Evaluation of Porosity Derivation from Neutron Logs
- R.H. Widmyer (The Texas Co.) | G.M. Wood (The Texas Co.)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- May 1958
- Document Type
- Journal Paper
- 57 - 60
- 1958. Original copyright American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc. Copyright has expired.
- 1.14 Casing and Cementing, 5.6.1 Open hole/cased hole log analysis, 6.5.4 Naturally Occurring Radioactive Materials, 2.4.3 Sand/Solids Control, 4.1.2 Separation and Treating, 4.1.5 Processing Equipment
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In order to evaluate the quantitative determination of porosity by various neutron logging systems, four test wells were constructed to simulate some of the possible variables found in field logging conditions. All of the test holes contain sections of limestone with one section of sandstone included in one well. Each well has a representative section of high, medium, and low porosity. Three borehole sizes are represented by three of the test wells, all containing fresh water. One test well contains saturated brine. Statistical and time constant checks are made for each system evaluated. Runs and repeat runs are made with the son de centered and eccentric. Factors evaluated are the effect of borehole size, sonde position, salinity, chemical composition of rock, and statistical variation. A semi-quantitative method of evaluation of these effects is used giving the average maximum per cent error possible for each. Drift and calibration procedures are also discussed. The effect of the various factors appears to be largely a function of the phenomena detected as well as the means of detection. Graphic illustrations of some of these effects are presented. It is concluded, that for accurate log interpretation, the hole size and sonde position should be known.
The use of neutron logs for the derivation of porosity has become an accepted practice in many areas. In recent years, many different neutron logging systems have been offered to the industry. The neutron sources presently available are of the radium-beryllium or polonium-beryllium type. Detector types used are ionization chambers, Geiger-Mueller-type counters, proportional counters, or scintillation detectors used to detect or measure gamma rays, thermal neutrons, epithermal neutrons, fast neutrons or combinations of these. It is well-known that all systems do not respond alike in all environments. As a result, testing facilities and an evaluation method were developed for use in evaluating the porosity discriminating ability of neutron logging systems using conditions simulating those normally encountered in the field. With such an evaluation, the suitability of a logging system can be determined for selected field conditions.
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