Tracer-Test Simulation on Discrete-Fracture- Network Models for Characterizing Fractured Reservoirs

Abstract

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 94344, “Tracer-Test Simulation on Discrete-Fracture-Network Models for the Characterization of Fractured Reservoirs,” by A. Lange, J. Bouzian, and B. Bourbiaux, Inst. Français du Pétrole, prepared for the 2005 SPE Europec/EAGE Annual Conference, Madrid, Spain, 13–16 June. Fractures affect flow dynamics within a reservoir, causing uncertainty in field-productivity and -reserves assessment. Geologically realistic models of the fault/fracture network can be constructed, and these simplified models can be used as field-scale simulations of multiphase production. A 3D discrete-fracture-network (DFN) simulator enables the critical intermediate step of validating the geometry of the geological fault/fracture network and characterizing it in terms of flow properties. An extension of this simulator to compute tracer tests is detailed in the full-length paper. Compared with pressure tests, tracer tests may provide additional information about the reservoir heterogeneity because of the convective nature of these tests. Introduction The increasing number of mature fields in which fractures caused unexpected production features gave rise to extensive efforts in better characterizing and integrating fracture properties into field-scale simulation models for production assessment and optimization. A workflow for integrating the multiscale fracture-flow properties in reservoir simulators was proposed earlier. This workflow consists of the following. - Geologically realistic models of the fault and fracture network are constructed from seismic, well, and outcrop data. - Models are validated from dynamic field information such as well tests, interference tests, and production logs. - An equivalent simulation model applicable at reservoir scale is constructed with innovative flow scale-up procedures. Multiphase field production is simulated at reservoir scale with this equivalent-model. Using this workflow, field-simulation models remain interpretable in geological terms and enable explanation of unexpected production features, such as early water breakthroughs. However, validation of the geological fault/fracture-network model in terms of geometry and flow properties remains a critical step. Fracture conductivities must be characterized, and uncertainties on some geometrical parameters may have a large effect on the flow properties of the fractured medium. For instance, small variations in the fracture-length distribution may lead to large differences in the fracture-network connectivity, thus affecting the flow properties of the medium. A 3D DFN flow simulator was developed to fulfill this validation and characterization step. This tool enables simulating fluid flow on DFN realizations obtained from the geological model. Well tests, interference tests, and production logs then can be simulated on geologically realistic fracture networks and calibrated from available field data for validating the geometry of the geological model and characterizing the fracture conductivities. The DFN flow simulator with a dual-permeability option can be used with any type of fractured reservoir, whatever the scale, density, and connectivity of fractures. Model This simulator uses the dual-medium concept applied to an explicit representation of the geological model of the fault/fracture network and matrix medium. The transport and dispersion of nonreactive tracers are modeled by introducing a convection/dispersion equation into the existing dual-porosity flow model. A hybrid scheme discretizes the convective term implicitly within densely fractured regions for stability purposes and explicitly by use of a double-upstream scheme everywhere else for reducing spurious numerical dispersion.