The Dummy's Guide to Density Logging

porosity from density

Someone woke up the staff petroleum geologist the other day to ask about a potential nominee, a process akin to poking a hibernating bear. It's not that said geology type is a grump – he certainly is – but that he's had to read through so much misinformation about the industry where he spent three decades that he's tired of it. Sadly, the WiseGEEK.com post "What Is Density Logging?" (now, for unknown reasons, at niche site AboutMechanics.com) which the site attributed to freelance contributor Mary McMahon, did not improve his mood...

Up front, the most important thing to know about density logging is that it is one of several methods for evaluating the porosity of the rocks around a wellbore. We know McMahon went straight to Wikipedia for her information, but curiously, the word "porosity" (which appears in the first paragraph at Wikipedia) appears only once in her post. That's what happens when freelancers "explain" processes and terms of which they are ignorant.
Mary opens by telling her readers that,

"Density logging generates information about the density of materials inside a wellbore, a hole drilled to extract or explore natural resources such as water, gas or oil. This information is important for exploratory drilling and research applications, where geologists want to know more about what is under the surface of the earth and how it is behaving."

Our corrections:

• A wellbore is an empty space. Explorationists want to know about the rocks adjacent to the wellbore.
• We would say "explore for," not just "explore."
• A density log isn't going to have much to tell anyone about how "it is behaving," whatever "it" is!

Mary then rewords someone's explanation of a density tool, again making several mistakes. Here's some of what she says about the behavior of gamma rays as they pass through the annulus between source and receiver:

• "...the rays... travel through the rock and fluids inside the well." – Once again, Mary, a well is a hole!
• "As they move, their behaviors change." – Ummm, no, that's not what happens: gamma rays are absorbed by the hydrogen in hydrocarbons and water, but pass through rocks without hydrogen. Density, then, is estimated by the percentage of the gamma rays that complete the trip from source to receiver. [Note: that's a middle-school level explanation; the process is more complicated]
• "Information from density logging provides data about the overall density of the materials and can also offer insight into the mineralogy of the wellbore." – Backwards, Mary: a petrophysicist uses known mineralogy to "tune" the results of the log and estimate porosity.

And here's what McMahon has to say about the data collected:

• "...technicians can calculate the porosity, a reflection of the density of the liquid inside the well." – How many times do we have to tell you, Mary, that the log measures the rock beside the wellbore, not "inside" it? And let us tell you, a logging engineer will not look kindly on being called a "technician"!
• "All of this information may be important for tasks like deciding how to proceed with drilling, determining if a given region will yield useful materials..." – "Region"??? No, dummy, a density log is not a regional tool. It's used to help decide whether an interval in a well will be economically productive. Nothing regional about it at all!

The remainder of Mary's post is a mishmash of other people's information, badly reworded. The outcome when you ask the ignorant to describe the technical? A Dumbass of the Day award.

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