The effect of fracture networks on the vertical permeability of a tight carbonate reservoir analogue: laminites from the Crato Formation, NE Brazil

Abstract

The structural characterization of fractures is crucial to understand the processes of fluid flow in tight reservoirs. This contribution focuses on the role played by vertical fractures on the permo-porosity properties of the Aptian tight carbonate sequence of the Crato Formation, Araripe Basin (NE Brazil). The study performed a structural analysis in two different scales: reservoir scale (approximately 19.000 m2) and outcrop scale (approximately 125 m2), focusing on the fracture networks in lacustrine laminates, which have been investigated as an analogue of carbonate facies observed within the pre-salt reservoir sequence of the marginal basins in Brazil. This study employed a combination of systematic outcrop-based fracture characterization involving digital outcrop models and mechanical stratigraphy analysis. To evaluate the influence of vertical fracture systems in the fracture porosity and the equivalent permeability we performed DFN (Discrete Fracture Network) models. We focused on vertical calcite-filled fractures, oriented in two principal directions: set 1 NNW-SSE and set 2 NE-SW. Each set shows different vertical linkage patterns due to the influence of the mechanical intervals. Results of the outcrop scale model show similar behaviour for both fracture porosity and equivalent permeability, indicating that storage capacity and fluid flow may not be affected by vertical linkage of fractures. The reservoir scale DFN models show that the fracture porosity was greater in models which consider through-going continuous fractures than in models that consider the segmentation (discontinuity) of vertical fractures. Considering the vertical connectivity of fractures, the equivalent horizontal permeability (Kxx and Kyy) showed similar values in both scale models. This implies that vertical segmentation of fractures does not impact fluid flow in horizontal directions. The calculated values of equivalent vertical fracture permeability (Kzz) at reservoir scale are one order of magnitude higher in the DFNs that consider continuous fractures. Our results suggested that vertically continuous fractures enhance preferential flow pathways allowing greater vertical fluid flow than segmented fracture networks in tight carbonate reservoirs.