ABSTRACT

Neutron porosity logging techniques have been used for many years to measure the porosity of a formation surrounding a borehole. It is well known that the measurement is adversely affected by changes in measurement geometry such as borehole size, shape and tool position within the borehole (tool standoff). Compensation techniques attempt to overcome these perturbing effects by using two detectors - one located far from the source to measure formation porosity and a second located near the source to measure the effects of changing measurement geometry. Taking the simple ratio of near-to-far (N/F) counting rates reduces the effects of changing geometry. However, this procedure does not provide complete compensation and the resulting porosity values must still be corrected for borehole size, shape, and tool position within the borehole. Much improved compensation for borehole geometry effects is achieved by modifying the simple near-to-far ratio. A function of the far-detector count rate can be determined that results in the two detectors having nearly identical radial responses in the proximity of the tool. The ratio of the near-detector count rate to this function of the far-detector count rate yields a modified ratio that is insensitive to geometric perturbations that occur near the tool. This modified ratio results in a porosity measurement that is borehole invariant - a measurement that virtually needs no correction for washouts, rugosity, borehole shape or tool standoff. The technique is applicable to both wireline and logging-while-drilling (LWD) neutron porosity measurements. The benefits of this new compensation technique will be described and illustrated with laboratory data and Monte Carlo simulation results. The ability of the technique to implicitly account for changing borehole geometry will be demonstrated with several well log examples.

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