Abstract
Fluid flow modeling of naturally fractured reservoirs remains a challenge because of the complex nature of fracture systems controlled by various chemical and physical phenomena. A discrete fracture network (DFN) model represents an approach to capturing the relationship of fractures in a fracture system. Topology represents the connectivity aspect of the fracture planes, which have a fundamental role in flow simulation in geomaterials involving fractures and the rock matrix. Therefore, one of the most-used methods to treat fractured reservoirs is the double porosity-double permeability model. This approach requires the shape factor calculation, a key parameter used to determine the effects of coupled fracture-matrix fluid flow on the mass transfer between different domains. This paper presents a numerical investigation that aimed to evaluate the impact of fracture topology on the shape factor and equivalent permeability through hydraulic connectivity (f). This study was based on numerical simulations of flow performed in discrete fracture network (DFN) models embedded in finite element meshes (FEM). Modeled cases represent four hypothetical examples of fractured media and three real scenarios extracted from a Brazilian pre-salt carbonate reservoir model. We have compared the results of the numerical simulations with data obtained using Oda’s analytical model and Oda’s correction approach, considering the hydraulic connectivity f. The simulations showed that the equivalent permeability and the shape factor are strongly influenced by the hydraulic connectivity (f) in synthetic scenarios for X and Y-node topological patterns, which showed the higher value for f (0.81) and more expressive values for upscaled permeability (kx-node = 0.1151 and ky-node = 0.1153) and shape factor (25.6 and 14.5), respectively. We have shown that the analytical methods are not efficient for estimating the equivalent permeability of the fractured medium, including when these methods were corrected using topological aspects.
Highlights
The scale of the fracture systems is an important aspect to be considered in the fluid flow analysis of naturally fractured reservoirs because it affects the connectivity of the fracture network, as discussed by Nelson [12], Laubach [13], Berkowitz et al [14], Roy et al [15], Laubach et al [16], Sahu and Roy [17], and Silva et al [18], among others
The main objective of this work is to investigate the impact of fracture systems geometry, based on analysis of topology, in the fluid flow in naturally fractured media
Our results showed the impact created by the arrangements of fracture nodes on the fluid transfer term between the rock matrix and the fracture network, which is directly related to the capacity of the fluid flow in the system, the shape factor, and the equivalent permeability
Summary
Fractured reservoirs (NFRs) are composed of various lithologies such as shales, sandstones, carbonates, and igneous rocks and correspond to over 30% of the global production [1]. Within this group, fractured carbonate reservoirs represent an important part of the world’s oil and gas reserves (e.g., the Campos and Santos Basins in Brazil and the Kwanza Basin in Angola) [2,3]. Characterization and numerical simulation of naturally fractured reservoirs represent a challenge because of their complex evolution (diagenesis, geomechanics) and the effect of the coupled fracture-matrix relationship on the fluid flow [4,5,6,7,8]. Large fractures represent a crucial aspect in fractured systems because it is often necessary to model them explicitly due to their singular effect in numerical models [21]
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