A generalised equivalent resistivity model in laminated and dispersed shaly sands

Abstract

Based on the assumption of parallel conduction between laminated and dispersed shaly sand patterns, and the theory of a symmetrical anisotropic effective medium for resistivity interpretation, a generalised equivalent resistivity model is proposed for predicting water saturation in complex shaly formations. This model assumes that the shaly formations contain laminated shales, conductive rock-matrix grains, nonconductive hydrocarbons, dispersed clay particles, and water. It shows that shale distribution has the most important effect on water saturation calculations. The curvature of a formation conductivity Ct versus water saturation Swt curve is greatly affected by clay conductivity Ccl, but is less affected by formation matrix conductivity Cma. Also, this curvature is affected by water percolation rate λw or percolation exponent γ, but it is less affected by matrix percolation rate λma. When Ct is kept constant, Swt increases as λma or γ increases, and decreases as λw increases. Rock sample data in dispersed shaly sands, and well logging data in laminated shaly sands, show that this model can be applied in both of these environments. Experiments with artificial samples with conductive rock grains also show that the model may be applied in clay-free porous rocks, if water conductivity is larger than rock grain conductivity. Therefore, the proposed resistivity model can be considered to be a generalised equivalent resistivity model for laminated and dispersed shaly sand formation evaluations.