An Improved Formation Density Measurement using PNC Tools

Abstract

Abs tra c t Several schemes have been described for removal of the capture gamma-ray contribution during a neutron burst in order to obtain a clean inelastic gamma measurement. However, even a clean inelastic count-rate ( CR) measurement -while substantially dependent on the gamma transport properties of the formation -still possesses a sizeable residual neutron transport effect. M onte-Carlo modeling has been used in the proj ect reported here to better understand the physics of the neutron-gamma transport problem. M onte Carlo modeling has also been used to develop a correction technique that leads to a compensated inelastic ratio dependent almost solely on the gamma transport ( density) properties of the formation surrounding the tool. This paper will discuss how a modeling database was constructed and how it has been used to develop the correction technique. An extensive database of modeling results validates the proposed technique.I n addition, since this technique does not employ any new measurements from the PNC tool ( it uses existing near and far detector inelastic and capture CR' s) , any existing field PNC log can be used to test the process where a suitable openhole density-neutron log is available. Several log examples demonstrate a reasonable correlation between this new PNC density technique and an OH density log. Introduc tionVarious physical arguments point to the inelastic gross countrates in the detectors of PNC/ PNS tools as having significant sensitivity to formation density. Thus, some measure of formation density in cased-holes ( CH) may be obtained by proper analysis of these inelastic count rates. Several schemes have been described for removal of the capture gamma-ray contribution during the neutron burst in order to obtain a clean inelastic gamma measurement. 1-5 However, as will be shown here, even a clean inelastic count-rate measurement -while substantially dependent on the gamma transport properties of the formation -still possesses a sizeable residual neutron transport effect. M onte-Carlo modeling has been used to better understand the physics of the neutron-gamma transport problem and to develop a correction technique that leads to a compensated inelastic ratio dependent almost solely on the gamma transport ( density) properties of the formation surrounding the tool. An extensive database of modeling results validates this correction technique.This correction technique employs measurements that have been on logs for quite some time, thus allowing evaluation of the technique on previously run field logs. A couple of examples will be examined where openhole ( OH) neutrondensity logs were available for comparison. Generally, a good correlation is observed; but it must be recognized that any CH density measurement will necessarily be somewhat inferior to an OH measurement because of the complexities and uncertainties of the CH environment. Precise information on tool standoff, BH size, cement quality, and casing corrosion may not be available when the CH log is run. Variation in thes...

Study on Monitoring Reservoir Gas Density by Pulsed Neutron Logging

“…The RPM pulse neutron logging tool uses the fast neutrons emitted by the pulse neutron generator to pass through the completion string to have a nuclear reaction with the formation nuclei. The receiving crystal of the tool records the gamma rays generated by the nuclear reaction, thereby detecting formation information [16,17]. The neutron nuclear reaction in pulsed neutron logging is divided into activation reaction, inelastic scattering, elastic scattering, and capture reaction according to the level of neutron energy [18].…”

Bulk Density Response and Experimental Study of Pulsed Neutron-Gamma Density Logging

“…Odom et al (2001) developed a cased-hole pulsed neutron- gamma density tool with a fast neutron detector. Jacobson et al (2004) pointed out that neutron capture gamma rays could be used for correction in density calculation. Schlumberger developed the first commercial logging-while-drilling (LWD) pulsed neutron-gamma density measurement tool with thermal and epithermal neutron detectors (Yu et al, 2011;Evans et al, 2012;and Reichel et al, 2012).…”