Wednesday, October 11, 2000

When do logs grow underground?

Trees are not the only place to find logs. Well logs are graphs that show geologists what is in the ground where a well has been drilled. It could be water, oil, natural gas, or just a hole to find out what is under your feet.

Wells are drilled by equipment that looks much like a giant power drill. The machinery turns a bit (a cutting tool used to drill the hole that has teeth designed to grind through rock and soil), mounted at the end of a long string of pipes screwed together, one piece at a time. When the drilling is finished, the pipes and bit are taken out of the hole and other instruments, known as logging tools, are lowered on a wire to gather information about the subsurface geology. It may rub against the side and listen for echoes, it may glow with radiation and measure how the rocks react, or it may pulse with electricity and measure whether there is water or oil in the rocks. Whatever tool is used, the data is reflected on a long paper strip marked with curved lines indicating the measurements that were made in the well.

Well logs are fascinating to look at. You can actually "see" the type of rocks, liquids, and gases that are underground just by the shapes of the lines on the log, and the way different kinds of logs cross one another or run parallel.

This first picture is a sample of well log curves just the way
they are recorded from an oil well.

This second picture shows what can be done with logs by processing the data with a computer to emphasize different
aspects of the geology, or the water or oil in the rock.

If you are going to work for an oil company and make your living by finding energy for our world, you will need to make well logs of the oil and gas wells you drill.

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