Tracer-Test Simulation on Discrete Fracture Network Models for the Characterization of Fractured Reservoirs

Abstract

Abstract The presence of fractures at various scales affects the flow dynamics within a reservoir, thus casting uncertainty on the assessment of the field productivity and reserves. Methodologies and tools are now available (i) to reconstruct geologically-realistic models of the fault/fracture network (ii) to turn these models into simplified conceptual models usable for field-scale simulations of multi-phase production methods. However, a critical intermediate step remains that of validating the geometry of the geological fault/fracture network and characterizing it in terms of flow properties. An original 3D Discrete Fracture Network simulator enabling one to fulfil this characterization step has already been presented by the authors'. This simulator can compute the single-phase flow response of any type of fractured reservoir, whatever the scale, density and connectivity of fractures. An extension of this simulator to compute tracer tests is presented in this paper. Compared with pressure tests, tracer tests may provide additional information about the reservoir heterogeneity due to the convective nature of these tests. This simulator is based on the dual-medium concept applied to an explicit representation of the geological model of fault/fracture network and matrix medium. The transport and dispersion of non-reactive tracers are modelled by introducing a convection-dispersion equation into the existing dualporosity flow model. A hybrid scheme discretizes the convective term implicitly within densely fractured regions for stability purposes, and explicitly via a double-upstream scheme everywhere else for reducing spurious numerical dispersion. After demonstrating the reliability of this tracer model on validation cases, a realistic case that combines diffuse fracturing and major objects such as seismic and sub-seismic faults is studied. Tracer tests are simulated for wells in a five-spot injectionproduction pattern. Effects of a fracture/matrix tracer exchange as well as a Taylor-Aris dispersion on breakthrough curves are analyzed. The potentiality of tracer responses for providing information on the distribution of reservoir heterogeneities is discussed.