Publication History: This article is based on "Crain's Petrophysical Pocket Pal" by E. R. (Ross) Crain, P.Eng., first published in 1987, and updated annually until 2016. Updated 2018. This webpage version is the copyrighted intellectual property of the author.

Do not copy or distribute in any form without explicit permission.

HEAVY OIL IN Shaly Sand (Ancient Logs)
One way to test the techniques for ancient logs is to run the math on a modern log suite and compare results to the same logs after eliminating all the curves that would not have been available in ancient times. In this well, we have conventional induction-electrical and density-neutron logs in a heavy oil well. By assuming that the induction resistivity is similar to a 64 inch normal and that the only shale indicator is the SP, we can compute results simulating an ancient log suite using the PHIMAX method. This "ancient log" result can then be compared with the modern version in the identical rock/fluid sequence using standard log analysis models. The technique is relatively straight forward in sand shale sequences where porosity is often proportional to (1 - Vsh), but may not be useful in carbonates, unless good core data is available.

Watch for the possibility of ancient gamma ray and neutron logs run through casing -- they can be very helpful in guiding the shale volume and porosity calculations.

Once you have satisfied yourself that your ancient log technique works reasonably well in a modern well, you can embark on a field wide search for bypassed pay or poor quality completions.

Here is an example: top illustration shows raw log data, middle shows results using modern porosity logs, bottom shows results using resistivity, SP, and PHIMAX only.


 


IES and CNL FDC for heavy oil case history 1978


Conventional log analysis using GR, CNL, and FDC. Pick water contact in GP sandstone. Is there a contact in the Sparky sandstone? Compare your answer to resistivity log.


Same well computed with IES, SP, and PHIMAX = 0.34. Compare to results in previous illustration.

Although the induction resistivity is focused better than a 64 inch normal, this example shows that the PHIMAX method is quite suitable in a shaly sand sequence.
 

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