Characterizing Dynamic Stress Sensitive Fracture Apertures in A DFN Representation: An Example From the Island of Pag (Croatia)

Abstract

Improving the understanding of fluid flow through fracture networks is crucial to the optimum recovery of hydrocarbons in naturally fractured reservoirs. In explicit dual continuum representations of matrix and fractures, it is not just the intensity of fracturing and the topological relationships in the network but also connectivity of hydraulically open fractures that determines effective permeability of the system. Fractured folds often form prolific reservoirs owing to the structural closure they afford and the additional porosity and permeability due to the fold related fracturing. The fracture patterns are of specific interest owing to the complex geometries associated with folding. In this work we utilize a combined outcrop based and numerical approach to characterize fracture patterns, fracture apertures and fluid flow sensitivities using a folded ‘box-type’ anticlinal structure example from the Pag Island, Croatia. The Pag Island is part of the main Adriatic-Dinaridic Carbonate Platform in the External Dinarides region that persisted from the Triassic to the Eocene with Paleocene-Miocene Dinaridic deformation phases. The current day geology consists of up to 1 km thick Cenomanian - Senonian shallow water rudists bearing carbonates and about 650m thick Eocene-Oligocene Nummulitic carbonate successions. We present a 3D structural model of the Pag Island with a Discrete Fracture Network (DFN) digitized representation built from drone photogrammetry. The multiscale fracture patterns in this multilayer folded reservoir analog are classified on the basis of folding stages (pre-, syn-, post folding). FEM geomechanical modeling is used to quantify stress-sensitive heterogeneous fracture apertures with the Barton-Bandis empirical model. Permeability sensitivities of the fracture network are tested using coupled single phase fluid flow and poroelasticity simulations. The results show that hydraulic apertures are extremely sensitive to fracture orientation with respect to the shortening direction and reservoir pressure transients. Our novel approach for coupled flow & geomechanics simulations yields effective permeability tensors that are functions of both matrix and fracture conductivities and also fracture aperture variations in time. We test dynamic closure & dilation of fractures in depletion and injection scenarios and our results stress the importance of dynamic fracture apertures in structurally complex fractured reservoirs.