An Improved Formation Density Measurement using PNC Tools
Authors: Jacobson Larry , Durbin Dennis , Reed Skip 
Published: January 2008

Several schemes have been described for removal of the capture gamma-raycontribution during a neutron burst in order to obtain a clean inelastic gammameasurement. However, even a clean inelastic count-rate (CR) measurement -while substantially dependent on the gamma transport properties of theformation - still possesses a sizeable residual neutron transport effect.Monte-Carlo modeling has been used in the project reported here to betterunderstand the physics of the neutron-gamma transport problem. Monte Carlomodeling has also been used to develop a correction technique that leads to acompensated inelastic ratio dependent almost solely on the gamma transport(density) properties of the formation surrounding the tool. This paper willdiscuss how a modeling database was constructed and how it has been used todevelop the correction technique. An extensive database of modeling resultsvalidates the proposed technique.

In addition, since this technique does not employ any new measurements fromthe PNC tool (it uses existing near and far detector inelastic and captureCR's), any existing field PNC log can be used to test the process where asuitable openhole density-neutron log is available. Several log examplesdemonstrate a reasonable correlation between this new PNC density technique andan OH density log.

Introduction

Various physical arguments point to the inelastic gross count-rates in thedetectors of PNC/PNS tools as having significant sensitivity to formationdensity. Thus, some measure of formation density in cased-holes (CH) may beobtained by proper analysis of these inelastic count rates. Several schemeshave been described for removal of the capture gamma-ray contribution duringthe neutron burst in order to obtain a clean inelastic gammameasurement.1–5 However, as will be shown here, even a cleaninelastic count-rate measurement - while substantially dependent on the gammatransport properties of the formation - still possesses a sizeable residualneutron transport effect. Monte-Carlo modeling has been used to betterunderstand the physics of the neutron-gamma transport problem and to develop acorrection technique that leads to a compensated inelastic ratio dependentalmost solely on the gamma transport (density) properties of the formationsurrounding the tool. An extensive database of modeling results validates thiscorrection technique.

This correction technique employs measurements that have been on logs forquite some time, thus allowing evaluation of the technique on previously runfield logs. A couple of examples will be examined where openhole (OH)neutron-density logs were available for comparison. Generally, a goodcorrelation is observed; but it must be recognized that any CH densitymeasurement will necessarily be somewhat inferior to an OH measurement becauseof the complexities and uncertainties of the CH environment. Preciseinformation on tool standoff, BH size, cement quality, and casing corrosion maynot be available when the CH log is run. Variation in these quantities can havesignificant impact on the accuracy of the density measurement in CH. Casingsize and casing fluid also have a significant effect, but these quantities areprobably better known and knowable.

Modeling Methodology

A Monte-Carlo model of Halliburton's Pulsed Neutron Capture (PNC) tool (theTMDL6) was developed; and all the modeling results reported herespecifically apply to that PNC tool. Since other service companies. PNC toolsare similar, the results for them would differ in detail but not in substance.Fig. 1 illustrates a typical model layout in cross section. The primarypurpose of this model was to determine what measures provided by this PNC toolhad the greatest sensitivity to formation density. Because this problem is acombined neutron - gamma transport problem, it is difficult to separate the twosignificant effects of hydrogen index (HI) and bulk density. The former willprimarily affect the neutron transport; and the latter, primarily the gammatransport. Unfortunately, both modes of transport vary at the same time inlaboratory and real formations. Thus, it is very difficult to create testformations of constant HI but variable density; and, conversely, constantdensity but variable HI. However, these situations are easily handled bymodeling.