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global flow of the gas phase may occur through the fracture network, while global
flow of the aqueous (wetting) phase may involve matrix-to-matrix flow. Pruess
The DPM implies a quasi-steady approximation for interporosity flow. In some
cases the characteristic times for interporosity flow and transport may be large, and the
quasi-steady approximation becomes inaccurate. Under these circumstances it may
be necessary to resolve the gradients driving fracture-matrix exchange, which can
be accomplished by sub-gridding of matrix blocks into a series of continua defined
according to distance from the matrix block surface, the so-called MINC-concept
(Figure 12.1c).

12.2.4 Example: Diffusive Effects in the Migration of
Volatile Tracers in Fractured Media
Tracer testing has been extensively applied in fractured geothermal reservoirs as a
means for mapping fast preferential pathways and characterizing reservoir processes
that involve boiling and condensation phenomena. Diffusive propagation of tracers
tends to be slow in comparison to advective transport, so that its contribution to
transport over interwell distances is negligible. However, diffusion can be a very
important process for fracture-matrix exchange, and as such can have a strong influ-
ence on tracer breakthrough curves (BTCs). As fluids (gas or liquid) are advecting
through a fracture, tracer will diffuse into the rock matrix, causing a delay in tracer
breakthrough and reduced peak concentrations (Figure 12.2). If tracer is injected as a
slug, the concentration gradient between fractures and matrix will reverse after peak
concentrations have passed, and tracer that previously entered the rock matrix will
diffuse back out into the fractures, giving rise to long tails in the BTCs.
Pruess (2002) examined the behavior of volatile tracers during water injection into
depleted vapor zones of a geothermal reservoir. In this case, injected water is subject
to vigorous vaporization. Much of the tracer will partition into the gas (vapor) phase,
and the exchange between fractures and matrix will involve diffusion in both gas and
liquid phases. Figure 12.3 shows simulated breakthrough curves for two different
phase-partitioning tracers with and without inclusion of diffusive effects. Diffusion

Rock matrix

Fluid flow

Fracture
Tracer diffusion

Figure 12.2. Schematic of tracer behavior in a fractured-porous medium