Abstract

The discrete fracture network (DFN) model simulation, in which the fracture network can have a natural heterogeneity, is one of the most effective approaches in fluid flow analyses for a fractured reservoir. In the DFN model simulation, the fracture is modeled by a pair of parallel smooth plates although real fractures have rough surfaces. However, numerous field and laboratory observations have suggested that fluid flow through a fracture occurred in specific preferential flow paths (channeling flow) due to a heterogeneous aperture distribution formed by the rough surfaces. The conventional DFN model simulation therefore gives us a serious concern about the reality. To address this concern, we have developed a new concept DFN model simulator, GeoFlow, in which the fracture can have the heterogeneous aperture distribution. Three dimensional fluid flow simulation was performed for a simple fracture network by both the conventional and the new concept DFN models. In the conventional DFN model simulation, the fracture had no aperture distribution, and fluid flow in the fracture plane was quite uniform. On the other hand, the GeoFlow simulation showed formation of three dimensional preferential flow paths in the fracture network. In addition, another GeoFlow simulation showed that productivities of the wells highly depended on their locations even when the wells intersected the same fracture. The productivities were considerably smaller when the wells intersected the regions with smaller aperture conductivities, where the preferential flow paths were difficult to form at the natural condition (no well condition). The results demonstrated occurrence of three dimensional channeling flow in fractured reservoirs, which should be addressed for effective developments of the reservoirs.

Introduction

Fluid flows through rock fractures in the Earth's crust have been a subject of interest for some time because rock fractures usually have much greater permeability than the rock matrix. Rock fractures are therefore recognized as the predominant pathways of resources and hazardous materials such as groundwater, oil/gas, geothermal fluids, and the high-level nuclear wastes. The fluid flow properties of rock fractures have been investigated with respect to the geological disposal of the high-level nuclear wastes. As a result, our understanding of the subsurface flow system has been greatly improved and has been applied to the development of geothermal and oil/gas fractured reservoirs. Recently, the prediction of flow and transport phenomena through rock fractures based on a precise modeling of the flow system in a fractured rock mass with natural heterogeneities has become increasingly important because recent environmental and energy problems require urgent solutions using underground space based on the safe and effective development of reservoirs.

A modeling with a natural heterogeneity of a fracture network has been established by the Discrete Fracture Network (DFN) modeling technique [1–5]. In the DFN modeling, rock fractures have been described by parallel smooth plates. However, field and laboratory studies have suggested that fluid flow through a rock fracture is far from the fluid flow through parallel smooth plates, due to channeling flow in a heterogeneous aperture distribution by rough surfaces [6–14]. When channeling flow occurs in a single fracture of granite, the area where flowing fluid exists is expected only 5–20% at confining pressures of up to 100 MPa, with various features in the preferential flow paths [14].

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