Why petrophysics is hard
Earlier this week we published our fourth cheatsheet, this time for well log analysis or petrophysics. (Have you seen our other cheatsheets?) Why did we think this was a subject tricky enough to need a cheatsheet in the back of your notebook? I think there are at least three things which make the interpretation of log data difficult:
Most of the tools do not directly measure properties we are interested in. For example, the radioactivity of the rocks is not important to us, but it does make a reliable clay and organic matter proxy, because these substances tend to have more uranium and other radioactive elements in them. Almost all of the logs are just proxies for the data we really need.
We only see the rocks through the filter of the method. Even if we could perfectly derive apparent reservoir properties from the logs, there are lots of reasons why they might be less than accurate. For example, the drilling fluid (usually some sort of brine- or oil-based suspension of mud) tends to invade the rocks, especially the more permeable formations, the very ones we are interested in. The drilling fluid can also interfere with some tools, depending on its composition: barite absorbs gamma-rays, for example.
The field is infested with jargon and historical baggage. Since Conrad and Marcel Schlumberger invented the technique almost 100 years ago, thousands of new tools and new methods have been invented. Every tool and log has its own name, method (usually proprietary these days) and idiosyncracies, making for a bewildering, intimidating even, menagerie. Worse still, lots of modern tools collect multi-dimensional data: for example, sonic spectra on multiple axes, magnetic resonance T2 distributions, dynamically-scaled image logs.
We drew from several sources to build our cheatsheet. We drew partly from our own experience, but also relied on input from some petrophysical specialists: Neil Watson of Atlantic Petrophysics, Andrea Creemer of Corridor Resources, and Ross Crain of Spectrum 2000. We also consulted the following references, synthesizing liberally where they disagreed (quite often, given the range of vintages of these works).
- Schlumberger (1991), Log Interpretation Principles/Applications, Schlumberger Educational Services, Houston
- Rider, M (1991), The Geological Interpretation of Well Logs, Whittles Publishing, Caithness
- Crain, ER (2011). Crain's Petrophysical Handbook.
- Schlumberger's online documentation
- Halliburton's online documentation
- Paul Glover's course notes
Despite referring to some of the best sources in the industry, we hereby assert that all errors are attributable to us, not our sources. If you find errors, please let us know. Get in touch on Twitter, use the contact form, or leave a comment.
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cheatsheet, data, petrophysics in Science 
Reader Comments (9)
As a novice at interpreting logs, I've found that you really do have to use every piece of information that you have! Maybe one thing that you could add to your cheat sheet is an example of a pitfall for each log. It's easy to say that a steepening-upward pattern in your dipmeter could be a point bar but you have to account for everything else- does your gamma ray show a mud or a sand and what is the effect of that on the dipmeter response?
I've found that the most important part of interpreting logs is having some defining factors to hang your hat on and sometimes the credibility of those factors can be affected by other logs.
Mostly what I'm getting at is that in my few months of log interpretation, I've learned that it's not easy!
@Ella,
Some good points. I originally envisioned a well log cheat sheet that would show pictures, log signatures, and real data. A template-based manifesto for wireline based information, with uncertainties, and common pitfalls. But just as you suggest, it would get very massive very quickly. I still would like to build a template for a specific environment, but it's a big job. It's amazing how many different expressions of a one dimensional earth there can be (of course, calling well-logs one dimensional would be an insult to people who deal with azimuthal variations).
As my recent work with well log QC for inversion will attest, petrophysics can be made more complicated by suboptimal well log acquisition, conditioning and interpretation. It's not just invasion, but whether the measurement was made in casing, is properly depth-corrected and meets some elementary geological and geophysical criteria. The best petrophysicist is well-rounded and experienced in geology, borehole geophysics, rock physics and well engineering.
@Maitri: I agree... it seems like we often end up wishing for that extra run, or a slightly different tool set. I liked what Jon said on the first post: 'Time is money,... but missed pay is more money'.
As for what to do about a poor set of logs... Only at one company (Statoil) have I had the luxury of a dedicated petrophysicist in the team with the time to ensure a reliable suite of logs, with corrections and even interpolations where necessary. It was great to be able to just focus on interpretation and rock physics. What a concept!
Wow, this is great. I have been looking for a reference document like Crain's Petrophysical Handbook for a long time. Thanks for putting this online!
The depth of investigation for NMR wireline tools is not always 28-38mm, for some of the decentralized tools the range can be 5-10 cm. Also it's important to mention in your cheatsheet that LWD measurements might have different parameters (and usually they do) and different aspects affect LWD log data.
@Alexander: Thanks for the clarification — and I love the idea of adding something around LWD. Maybe we'll update it soon.
@Matt ..
Why you think we have few Petrophysicist as compare to geophysicist.
I liked petrophysics when I was in university took some advance courses in that area. But, it's impossible to get opportunity as junior in this area.
@Aka: I don't quite know. The strongest petrophysicists I've worked with have some experience with logging crews — so many of the tools have idiosyncrasies that only field engineers seem to really understand. They also need a sound understanding of geology, especially petrology, and be comfortable with physics and mathematics. People with these skills are quite sought after, at least in the US and Canada, especially if they like shale!