The petrophysical evaluation of tight gas formations has traditionally been centered on calculations of porosity and water saturation. These two parameters are used to quantify the original volumes in place but they do not provide information about phase mobility except at the saturation endpoints in high porosities. Low porosity affects the accuracy of water saturation calculations and can often make them ambiguous, leading to wrong decisions and unwanted water production.
We found dielectric dispersion logging to be a robust technique for determining gas pay zones independently from saturation equation input parameters. Dispersion analysis of the conductivity and permittivity measurements acquired by these tools is a function of the water tortuosity factor (mn). This factor is vitally important for accurate water saturation evaluation, but is often unknown or variable.
Nuclear magnetic resonance (NMR) measurement has the potential to enhance traditional formation evaluation techniques by providing estimation of the irreducible water saturation (Swirr) and permeability throughout the interval of interest. Accurate determination of these parameters benefits the selection of perforation intervals and improves the chances of producing maximum hydrocarbon with minimum water. NMR logging of deep tight gas formations poses unique challenges with regards to data acquisition due to low porosity, high temperature, and frequently saline muds. Pulse sequences and quality control procedures are used to validate the NMR measurements at high temperatures and high salinities.
An interpretation workflow was developed to integrate dielectric dispersion and NMR data and the results compared with more traditional formation evaluation techniques. There were significant improvements in the prediction of hydrocarbon- and water-producing intervals. The technique has been applied in several deep, high-temperature, low-porosity gas wells. These analyses are made in a timely fashion to provide operators with information for making better completion decisions.
