A Refined Approach to Estimating Effective Flow Porosity from Cross-Hole Tracer Tests in Fractured Media

摘要

Simulations of flow and transport in two-dimensional representations of heterogeneous fractured media are used torninvestigate the errors and biases associated with effective flow porosity estimates derived from cross-hole tracer tests. Arnmethod is presented for constructing probability distributions of “correction factors” that can be used to correct apparent flowrnporosities obtained from tracer tests to obtain “true” flow porosities in fracture systems. Although only a limited number ofrnthe many possible variations in fracture flow system properties is investigated, it is concluded that effective flow porositiesrnderived from cross-hole tracer tests have a strong tendency to overpredict true flow porosities in fracture flow systems. Thisrntendency toward overprediction decreases as the fracture conductivity relative to the background conductivity field decreasesrnand as the orientation of the most conductive fractures becomes better aligned with the two wells. Tracer tests with smallrnamounts of recirculation of water from the production well to the injection well are predicted to result in much betterrnestimates of true flow porosity (on average), and with much less variability in the estimates, than tests with no recirculation.rnHowever, the advantage offered by recirculation decreases as the fracture conductivity relative to the backgroundrnconductivity decreases.