--> --> Lessons for Subsurface Fractured Reservoir Studies from an Outcrop-based 3-D Model of Fluid Flow in a Fractured Carbonate Fold

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Lessons for Subsurface Fractured Reservoir Studies from an Outcrop-based 3-D Model of Fluid Flow in a Fractured Carbonate Fold

Abstract

The characterization of heterogeneous fracture patterns in complex reservoirs, such as folds, is difficult, as multiple stages of fracturing and fracture reactivation take place pre-, syn- and post-folding. Outcrop studies help understanding the behavior of fractures and subsequent fluid flow in folds, as outcrops provide both small-scale fracture patterns and the large-scale fold geometry. We aim at characterizing these patterns in terms of stresses, deriving from outcrop fracture data the stress conditions under which the fractures were formed, and linking this to large-scale development of the fold. From these relations, we construct 3D fracture networks, conditioned to outcrop data, which serve as the basis for upscaling and fluid flow simulations. Through these simulations, we quantify the relative fluid flow trends in different domains of a fold. We map the large-scale geometry of and fracture patterns in two formations, one Eocene and the other Upper Cretaceous, in different domains of a reservoir-scale outcropping fold in central Tunisia. Both formations are limestones with comparable bed thicknesses, but the Eocene formation has cm-thick shale layers in between the beds, while the Upper Cretaceous formation has no shale. During subsidence, regional fracturing resulted in a system of two bed-perpendicular conjugate orientation families. The bisection of the small angle of the conjugates corresponds to the tectonic stress direction. The angle between the two conjugate families is 40°, and the fractures have an opening and shear component, indicating that they are hybrids between Mode I and Mode II. During folding, the lack of shale in the Upper Cretaceous layer resulted in pure flexural slip, creating oblique-to-bedding stylolites and veins. In the Eocene layer, the presence of shale causes the maximum principle stress to remain parallel to the layers, creating bed-perpendicular fractures. These fractures are organized in two conjugate families which are perpendicular to the pre-folding conjugate system. Their spatial arrangement is determined by the pre-folding fractures, which created local weaknesses and stress heterogeneities in the rock. The small-scale stresses are correlated to large-scale stresses, using mechanical modelling, to construct a fold-scale fracture frequency model. The resulting model is upscaled to an equivalent fracture permeability model for fluid flow simulations, to analyze the behavior of flow through fractures in folds.