Modern pulsed neutron capture (PNC) and pulsed neutron spectrometry (PNS) tools provide new information about lithology that can be important in gas and oil reservoir evaluation. New PNC tools, besides the usual capture cross section measurement, also provide useful lithology information and can pass through-tubing. The PNS tools provide highly accurate complex lithology information in addition to the primary oil saturation (carbon-oxygen) measurement. Discrimination between tight limestone and gas-saturated sandstone is enhanced through application of either of these two logging tools.
The newest PNC tools also provide several neutron count-rate ratios sensitive to formation gas. A near-far capture ratio, a near-far inelastic ratio, and an inelastic-to-capture ratio provide an array of measurements that have slightly different responses to gas usually allowing for definitive identification of gas saturated reservoirs. Production problems in reservoirs caused by water movement in the casing or cement annulus between zones can often be detected by oxygen activation logging using either tool. Furthermore, flow can often be determined as inside or outside the casing.
Field examples illustrate the benefits of using pulsed neutron logging tools for oil or water saturation determination and lithology evaluation in mixed limestone/dolomite carbonates. Additional field examples will demonstrate techniques for gas detection using the numerous PNC measurements. Finally, log examples will illustrate the use of oxygen activation for detecting undesirable water flow both inside and outside the casing.
An early PNC service introduced in 1982 (TMD*) provided a formation capture cross section measurement (FM), the near/far countrate ratio (RTMD) which is sensitive to porosity. and the total near and far detector countrates (NTMD and FMD) which when used together provide a gas indication. Modifications to the original TMD tools have added the total inelastic countrate from the far detector which provides another measurement with gas sensitivity. In 1994 Halliburton introduced the TMD-L,* which provided all the earlier measures plus spectral measurements obtained from the far-spaced detector. In a 1-11/16" (43 mm) diameter tool the detectors are necessarily small, so the total count in the spectrum above 1 MeV is low. Usable results require slow logging speeds and/or merging multiple passes to improve the statistical precision of the answers. Where spectral information is needed, slowing down from the standard logging speed of 20 ft/min. (6.1m/min.) to 10 ft/min. (3.1 m/min.) is required. Best results are obtained if three passes recorded at 10 ft/min. (3.1 m/min.) are merged. These spectral measurements can be used for lithologic identification. Spectral measurements obtained during the background measurement period provide a determination of the oxygen activation. This is a useful measure of upward water flow during normal logging operations.