--> Fracture Potential of Evaporite Seals

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Fracture Potential of Evaporite Seals


Cap-rocks or seals are fine grained, high capillary-entry pressure units that serve as aquitards preventing the upward migration of fluids (Gluyas & Swarbrick, 2004)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, the chance preservation of transient, fluid-induced fractures in low permeability rock types such as the Mercia mudstone by the injection of sandstone dykes or by the precipitation of gypsum, demonstrates the tendency of these potential cap rocks to undergo brittle failure in response to high fluid pressures (Cosgrove, 2001)When considering potential CO2 storage sites, one major concern is the sealing efficiency of the cap-rock overlying the reservoir into which 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 induced by an excess in pressure resulting from the injection of CO2 and the related buoyancy forces is considered to be the main risk scenario.

Studies show that the two key parameters influencing 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, UK) showed that the layered anhydrite contains similar features to those observed in the Mercia Mudstone (i.e. bedding–parallel gypsum veins) confirming the existence of past high fluid pressures. It's clear that within evaporite-rich cap-rocks these are common secondary features and it is important to determine what impact they may have on the propagation of fractures through such cap rock complexes. In this project we investigate the loss of cap-rock integrity through fluid induced failure 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, UK) and numerical modelling techniques is used to determine the key parameters that control the development and propagation of fractures in layered successions.

We have used the combined finite-discrete element method (FEMDEM) where, continuum behaviour is modelled by finite elements and discontinuum behaviour is modelled by discrete elements. The transition from continuum to discontinuum is modelled through fracturing and fragmentation processes using a combined single and smeared crack model where the behaviour of the material up to the ultimate tensile strength is modelled through finite elements and a discrete crack model is implemented on the post peak (strain softening) part (Munjiza A., 2004).

We are using a linear–elastic, three-layered model undergoing plane strain with an implemented Coulomb friction law to govern the deformation of the layer interfaces and the sliding between opposite sides of fractures, and where parameters such as the mechanical properties of the layers (E, T, C, υ, ρ), the coefficient of sliding friction (μ) and normal stress (s) on the layer boundaries can be defined by the user. We are currently examining the influence of the variation of the normal stress, the Young's modulus variation, and the coefficient of sliding friction on the propagation of a fracture through an interface.

Preliminary results demonstrate that the increase in confining pressure (vertical stress normal to the layer boundaries) promotes the formation of through going fractures by increasing the coupling of the layers and allowing the horizontal stress to propagate in an homogenous way. Lower values of confining pressure are responsible for strata bound fractures. Experiments with the variation of the Young's modulus have confirmed the theoretical predictions with stiffer layers (higher modulus) generating fractures at an earlier stage in the simulation when compared with lower modulus layers subjected to the same extensional stress.