A Method to Compute Water Saturation in Tight Rocks Accounting for Conductivity of Clay Minerals

摘要

The objective of this paper is to determine the water saturation applying the Waxman-Smits equation and using mineralogy analysis obtained from XRD/CT scanning and resistivity data for a tight sandstone formation. Contrarious to using Archie's equation, the suggested approach takes conductivity into account and provides more accurate water saturation calculations, in order to correctly predict oil and gas in place. Accurate computing of the correlations between rock resistivity and fluid saturations is vital to constructing 2D and 3D models of water saturation to understand the effect of microstructure on petrophysical properties of tight formations. The negative charges on the surface of monoclinic and triclinic clay minerals, with layered silica tetrahedra in shale rocks, hold electric interactions with bipolar molecules of water and free cations. The presence of clays in tight gas sandstone reservoirs may lead to the overestimation of water saturation, if the mineralogy is not properly included into the computations. This implies that proper water saturation characterization of tight gas reservoirs could lead to an increase of evaluated hydrocarbon resources. Such an increase may be significant in large newly discovered tight rock gas bearing basins with multiple shaley-sand sweet spots. The case study in this paper shows a computational analysis made by comparing Archie versus Waxman-Smits equations. Results from tight rock samples illustrate a proportional relationship between the clay content and the quantity of exchangeable clay. A new term, named the clay factor, Cf, has been obtained from this linear relationship, and was found that it could possibly replace the BQV term in the Waxman-Smits equation. These results show a promising workflow method to calculate water saturation considering the conductivity and fraction volume of clay minerals. As the mineralogy of a rock can be accurately identified by high resolution imaging techniques, an approach based on mineralogy, as the proposed method can be utilized as a better way to compute saturation using modification of the Waxman-Smits model. Good assessment of saturation can have a large impact in the computation of transition zones and gas resources in tight rocks, revisiting volumes that were counted as water in past models.