Calibration of water saturation models derived from traditional wireline data in tight shaly gas sandstones, Part 1: Modelling

Abstract

Relative permeability studies performed on tight gas sand core show that water saturation has much influence upon effective gas permeability. This makes water saturation in tight gas sand a critical reservoir parameter. The aim of this study is to introduce a new approach to reduce the uncertainty in water saturation estimation by well logs models in tight shaly gas sandstones. İnasmuch as all of water saturation approaches were fundamentally derived from Archie's classic work in high-porosity and high-permeability systems, calibration of these models for tight sandstones (low-porosity and low-permeability systems) is required. We introduce a new method by using the Porosity-Resistivity cross plot to calibrate water saturation models for tight shaly sandstones. This method represent an equation that comes through resistivity, gamma ray, neutron and density logs (to take porosity), which are the commonly available logs in most wells and formations. For calibrating the conventional water saturation models with tight shaly sandstones conditions, the introduced equation should be multiplied by traditional water saturation models.To validate the introduced method, one traditional model is tested to predict water saturation in fourteen wells (39 different zones) from a basin in western USA. It is observed that after calibration, the value of uncertainty calculated using least-square error method (LSE) to be reduced significantly.