Computer Simulation of Two Nuclear Well Logging Methods

Monte Carlo techniques have been used to simulate the response of a multi window natural gamma ray spectral log to varying borehole conditions and to investigate the potential of a porosity logging concept using a ratio of fast to epithermal neutrons. Introduction There are many situations in nuclear well logging research when it would be highly desirable to have a theoretical method of realistically simulating the measurements made by a downhole logging instrument. In the early stages of research into the feasibility of a logging concept, it would be possible to make an accurate theoretical determination of whether the idea should be pursued. Should the concept turn out to be unsound, the expense and time required to design, build, and test a logging system would be avoided. In later stages of log development, the theoretical model could be used to assist in optimizing sonde design and specifications. In situations where a logging device already has been developed fully, such a model also could be used to ascertain much information about the way the physical device works. As an illustration, a theoretical simulation of a neutron (n, ?) logging sonde could provide information regarding the relative importance of the gamma rays scattered into the detector from the various downhole materials (Le., toolcase, borehole fluid, formation, cement, and well casing). The depth of investigation into the formation and the significance of neutrons in the gamma ray detector could be estimated also. Few of these parameters could easily be determined experimentally. Monte Carlo radiation transport techniques,1,2 which long have been used by scientists involved in nuclear reactor shielding design and radiation dosimetry, perhaps provide the best methods for fulfilling the theoretical nuclear logging objectives mentioned. Several papers in this area have been published.3–6 Our paper briefly describes both the Monte Carlo method and the specific Monte Carlo computer programs - SAM-C7 and an updated version SAM-CE8 - which we have been using in nuclear logging applications. Two specific illustrations are discussed. The first example involves the simulation of a natural gamma ray spectral logging device. Monte Carlo results are presented which indicate that significant errors can be introduced into the elemental concentrations obtained from these instruments unless compensations are made for differing borehole conditions. The second example describes how we have used Monte Carlo program SAM-CE to simulate an untested porosity logging technique that uses a ratio of fast to epithermal neutrons. The sensitivity and linearity of this ratio to changes in porosity were determined from the program, as were the effects of changing matrix type. These results then are compared with calculations simulating a conventional dual-spaced epithermal porosity measurement under identical lithologic conditions. The two examples presented in this paper involve relatively simple geometries to facilitate concept comprehension by the reader. Much more complex geometries also are well within the capabilities of SAM-CE or other current Monte Carlo programs.