Intrinsic Capture Cross-Section and Porosity Transforms for the TMD-L Pulsed Neutron Capture Tool

Abstract

Abstract Environmental corrections to formation capture cross-section values (∑nf) measured with the new TMD-L tool are illustrated for a wide range of borehole conditions. A porosity transform for the near-far count-rate ratio is also developed. Test-pit measurements cover a range of intrinsic capture cross-section (∑int) values from 8 c.u. to 37 c.u. while porosities ranged from 2 p.u. to 38 p.u. Borehole sizes were 6, 8, 10, and 14 inches with casing sizes of 4-1/2, 5-1/2, 7, and 9-5/8 inches. Cement annuli up to 2-inch thickness were used. Effects of saltwater, freshwater, and gas (air) filled casings were determined. Regression techniques were employed in developing the algorithms to apply these corrections. They are typically capable of obtaining ∑int from the measured ∑fm, values to ± 1 c.u. over the range of conditions explored, even for gas-filled casings. The porosity transform for fluid-filled casing typically yielded the assigned test-pit porosities within ± 2 p.u. for casing sizes of 7 inches or less in boreholes of up to 10 inches. These transforms, which can be run in real-time, require as inputs the casing size, borehole size, casing liquid salinity (or gas), and cement salinity. One field log illustrates the utility of the intrinsic transform in removing the borehole effects such as at casing liquid-gas interfaces. Another example compares porosity from the TMD-L with neutron-density openhole porosity.