Shape Factor for Dual-permeability Reservoir Simulation - Effect of Non-uniform Flow in Fracture Network

Abstract

The flow properties of naturally fractured reservoirs are dominated by flow through the fractures. In a previous study we showed that even a well-connected fracture network behaves as one near the percolation threshold in some cases: i.e., most fractures can be eliminated but still form a percolating sub-network with virtually the same permeability as the original fracture network. In this study, we focus on the influence of eliminating unimportant fractures on the inferred characteristic matrix-block size. We model a two-dimensional fractured reservoir in which the fractures are well-connected. The fractures obey a power-law length distribution, as observed in natural fracture networks. For the aperture distribution, because information from the subsurface is limited, we test a number of cases: narrow and broad log-normal and power-law distributions and one where aperture correlates with fracture length. The matrix blocks in fractured reservoirs are of varying sizes and shapes; we calculate the characteristic matrix-block size from the ratio of matrix-block area to its perimeter. We test different criteria to determine the critical sub-network, such as aperture, flow simulation results, etc. We show how the characteristic matrix-block size increases from the original fracture network to the critical sub-network. An implication of this work is that the matrix-block size, or shape factor, used in dual-porosity or dual-permeability waterflood or EOR simulations should be based not on the entire fracture population but on the sub-network that carries almost of all the flow.