A Model for Estimating the Saturation Exponent Based on NMR in Tight Sandy Conglomerate Reservoirs

Abstract

The electrical resistivity of a porous medium, which is used for water or oil saturation evaluation through combination with Archie’s law, strongly depends on the wettability and geometry of the pore system. In tight sandy conglomerate reservoirs, the pore-throat structure is so complicated that Archie’s equation is non-applicable. Analysis shows that the saturation exponent n is a function of the water saturation $$(S_\mathrm{w})$$ , microstructure, and wettability. Here, a novel method based on nuclear magnetic resonance is proposed to determine the saturation exponent (n) under water- or oil-wet conditions. The Schlumberger Doll Research model is also introduced to estimate and analyze the fluid distribution in an irregular pore space. To validate the model, 21 water-wet core samples from the tight conglomerate reservoir in the Junggar Basin, Northwestern China, were selected to measure the electrical resistivity at varying water saturation levels. The absolute errors between the predicted and measured saturation exponents range from −0.153 to 0.131, and the absolute errors for the water saturation between the predicted and measured values vary from −0.032 to 0.023. These results indicate that the proposed model could be applied to accurately predict the water saturation in tight sandy conglomerate reservoirs.