The effective conductivity Kef of porous formations of spatially variable permeability K is determined for media of random two-dimensional and isotropic structures. The medium is modeled as an ensemble of multiphase circular inclusions of different Y = lnK, characterized by a pdf f(Y ), and of different radii R (polydisperse medium), of pdf f(R|Y ), which are implanted in a matrix of K = K0. A ...