Sidewall neutron tools have been the standard neutron porosity tool in air-filled boreholes for 25 years. The introduction of combinable neutron tools with simultaneous dual-detector thermal and epithermal neutron measurements provided the possibility for improved porosity evaluations in both air- and liquid-filled boreholes. Obtaining thermal neutron porosity measurement in air-filled boreholes is impractical because of poor porosity sensitivity and the combined effects of borehole size and thermal neutron absorption. In contrast, the epithermal ratio possesses good sensitivity in the low-to-medium porosity range encountered in air-drilled formations. A significant, but manageable borehole size effect also influences the epithermal ratio. In addition, the epithermal ratio porosity has better statistical precision than sidewall measurements because a stronger neutron source is used with the dual-detector design.

New responses for the dual-detector epithermal neutron measurement in air-filled boreholes have been developed from data obtained in recent laboratory experiments and Monte Carlo modeling calculations. A data base consisting of 47 laboratory measurements and 164 Monte Carlo simulations was used to derive basic responses to the three principal formation matrix materials and to define a correction for variations in borehole diameter ranging from 4 to 14 in. [10 to 36 cm]. In addition, Monte Carlo simulations and laboratory experiments were used to investigate the effects caused by tool standoff.

In the past, parameterizations of the dual-detector epithermal response in air-filled boreholes were obtained through empirical relationships with measurements in liquid-filled boreholes and comparisons with sidewall neutron measurements in air-filled boreholes. Because the previous relationships were "calibrated" to other measurements made under specific borehole conditions, the new algorithms significantly improve the accuracy of the porosity determination in air-filled boreholes over a greater range of porosity, lithology and borehole diameters.

Example logs from different areas of the country are presented to demonstrate the effectiveness of the new algorithms and the improvement in statistical precision. A new porosity crossplot chart is included to estimate gas saturation and determine porosity in air-filled boreholes.

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