A Connectivity-based Hierarchical Model for Naturally Fractured Reservoir Simulation

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2018-12

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Abstract

Naturally fractured reservoirs usually contain multi-scale fractures with a wide range of fracture lengths and various topologies. Modeling of such fractured systems has traditionally followed a multi-scale approach, in which the selection of the numerical models is purely based on the fracture length. However, fractures with different length scales and connectivity degrees usually intercross each other. Thus, the simple clustering and modeling of fractures by geometrical size will fail to address the physical communication among them, resulting in significant errors when modeling the flow dynamics. In order to accurately characterize multi-scale fractures without dramatically increasing the number of simulation grids and time costs, a connectivity-based hierarchical model is proposed that integrates different numerical models according to the geometrical size and connectivities of different types of fractures. We employed the discrete fracture model (DFM) for large-scale fractures, which usually dominate the flow pattern, the dual porosity/dual permeability (DP) model for fractures that form strongly connected networks, and the effective matrix medium (EMM) model for smaller fractures with weak connectivities. Large-scale fractures, which are usually characterized by seismic interpretations, are referred to as Type I fractures. They are used as geometrical constraints for unstructured grid construction; thus, the geometrical and petrophysical properties of each large-scale fracture are explicitly represented. The remaining smaller fractures are classified as Type II and Type III fractures according to their level of connectivity to neighboring fractures. We propose a flow-based methodology to quantify the fracture communication volume and units influenced by fracture connectivities. The resulting Type II fractures with relatively strong communications will form a DP system, and Type III fractures with weak communications will contribute to an equivalent matrix system with enhanced porosity and permeability. The proposed approach is applied to a conceptual model and a highly fractured carbonate reservoir, respectively. In the conceptual model case, the simulation results of the proposed hierarchical model are compared with different models, including full DFM (as the reference solution), full DP model, and hybrid models. The comparisons demonstrate that the proposed hierarchical model provides highly accurate results while it can achieve a speedup factor of 5–20. In contrast, the full DP model results in significantly larger error due to their failure to accurately model large-scale fractures. Moreover, the traditional hybrid models lead to large simulation error since they fail to distinguish the connectivity level among fractures. In the field study on an actual offshore reservoir with four production wells and strong water drive, water cuts were observed to increase rapidly within a short period of time. The proposed model is capable of capturing such drastic flow dynamics accurately and efficiently, while the classic DP model, which is prevalent in current filed applications, yields significant errors compared to the well history.

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Hierarchical model, Connectivity, Multi-scale fracture system, Reservoir simulation

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