204-27 - A METHOD for NEUTRON DERIVED POROSITY...

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A METHOD for NEUTRON DERIVED POROSITY DETERMINATION for THIN BEDS ABSTRACT J. M. EDWARDS A. 1. SIMPSON The application of the Scintillotneter to rudioactivrty surveys in boreholes of oil wells has made it possible to determine bed rhickness with a high degree of accuracy. This fact, combined with known relation of logging variables, makes thin bed porosity determination pruc- tical. With the use of the Scintillonzeter a /nethod hot been derived whereby the apparent lrvel of radiactivity, as recorded on radiation logs, rnay be converted to the true level. The true level of radioactivity is that level to which the recording pen would have traveled had the detector remained at a given depth for on infinitely long period of time. Thi.7 factor is: McCUllOUGH TOO1 CO 10s ANGELES, CAlIF. RJR, - --CID e~3~ x e~~' where R, i~ the true Log - @p + eTD-1 radioactivity levcl, is the apparent radioactivity level, T is the bed thickness, D is drag, and L is the length of the detecting system. This method depends entirely on accurate measurements of bed thickness and drag. A thorough discussion of the concept of time constant and logging speed as related to drag is under- taken. INTRODUCTION Neutron curve interpretation has become one of the accepted methods for porosity determination in oil field 'References given at end of paper. Paper presented at Pacific Petroleum Chapter Fall Meeting in Los Angeles Oct. 7-8. 1954. and at Petroleum Braneh Fall Meeting in San Antonio Oct. 17-20. 1954. Original manuscript received in Petroleum Braneh office on Aug. 3, 1964. Revised manuscript received May 11. 1966. practices. A great deal of literature has been published about neutron derived porosities, but this has been concerned with thick beds, or possibly even averaging of a series of thin beds as a single unit. There has been a lack of written information on methods whereby porosity can be evaluated for beds less than 10 ft in thickness. In certain areas, production is derived from rela- tively thin formations. The importance of thin bed evaluation, the fact that the Scintillometer is an ade- quate tool for this application, and the general lack of published information concerning neutron porosity for thin beds are the fundamental reasons for pre- senting this method of porosity determination. This concept is not put forth with the idea of discounting existing methods, but is merely suggested as a modi- fication of other accepted methods of porosity deter- mination which makes no allowance for quantitative cvaluation of thin beds. The effects on the neutron log of counting statistics, statistical variation in the porosity of earth materials, and hole size must be taken into account to obtain neutron derived porosities. Since these effects have been described in past litera- ture, the authors have not made an attempt to go into the subject in detail. With the use of statistical and repeated sections on the log, it may be determined which responses on the log actually correspond with lithologic changes.
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204-27 - A METHOD for NEUTRON DERIVED POROSITY...

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