Conventional reservoir calculations using resistivity and density/neutron porosity have been the industry standard for defining reservoir characteristics. The calculation of water saturation from these measurements is used in every reservoir situation. Many reservoirs are not easily analyzed; therefore, various and complicated analysis techniques have been developed.
In some cases, years were spent in characterization before these log measurements could be applied with confidence. This time and expenditure of resources drained capital from exploration to research processes and produced many inefficient completion attempts in reservoirs that were incorrectly characterized as productive.
The great hope with the introduction of NMR tools to the industry was that reservoir parameters could be established and applied in an uncomplicated manner. Advances in technology of NMR responses have produced the ability to capture polarization time and relaxation time in a single pass through the reservoir. Nuclear magnetic polarization time has been demonstrated to have a relationship to the fluid type in the reservoir. The relaxation time (T2) has been used to characterize permeability with an excellent relationship to production. The Bray-Smith permeability equation was developed from this observation. This equation incorporates relaxation time and calculates permeability. No alterations to the equation for rock type or texture changes were needed.
This paper presents examples of logs in three different and difficult-to-evaluate reservoirs. A reservoir with lenticular sand, another with a thinly-bedded sand and shale sequence, and a conglomerate/mudstone reservoir are each evaluated for fluid type and permeability. Effective porosity, fluid type, and permeability are all resolved accurately by the NMR. Comparisons to production results are presented.