A Study of the Effects of Diffusion on Pulsed Neutron Capture Logs(includes associated papers 7670 and 7901 )
- Russel Randall (Dresser Atlas) | Eric Hopkinson (Dresser Atlas) | A.H. Youmans (Dresser Atlas)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- December 1978
- Document Type
- Journal Paper
- 1,788 - 1,794
- 1978. Society of Petroleum Engineers
- 5.3.4 Reduction of Residual Oil Saturation, 4.1.5 Processing Equipment, 4.1.2 Separation and Treating, 5.2 Reservoir Fluid Dynamics, 4.3.4 Scale, 5.6.1 Open hole/cased hole log analysis, 5.2.1 Phase Behavior and PVT Measurements, 2.4.3 Sand/Solids Control
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The response of the Neutron Lifetime Log TM is studied in view of the diffusion of thermal neutrons in a wellbore geometry. Qualitative theoretical arguments are given for the diffusion effects of a log-inject-log (LIL) procedure. Experimental verification that diffusion exactly cancels out the LIL procedure is presented.
One application of pulsed neutron logging has been to distinguish qualitatively among gas-, oil- and saltwater bearing formations. The parameter ordinarily measured is the macroscopic, thermal neutron-absorption crosssection,, which is sensitive to the chlorine content of the formation and can sense the saltwater saturation of reservoir rock. A particularly useful advantage of pulsed neutron logs is that they can measure cased wells quantitatively. One presumed theoretical deficiency has been much discussed and contended. Neutron diffusion, a well known physical phenomenon, has been recognized or at least proposed - as a possible source of error when measuring cased wells. Certainly this phenomenon sullies the otherwise simple relationship between the inherent nuclear properties of a formation and , measured by commercial instruments. This expected influence of thermal neutron diffusion on the Neutron Lifetime Log TM (NLL) for a particular case was investigated experimentally and is discussed in the first part of this paper.
Another application of pulsed neutron logs has received considerable attention in recent years. Various people have tried to use pulsed neutron capture logs in a log-inject-log (LIL) procedure to ascertain residual oil saturation quantitatively in formations that have been depleted by secondary recovery operations. It has been argued that the effect of neutron diffusion on the measured must be accounted for to attain the limiting accuracy of the method. On the other hand, it has been recognized that the LIL procedure involves the comparison of two separate measurements, each of which is influenced equally by diffusion effects (if any exist). This being the case, the net result of neutron diffusion is zero. Demonstration of this cancellation is verified experimentally and discussed in the last pan of our paper.
Theory of Measurement
Thermal neutrons are not detected directly in the system that produces the NLL. Instead, gamma rays with energy greater than 2.2 million electron volts (MeV) are measured. This procedure effectively samples the thermal neutron population in the formations surrounding the borehole. The NLL instrument emits cyclical bursts of 14-MeV neutrons, which suffer subsequent elastic and inelastic collisions and slow down to thermal velocities. Since protons are the best moderators of fast neutrons, any fluid-filled porosity results in a rapid slowdown. Typical sand formations saturated with water have slowdown times of 10 to 20 seconds for 14-MeV neutrons. Thus, waiting an appropriate time after the fast neutron burst assures that observed gamma rays are caused by thermal neutron reactions.
The detection of gamma rays generally is conceded to be superior to neutron detection. This advantage is primarily because most gamma rays can penetrate farther to reach the detector than thermal neutrons. Furthermore, gamma rays travel with the speed of light, while neutrons diffuse slowly, especially through the highly absorbing borehole fluids that come between the formation and the detector.
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