Advances in Neutron Lifetime Logging

Abstract

Abstract The Neutron Lifetime Log (NLL)* has gained industry-wideacceptance as a superior cased-hole logging technique. Quantitative evaluation of hydrocarbon deposits has beenproved to be feasible where conditions are favorable. Under less favorable conditions, where very small contrastsare observed, it is usually possible to make usefulqualitative judgment of formation fluid content. Experiencegained from studying many Neutron Lifetime Logs hasprovided a better knowledge of the accuracy and reliabilityof interpretive techniques. The expected range of magnitudeof R and w in various areas and conditions will bediscussed. The API neutron test pits have been logged andR determined for the three limestone blocks in thatfacility. Studies of the chemical composition of rocks andfluids, including the elects of trace elements having veryhigh thermal neutron capture cross-section, have added tothe understanding of variations in R. Activation logging for silicon content is a new techniquewhich is made possible by the NLL instrumentation. Lithologicdifferences due to variations in sand, shale and carbonatecontent can be recognized by this means. This log has provenin many cases to be a valuable companion interpretation toolwhen used with the lifetime log. Other companion logs previouslycommercially available for simultaneous recording with thelifetime log are the gamma ray or epithermal neutron logs. Useof the log as a means for reservoir monitoring has improved theaccuracy and precision with which oil/water/gas interfaces canbe followed during the producing life of a reservoir. When usedin this fashion, the log provides data which improve knowledgeand understanding of the mechanism of production and the problemsrelated there to Introduction The NLL, introduced as a commercial logging service over2 1/2 years ago, has already gained industry-wide acceptanceas the most outstanding cased-hole logging service availabletoday. Advances in both logging capabilities and interpretivetechniques, combined with the sheer bulk of logs run duringthis period, have been continuously increasing the value ofthe service. In addition to the United States and Canada, outstanding results have been achieved with the log inVenezuela, Nigeria and Libya. Several previously published papers have discussed thephysical aspects of the tool, the basic measured parameterand how this parameter, the thermal neutron capture cross-section, is related to reservoir rocks and their contained fluids. Briefly, the NLL is made with an instrument containing a source of very highenergy pulsed neutrons. This source is actuated to produce shortneutron pulses at 1,000 microsecond intervals. After each pulse, while the source is inactive, the rate of neutron die-away isexamined by means of a scintillation gamma ray counter at eachof two preselected time intervals (or gates) following each pulse. The value of thermal neutron capture cross-section isdetermined from the measured counting rate during these intervalsfrom the relation ........................(1) The units of in this equation are 10(-3) sq cm/cu cm. Thetime interval elapsed between the first and second measurementsis represented by T measured in microseconds. The neutronpopulation in the two time intervals is proportional to the gammaray counting rates N1 and N2 measured during gates 1 and 2, respectively. Fig. 1 is a schematic representation of the rate of thermalneutron generation and capture in a typical formation. Note thatthe two gates of equal length, where N1 and N2 are counted, arelocated on the straight-line portion of the semi-logarithmic plot. The macroscopic or composite thermal neutron capture cross-sectionof the formation and contained fluids is a direct function ofthe slope of this straight-line section of the die-away curve. Since is the summation of the capture cross-sections of theindividual atoms in a cubic centimeter of material, a reservoir rockcontaining both hydrocarbons and water may be correctly representedby the following equation. ...........(2) In this equation the sub scripts refer to the rock matrix, waterand hydrocarbon phases; is porosity; and Sw is watersaturation. When reliable values can be assigned to the otherparameters, Sw can be accurately determined from the measurementof provided by the log. JPT P. 479ˆ