An analytical solution for slug tracer tests in fractured reservoirs

Abstract

The transport of chemicals or heat in fractured reservoirs is strongly affected by the fracture‐matrix interfacial area. Under unsaturated conditions, such as in vapor‐dominated geothermal reservoirs, this area can be estimated from inert gas tracer tests that produce a characteristic tail in tracer breakthrough curves (BTCs). For water‐saturated conditions, molecular diffusion is orders of magnitude smaller, and tails in BTCs from diffusive exchange between fractures and matrix are too weak to be practically useful as a characterization tool for fracture spacings of practical interest. However, recent studies suggest that reversibly sorbing solute tracers can generate strong tails in BTCs that may allow a determination of the fracture‐matrix interfacial area. To theoretically explore such a useful phenomenon, this paper develops an analytical solution for BTCs in slug tracer tests in a water‐saturated fractured reservoir, idealized here as a set of uniformly spaced plane, parallel, identical fractures. The solution shows that increased sorption should have the same effect on BTCs as an increase of the diffusion coefficient. The solution is useful for understanding transport mechanisms, verifying numerical codes, and identifying appropriate chemicals as tracers for the characterization of fractured reservoirs.