Drill Bits and Well Samples (cuttings)
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. The drill bit and pipe are hollow. A chemical that looks like thick 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.
At the right is a picture of a typical drill bit, called a "three-coned bit". As
the bit rotates at the end of the drill pipe, the sharp teeth cut pieces of rock from the hole
After the cuttings are carried by the mud to 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.
A well may be only a few hundred feet deep, or over 20,000 feet
deep. The 20,000-footers cut through a lot
of rocks that have to be examined carefully, and it takes months to drill a deep 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 cuttings are carried to the surface by the drilling mud. These samples have had the thick "mud" washed off of them.
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. The drill bit and pipe are hollow. A chemical that looks like thick 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.
At the right is a picture of a typical drill bit, called a "three-coned bit". As
After the cuttings are carried by the mud to 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.
A well may be only a few hundred feet deep, or over 20,000 feet
deep. The 20,000-footers cut through a lot
of rocks that have to be examined carefully, and it takes months to drill a deep 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 cuttings are carried to the surface by the drilling mud. These samples have had the thick "mud" washed off of them.
