SORBING TRACERS- A POTENTIAL TOOL FOR DETERMINING EFFECTIVE HEAT TRANSFER AREA IN HOT FRACTURED ROCK SYSTEMS

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, based on the unique signature of a long tail in a typical tracer breakthrough curve (BTC) due to diffusion of tracers between fractures and matrix rocks. The tailing strength increases systematically with the fracture-matrix interface area. In a water-dominated system, however, aqueous phase diffusivities are too small to generate a meaningful tail in the BTC of an inert solute tracer. Recent numerical studies have shown that reversible tracer sorption on matrix rocks will enhance tailing in BTCs for aqueous solutes in fracture-matrix systems. In this paper we develop an analytical solution to theoretically explore 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 on BTCs as increase of the diffusion coefficient (that is practically restricted). The strong enhancement in the BTC tails of sorbing tracers 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