Sorbing Tracers - A Potential Tool for Determining Effective Heat Transfer Area in Hot Fractured Rock Systems

Abstract

A key parameter controlling the performance and lifetime of a Hot Fractured Rock (HFR) reservoir is the effective heat transfer area between the fracture network and the matrix rock. In a vapor-dominated system, this area can be estimated by conducting a tracer test of non-adsorptive chemical. Such estimate is usually based on the unique signature of a long tail on a typical tracer breakthrough curve (BTC), caused by the diffusive exchange of tracers between fractures and matrix rocks. The tailing strength increases systematically with the fracture-matrix interface area. In a water-dominated system, however, the diffusion effect is too small to generate a meaningful tail on the BTC of a non-sorbing tracer test.

Recent numerical studies have shown that a sorbing tracer test will generate a tailed BTC in a single-phase liquid fracture-matrix system. In this paper we develop an analytical solution to theoretically confirm such a useful phenomenon. In deriving the solution, we used a boundary condition of a finite-length tracer slug, and neglected the diffusion along fractures. The solution shows that in a water saturated fractured rock system, increase of the retardation factor (that is practically manageable) should have the same effect as that of increase of the diffusion coefficient (that is practically restricted).

The strong enhancement in the BTC tails of a sorbing tracer test provides adequate sensitivity for determining the heat transfer area. The solution is useful for understanding transport mechanisms, verifying numerical codes, and for identifying chemicals with appropriate sorption properties as tracers for the characterization of a fractured reservoir.