A GEOLOGICALLY-BASED APPROACH FOR DETERMINING DISTRIBUTIONS OF MASS TRANSFER RATES IN FRACTURED ROCK SYSTEMS

In most fractured aquifer systems, pore space is present in both interconnected networks of fractures and within fracture-bounded blocks. Often several scales of fracturing are present, and several types of porosity occur within the fracture-bounded blocks. Multiple-rate, double-porosity transport models can accommodate this level of complexity. These models allow mass transfer into a number of zones each defined by different first-order mass-transfer coefficients and capacity coefficients. With proper parameterization, multiple-rate models can preserve transport behavior at all scales.

Current approaches for parameterizing multiple rate models rely on inverse modeling of tracer test data. Estimated multiple-rate parameters from tracer tests are most sensitive to short time and length scales, while parameters that describe slower mass transfer into larger blocks are poorly estimated. This is problematic because flow and transport predictions are often required over time and length scales that greatly exceed those of tracer tests.

We have developed a geologically based approach for parameterizing multiple-rate transport models. Using size and shape distributions for fracture blocks determined from outcrops and cores with laboratory measurements of porosity and formation factor, continuous distributions of first-order mass transfer coefficients and capacity coefficients can be determined for fractured aquifer systems. These distributions are independent of experimental time-scale limitations, are germane to experimental design, and are useful for transport predictions at large time and length scales. Examples are provided from fractured basalt and dolomite aquifers.