1. B ASIC WELL L OGGING A NALYSIS – N EUTRON L OG Hsieh, Bieng-Zih Fall 2009 1

2. N EUTRON L OG Neutron logs are porosity logs that measure the hydrogen ion concentration ( ) in a formation. In clean formations (i.e. shale-free) where the porosity is filled with water or oil, the neutron log measures liquid- filled porosity. 2

3. N EUTRON L OG (C ONT.) Neutrons are created from a chemical source in the neutron logging tool. The chemical source may be a mixture of americium ( ) and beryllium ( ) which will continuously emit neutrons. These neutrons collide with the nuclei ( ) of the formation material, and result in a neutron losing some of its energy. 3

4. N EUTRON L OG (C ONT.) Because the hydrogen atom is almost equal in mass to the neutron, maximum energy loss occurs when the neutron collides with a hydrogen atom. Therefore, the maximum amount of energy loss is a function of a formations hydrogen concentration. Because hydrogen in a porous formation is concentrated in the fluid-filled pores, energy loss can be related to the formations porosity. 4

5. (N EUTRON L OG ) Thermal velocity

6. G AS EFFECT Whenever pores are filled with gas rather than oil or water, neutron porosity will be lowered. This occurs because there is less concentration of hydrogen in gas compared to oil or water. A lowering of neutron porosity by gas is called gas effect. 6

7. N EUTRON L OG (C ONT.) Neutron log response vary, depending on (1) differences in detector types, (2) spacing between source and detector, and (3) lithology i.e. sandstone, limestone, and dolomite. These variations in response can be compensated for by using the appropriate charts. A geologist should remember that neutron logs (unlike all other logs) must be interpreted from the specific chart designed for a specific log (i.e. Schlumberger charts for Schlumberger logs and Dresser Atlas charts for Dresser Atlas logs). 7

8. N EUTRON L OG (C ONT.) The first modern neutron log was the Sidewall Neutron Log ( ). The Sidewall Neutron Log has both the source and detector in a pad which is pushed against the side of the borehole. The most modern of the neutron logs is a Compensated Neutron Log ( ) which has a neutron source and two detectors. The advantage of Compensated Neutron logs over Sidewall Neutron logs is that they are less affected by borehole irregularities. 8

9. N EUTRON L OG (C ONT.) Both the Sidewall and Compensated Neutron logs can be recorded in apparent limestone, sandstone, or dolomite porosity units. If a formation is limestone and the neutron log is recorded in apparent limestone porosity units, apparent porosity is equal to true porosity. However, when the lithology for a formation is sandstone or dolomite, apparent limestone porosity must be corrected to true porosity by using the appropriate chart. 9

10. C HART FOR CORRECTING CNL FOR LITHOLOGY 10

11. N EUTRON POROSITY FOR SHALY SANDS 11

12. N EUTRON POROSITY FOR SHALY SANDS After the volume of shale (Vsh) is determined, it can be used to correct the porosity log for shale effect. The formula for correcting the density log for volume of shale is (Schlumberger, 1975): Where, Ф N-sh neutron porosity corrected for shale Ф N neutron porosity uncorrected for shale Ф N clay neutron porosity of adjacent shale V sh volume of shale 12

13. C OMBINATION N EUTRON -D ENSITY L OG 13

14. C OMBINATION N EUTRON -D ENSITY L OG The Combination Neutron-Density Log is a combination porosity log. Besides its use as a porosity device, it is also used to determine lithology and to detect gas-bearing zones. 14

15. 15

16. G AS EFFECT Gas effect is created by gas in the pores. Gas in the pores causes the density log to record too high a porosity (i.e. gas is lighter than oil or water), and causes the neutron log to record too low a porosity (i.e. gas has a lower concentration of hydrogen atoms than oil or water). Where an increase in density porosity occurs along with a decrease in neutron porosity in a gas-bearing zone, it is called gas effect. The effect of gas on the Neutron-Density Log is a very important log response because it helps a geologist to detect gas-bearing zones. 16

17. T RUE POROSITY -- NEUTRON - DENSITY POROSITY 17 The method of determining neutron-density porosity is: For gas-bearing formation For oil- or water-bearing formation

18. H OMEWORK #5 -- N EUTRON LOG (CNL) 18

19. H OMEWORK #5 – N EUTRON LOG 19 CNL

20. H OMEWORK #5 – N EUTRON LOG DepthNPHI ФNФN Vsh Ф N-sh 7600 7610 7620 …. 7840 7850 20 Information: The formation is Sandstone and the neutron log is recorded in apparent limestone porosity units

21. C HART FOR CORRECTING CNL FOR LITHOLOGY 21

22. H OMEWORK #5 – N EUTRON -D ENSITY LOG Depth ФNФN Ф N-sh ФDФD Ф D-sh Ф N-D Ф N-D-sh 7600 7610 7620 …. 7840 7850 22