Steady flows driven by sources of random strength in heterogeneous aquifers with application to partially penetrating wells

摘要

Average steady source flow in heterogeneous porous formations is modelled by regarding the hydraulic conductivity K(x) as a stationary random space function (RSF). As a consequence, the flow variables become RSFs as well, and we are interested into calculating their moments. This problem has been intensively studied in the case of a Neumann type boundary condition at the source. However, there are many applications (such as well-type flows) for which the required boundary condition is that of Dirichlet. In order to fulfill such a requirement the strength of the source must be proportional to K(x), and therefore the source itself results a RSF. To solve flows driven by sources whose strength is spatially variable, we have used a perturbation procedure similar to that developed by Indel-man and Abramovich (Water Resour Res 30:3385-3393, 1994) to analyze flows generated by sources of deterministic strength. Due to the linearity of the mathematical problem, we have focused on the explicit derivation of the mean head distribution G_d(x) generated by a unit pulse. Such a distribution represents the fundamental solution to the average flow equations, and it is termed as mean Green function. The function G_d(x) is derived here at the second order of approximation in the variance σ~2 of the fluctuation ε(x) = 1 -K(x)/K_A (where K_A is the mean value of K(x)), for arbitrary correlation function p(x), and any dimensionality d of the flow domain. We represent G_d(x) as product between the homogeneous Green function G_d~(0){x) valid in a domain with constant K_A, and a distortion term = 1 +σ~2ψd(x) which modifies G_d~(0)(x) to account for the medium heterogeneity. In the case of isotropic formations ψd(x) is expressed via one quadrature. This quadrature can be analytically calculated after adopting specific (e.g.. exponential and Gaussian) shape for p(x). These general results are subsequently used to investigate flow toward a partially-penetrating well in a semi-infinite domain. Indeed, we construct a σ~2-order approximation to the mean as well as variance of the head by replacing the well with a singular segment. It is shown how the well-length combined with the medium heterogeneity affects the head distribution. We have introduced the concept of equivalent conductivity K~(dq)(r,z). The main result is the relationship J~(eq)(r,z)/K_A = 1 -σ~2ψ~(w)(r,z) where the characteristic function ψ~(w)(r,z) adjusts the homogeneous conductivity K_A to account for the impact of the heterogeneity. In this way, a procedure can be developed to identify the aquifer hydraulic properties by means of field-scale head measurements. Finally, in the case of a fully penetrating well we have expressed the equivalent conductivity in analytical form, and we have shown that K~(efu)≤ K~(eq)(r) ≤ K_A (being K(efu)the effective conductivity for mean uniform flow), in agreement with the numerical simulations of Firmani et al. (Water Resour Res 42:W03422, 2006).