The Mississippian formation in Oklahoma and Kansas significantly contributes to oil and gas production in the region. This formation is a lime deposition with substantial secondary porosity that can be created by fractures and vugs. These alterations can change the capacity of the formation to produce fluids. Characterization of the primary porosity condition and the altered condition of the reservoir are not possible using conventional openhole logging practices.

Nuclear magnetic resonance (NMR) logs were added to logging programs to attempt to resolve these issues. This device only measures fluid, and the sum of all fluids present results in a direct measurement of the total and effective porosity. The evaluation of relaxation from the NMR measurement and calculation of permeability provides an estimate of permeability that is consistent in conventional reservoirs.

Full cores were taken in Mississippian wells, and a conventional core analysis of porosity and permeability was performed in 1-ft increments. This study compares the results of porosity and permeability from NMR to these same values, as determined by laboratory work on the cores. Some of the sections are unaltered; other portions have significant secondary alteration.

In all portions of the reservoir considered, permeability thickness (Kh) and porosity from both core and NMR are comparable. Operators attempting to evaluate this reservoir using openhole logging techniques now have a powerful and accurate technique to determine these reservoir properties before incurring expenses associated with completion.

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