Drill Bits and Well Samples (cuttings)

Well Samples

Every time a well is drilled, PG's watch the drilling of the well and make careful records of all the rocks and formations that the well drills through.  A PG is able to examine the rocks cut by a well because wells are drilled using a "mud system".  It's quite simple.  Look at the picture on the left.  The drill bit and pipe are hollow.  A chemical that looks like mud (that's why we call it "mud") is forced down the drill pipe to the bottom of the hole.  As the rock bit grinds its way down, it cuts off small pieces of rock.  The mud comes out the drill bit and flows back to the surface, carrying the well samples, or "cuttings", with it.  The mud serves to lubricate and cool the bit, and also bring the important rock cuttings to the surface..

At the right is a picture of a typical drill bit, called a "three-coned bit," or a "Tricone (try-cone)." As the bit rotates at the end of the drill pipe, the sharp teeth cut pieces of rock from the hole.

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.

Tricone Bit - Head-On View

 For very fast penetration, particularly in shales, a special (and expensive) bit called a "PDC" (polycrystalline diamond compact) is sometimes used.  This bit has carbide or man-made industrial diamonds set in fixed blades.  The cutters simply scrape the rock away during drilling, and do not rotate and grind the way the cones of a tri-cone bit do.

A well may be only a few hundred feet deep, or over 30,000 feet deep.  The 30,000-footers cut through a lot of rocks that have to be examined carefully, and it takes many months to drill a 30,000' well! 

The PG's assistant records all the information on a very long piece of paper called a "mud log.".  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