Hydraulically equivalent fractures may show striking differences when a
gas-migration experiment is performed because of the different
correlations between transmissivity, pore volume and entry pressure. We
numerically simulate gas migration between injection and extraction
boreholes in a parallel plate fracture with a heterogeneous fault
gouge, in a rough-walled fracture filled with homogeneous material, and
in a rough-walled empty fracture. The parallel plate model and the
empty model clearly show the existence of preferential paths; for high
variance of the transmissivity field, gas flow takes place only in few
discrete channels separated by water-saturated regions. In contrast, in
the fracture filled with homogeneous fault gouge, the gas saturation is
continuous and more uniformly distributed. It appears a fundamental
issue to be able to discriminate in situ among conceptual models that
can yield such a different gas-saturation distribution. As in practice,
the saturation distribution cannot be directly observed, tracer
experiments are performed to characterize a fracture. For these
reasons, we simulate the transport of tracers, which are added to the
Gas phase as soon as quasi-steady saturation distribution and
extraction rate are achieved, and we compare the breakthrough Curves
obtained assuming different models. Our numerical simulations suggest
that discrimination among the models on the basis of single-tracer
tests is unlikely. A better tool to investigate fracture properties is
provided by a gas-tracer test, in which a cocktail of gases with
different water Solubility is employed. These gases behave as
partitioning tracers and allow us to estimate the gas saturation in the
fracture. Indeed, by comparison of the residence-time distributions of
different gases, we are able to compute a streamline effective
saturation, which is an excellent estimate of fracture saturation. In
addition, the streamline effective saturation curve contains
information that is useful to identify the conceptual model that more
likely applies to the fracture. (C) 2004 Elsevier B.V. All rights
reserved.