Well test-derived permeability in naturally fractured reservoirs with heterogeneous and disconnected fractures

Abstract

The estimation of permeabilities is a key objective of a well test. In Naturally Fractured Reservoirs (NFRs), this is particularly important because the permeability derived from pressure transient analysis can yield quantitatve information about the characteristics of both, the fractured and matrix system. In this study, we present new insights that enable us to understand the nature of the permeability values obtained from well testing in NFRs. We considered cases that are little explored in the literature like the impact of varying apertures along the fracture planes or disconnected fractures on the effective permeability of the fracture-matrix system. We showed that if only 5% of the fracture connections are closed, the fracture network no longer percolates, that is the fractures begin to act as isolated features and the effective total permeability of the fracture-matrix system is reduced significantly. We propose a way forward for distinguishing between connected and unconnected fracture networks, which uses the ratio of well test-derived permeability and the average core permeability. We suggest that if this ratio is larger than 10, the fracture network is likely to be connected while values below 10 likely correspond to either disconnected, poorly connected or very low-conductivity fracture networks.