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Drill Bits and Well Samples (cuttings) Well Samples At the right is a picture of a typical drill bit, called a "three-coned
bit". As After the cuttings reach the surface, the PG or his assistant look at them through a microscope to see whether the rocks being drilled are sandstone, limestone, or shale, whether they have porosity, and whether any oil can be seen in them. They also do some simple chemical tests to help them decide what they are seeing. Usually, a sample is examined every 10 feet.
For very fast
penetration, particularly in shales, a special (and
expensive) bit called
A well may be only a few hundred feet deep, or over 20,000 feet deep. The
20,000-footers cut through a The PG's assistant records all the information on a very long piece of paper called a "mud log" (what else?). He uses special symbols to illustrate what he sees in the rock samples. Once a PG has several mud logs, he can compare one to another and start deciding where the tops and bottoms of the rocks he is interested in (the ones with oil in them) are located.
At the right are well samples, or "cuttings", from the bottom of the hole. The cutting are carried to the surface by the drilling mud. These samples have had the mud washed off of them.
Most photos, A Primer of Oil Well Drilling
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