Petroleum Engineers

NUCLEAR LOGGING TECHNIQUES 50-15are basically thermal neutron detectors. For some loggingapplications, it is desirable to measure the cpithcrmal neutronflux while being insensitive to thermal neutrons. Thiscan be achieved by making a minor modification to anyof the three types of detectors previously mentioned. Itconsists of using an exterior thermal-neutron-absorbingmaterial with a large cross section, such as cadmium. toshield the detector. Thermal neutrons will be absorbedin the shield, but the reaction particles. whose range issmall (on the order of tenths of millimeters). will not reachthe counter. The higher-energy neutrons that manage topenetrate the shield will be detected by the thermal neutrondetector with somewhat reduced efficiency.Nuclear Radiation Logging DevicesThe logging devices discussed in the following section fallunder two general categories: those that measure naturalradiation fields and those that produce radiation fields andmeasure some aspect of their interaction with the formation.The first group contains tools that measure the naturalgamma ray activity of earth formations resulting fromthe spontaneous decay of radioactive materials. The secondcategory can be broken down into the type of radiationused-gamma rays or neutrons. The latter may besubdivided further into the use of chemical or steady-stateneutron sources or pulsed particle accelerator-basedsources described earlier.Rather than trace the historical development, which hasbeen well documented by Segesman,” only the most recentlogging devices will be discussed. Both neutronporosity and gamma-gamma density devices have undergonesubstantial evolution since their respective introductionsas commercial services. The earliest devicesinvariably used a single detector. As the use of these typesof measurements grew, more emphasis was put on improvingthe quantitative nature of the measurements anda better appreciation of environmental effects was gained.This led to the development of borehole-compensateddevices generally using a second detector at a lesser spatingfrom the source that, because of its larger sensitivityto environmental effects, provides a correction to be appliedto the principal detector.Gamma Ray DevicesThere are two series of naturally occurring radioactiveisotopes that occur in significant quantities in sedimentaryrocks: the uranium and thorium series. The only othersignificant naturally occurring radioisotope is that ofpotassium (40K). Clay minerals that are formed duringthe decomposition of igneous rocks in general have a veryhigh cation exchange capacity. Because of this propertythey are able to retain trace amounts of radioactive mineralsthat originally may have been components of thefeldspars and micas that go into the production of clayminerals. This process generally results in a higher concentrationof radioactive elements in shales than in sandstonesor carbonate rocks not produced by weathering.However, some radioactivity can be associated with carbonaterock and sandstones because of transport of radioactiveminerals in solution in the formation waters.The principal use of the gamma ray log is to distinguishbetween the shales and the nonshales. Historically, thefirst gamma ray devices measured only the total gamma1.460 0.5 1.5 2 2.5 3Gamma Ray Energy, MeVFig. 50.18-Theoretical gamma ray emission spectra from thethree naturally occurring radioactive products.ray flux emanating from the formation. However, it isnow known that different types of shale have different totalgamma ray activity because of the Th, U, and K concentrations.Fig. 50.18 shows the various gamma ray lineemissions associated with each. This indicates that by determiningthe intensity of the particular gamma ray energiesit is possible to identify the quantity of eachradioactive emitter in the formation. With the developmentof improved spectroscopic-quality gamma ray detectors,it became natural to refine the gamma raymeasurement into a measurement of the actual concentrationsof the three components.The measurement element for recent gamma ray orspectral gamma ray logging devices is the NaI detector.The gamma ray devices measure the total number of gammarays above some practical lower limit (on the orderof 100 keV). This total counting rate will be (I) a functionof the distribution and quantity of radioactive materialin the formation and (2) influenced by the size andefficiency of the detector used. For this reason somecalibration standards have been established by the API,and all total-intensity gamma ray logs are recorded in APIunits.The definition of the API unit of radioactivity comesfrom the artificially radioactive formation constructed atthe U. of Houston facility. A formation containing approximately4% K, 24 ppm Th, and 12 ppm U was constructedand defined to be 200 API units. The details ofthis calibration facility can be found in Ref. 6.Spectral gamma ray devices basically use the same typeof detection system as the total gamma ray devices, butinstead of one broad energy region for detection. the gammarays are analyzed into several different energy bins.