Tracer Flow in Naturally Fractured Reservoirs

Abstract

This paper presents solutions for the continuous, finite step and spike injection of radioactive tracers in naturally fractured reservoirs. Solutions are presented for linear flow-vertical fractures, and for the radial flow cases of horizontal fractures and cubic block matrix-fracture geometry. The three derived solutions consider as particular cases the flow of a chemical tracer. The reservoir is treated as being composed of two regions: a mobile (fractures) where dispersion and convection take place and a. stagnant (matrix) where only diffusion and adsorption are allowed. Radioactive decay is considered in both regions. The solution that considers vertical fractures is analytical, thus avoiding the double Laplace space numerical inversion used in previous studies. Another main advantage is that the important numerical dispersion reported by previous investigators when using the Stehfest Laplace transform inversion algorithm is avoided. The radial coupled matrix to fracture solutions are presented in Laplace space, and are accurately inverted by means of the Crump algorithm. The influence of the main dimensionless parameters that enter into the solutions was carefully investigated. A comparison of results for the three different naturally fractured systems investigated, indicates that a uniqueness problem may arise in the interpretation of a test, especially to distinguish between the radial cases. As expected, this problem alleviates for the finite step injection cases. The results of this study can be applied to interpret tracer tests in naturally fractured reservoirs, allowing the estimation of fracture and matrix practical parameters of interest.

Practical methodology for interwell tracer applications

“…The qualitative interpretation is made by observing the response curves (tracer concentration recovered against time) looking for qualitative properties of the pattern, such as the existence of high-permeability channels, barriers and fractures between wells; communication between different layers, stratification in the reservoir; and preferential flow directions [24][49]- [51].…”