movement of fluids both out of and through these potential seals. When considering potential CO2 storage sites, one major concern is the sealing efficiency of the cap-rock overlying the reservoir where the gas is to be injected. In such sequestration projects, the long-term integrity of these rocks is essential if the CO2 is to be prevented from escaping to the atmosphere. Failure by fracturing can be caused by an excess in pressure resulting from buoyancy forces and injection-related overpressure and is considered to be the main risk scenario. Studies show that two parameters that influence the propagation of fractures across an interface are the difference in Young's modulus of the two materials and the shear strength of the interface separating them (primary properties). Field work at Brightling Mine (East Sussex, U.K.) confirms that the layered anhydrite contains similar features to those observed in the Mercia Mudstone (i.e. bedding–parallel gypsum veins). It's clear that within evaporite-rich cap-rocks these are common secondary features and we must also consider their impact on the fracture propagation/arrest process. In this paper we investigate the loss of cap-rock integrity through fluid induced failure (hydrofractures) by studying what specific rock properties control fracture propagation through a layered media and what influences fluid flow in these low permeability units. A combination of field work at Brightling (East Sussex, U.K.) mine mmechanical tests conducted on rock samples collected at the field to determine their mechanical parameters (E,υ, UTS) and numerical modelling techniques (FEMEDEM code) is used to determine the key parameters that control the development of fractures in layered successions.