THE AIM OF THIS PROJECT IS TO DEVELOP A COMPUTER PROGRAMME, CONTAINING MODELS FOR THE VARIOUS NUCLEAR BOREHOLE TOOLS, WHICH WILL PERMIT A MORE CORRECT INTERPRETATION OF THE RESULTS OBTAINED BY THESE TOOLS.
The most used nuclear logging probes are the natural gamma probe, the neutron porosity probe and the gamma density probe. Less frequently used is the pulsed neutron capture gamma probe.
4 theoretical models have been developed for the 4 probes to use in a computer programme which gives a log interpretation consistent with the multiparameter dependence of the 4 probe signals.
The model developed for the neutron porosity probe is based on 3 group diffusion theory. The model is 1-dimensional, and assumes the use of 2 identical thermal neutron detectors. It uses analytical solutions to the diffusion equation.
The model developed for the gamma density probe is based on multigroup, modified diffusion theory which takes into account the strong anisotropy of the scattering of high energy gamma quanta. The model is 1-dimensional, and it uses analytical solutions of the diffusion equation.
The model developed for the pulsed neutron capture gamma for the natural gamma probe is based on Fermi-age theory for the fast neutrons and diffusion theory for thethermal. It is 1-dimensional, and it involves a double numerical integration.
The model developed for the natural gamma probe undertakes a calculation of the correction factor by which the measured count rate shall be multiplied to become normalized. The model is based on numerical integration over the formation and the use of a build up factor. It can handle a probe tool in the middle of the borehole or a probe pressed against the wall of the borehole.
The log interpretation programme NULIP is based on these 4 models. Through iteration the programme calculates from the probe signals a set of formation properties which is consistent with all the measurements. The formation properties are the bulk density (ROB), the water and oil volume fraction (V), the macroscopic absorption cross section (SIG) and the formation typr. In addition the volume fraction of matrix, water and oil, and gas may be obtained. Such a calc ulation is performed at each depth value.
THE INTERPRETATION OF FIELD LOGGING DATA OBTAINED BY THE USE OF NUCLEAR PROBES IS MADE DIFFICULT BY THE FACT THAT THE EXPERIMENTAL RESULTS FOR A GIVEN PROBE DO NOT ONLY DEPEND UPON THE SINGLE PARAMETER THE PROBE IS DESIGNED TO MEASURE BUT ALSO ON OTHER SECONDARY PARAMETERS (OR PROPERTIES).
THE PROPOSED RESEARCH WORK POINTS TO THE DEVELOPMENT OF THEORETICAL MODELS FOR EACH OF THE NUCLEAR PROBES USED, ALLOWING CORRECTIONS TO BE MADE FOR THE EFFECT OF THE SECONDARY PARAMETERS. MORE CORRECT VALUES OF THE MAIN PARAMETERS WILL THUS RESULT MAKING POSSIBLE A MORE RELIABLE INTERPRETATION OF THE LOGGING DATA.
AN ESSENTIALLY ONE-DIMENSIONAL MODEL FOR A NEUTRON-NEUTRON DOUBLE DETECTOR POROSITY PROBE HAS ALREADY BEEN DEVELOPED, BASED ON THREE-GROUP DIFFUSION THEORY, A JOINT SOURCE OF FAST NEUTRONS AND A LINEAR DETECTOR. THE NEAR/FAR DOUBLE-DETECTOR SYSTEM IS USED, THUS CANCELLING OUT THE BOREHOLE EFFECT.
AS TO THE GAMMA-GAMMA DOUBLE-DETECTOR DENSITY PROBE, A SEMI-EMPIRICAL THEORETICAL MODEL WILL BE DEVELOPED FOR THE NEAR DETECTOR, TAKING INTO ACCOUNT UP TO FOUR-TIMES SCATTERED QUANTA. FOR THE FAR DETECTOR, APPROACHES BASED EITHER ON THE USE OF BUILD-UP FACTORS OR ON THE MULTI-GROUP DIFFUSION THEORY FOR GAMMA QUANTA WILL BE DEVELOPED. TWO MODELS OF DIFFERENT DEGREE OF ACCURACY FOR A PULSED-NEUTRON CAPTURE GAMMA PROBE HAVE ALREADY BEEN DEVELOPED. TESTING AGAINST ACTUAL BOREHOLE LOGGING DATA WILL PERMIT A CHOICE TO BE MADE BETWEEN THEM. A MODEL FOR AN INTEGRAL GAMMA PROBE WILL BE DEVELOPED AND POSSIBLY EXTENDED TO A NATURAL GAMMA PROBE. AN ATTEMPT WILL BE MADE TO SET UP A MODEL FOR A GAMMA-GAMMA ZEFF PROBE.
TESTING AGAINST ACTUAL BOREHOLE LOGGING DATA WILL BE EFFECTED FOR EVERY OF THE AFOREMENTIONED MODELS. THEY WILL THEN BE COMBINED INTO AN OVERALL PROGRAMME. THIS WILL ALLOW: THE TYPE OF ROCK(TOR), THE EQUIVALENT WATER VOLUME PERCENT (VW), THE MATRIX DENSITY (M) AND THE MACROSCOPIC ABSORBTION CROSS-SECTION OF THE STRATA (A) TO BE CORRECTLY CALCULATED FROM THE MEASURED RESULTS OF THE WHOLE SERIES OF NUCLEAR PROBES AS INPUT DATA. TESTING OF THE OVERALL PROGRAMME AGAINST ACTUAL BOREHOLE LOGGING DATA WILL FINALLY BE MADE AND THE APPROPRIATE MODIFICATIONS INTRODUCED. .