TRANSIENT SOLUTE TRANSPORT IN A DUAL-POROSITY MEDIUM WITH FRACTURE SKIN

Fracture skins are layers along fracture surfaces that consist of alteration zones or coatings and that change the hydraulic and transport properties of a dual porosity medium. We extend previous studies of solute transport in such systems by mathematical analysis of transient flow from pulses of solutes in a set of parallel fractures. We utilize field data from the Brushy Canyon Sandstone of West Texas and consider both a reactive (¹³⁷Cs) and a conservative (Cl^-1) tracer with step-function solute pulses on the order of 100 days. This eliminated the steady state components of the previous solutions and resulted in different Laplace transform solution that adds an extra term (1-e^-kp). Sensitivity factors for skin thickness and fracture aperture were used to compare the effects of fracture skins. These are: for skin thickness and fracture aperture - B_s=[(d-b)/b], for matrix and skin porosities and diffusion coefficients - B_fD=[(f_sD_s)/(f_mD_m)], and for matrix and skin sorptivities - B_RD=[(R_s/D_s)/(R_m/D_m)]. b=fracture half aperture; d=skin thickness, f=porosity; D=diffusion coefficient; R=retardation coefficient; and the subscripts s and m refer to the skin and matrix, respectively.

For the Brushy Canyon data, sensitivity analyses demonstrate that B_fD is the most critical factor for both reactive and conservative species. For solutes that sorb, B_RD is the second most critical factor. For a wider range of fracture properties, skin thickness may become critical. Back diffusion is demonstrated to be an important process controlling the fracture concentration with fractures subject to pulses of solute. In the sensitivity analyses, we find that there are ranges where, with little error, the medium can be treated as a dual porosity medium using solely skin or matrix properties. These are:1) for the conservative solute (Cl^-1), B_s<0.01 and B_s>100; and B_fD<10^-5 and B_fD>5; and 2) for the reactive solute (¹³⁷Cs), B_s<0.01 and B_s>5; B_fD<10^-4 and B_fD>0.1; and B_RD<1 and B_RD>10⁴.