Drilling Fluid: Lifeblood of the Well
Drilling fluids are referred to as mud. You can see why.
In 1900, while drilling an oil well in Spindletop, Texas, workers ran a herd of cattle through a pit filled with water. The mud that resulted, a viscous, muddy slurry of water and clay, was pumped into the borehole. Drilling fluids are still called mud, but engineers no longer rely only on water and clay. Instead, they carefully design compounds and mixtures to meet specific needs under various drilling conditions. Modern drilling fluids are truly the lifeblood of the well. Today's deep wells could not exist without them.
Rotary drilling from an offshore rig.
Long ago, people drilled for water, not for oil. In fact, they were annoyed when they accidentally found oil since it contaminated the water! Early wells were drilled to provide water for drinking, washing, and irrigation, and for brine, used as a source of salt. It was only in the 19th century that drilling for oil became widespread as industrialization increased the need for petroleum products.
The earliest records of well drilling date back to the 3rd century B.C.E. in China. The technique—cable-tool drilling—involved dropping a heavy metal drilling tool and removing the pulverized rock with a tubular container. The Chinese were relatively advanced in this art and are credited with the first intentional use of fluids in the process of drilling. The fluid used was water. It softened the rock, making penetration easier, and aided in the removal of the pieces of pulverized rock known as cuttings. (It is important to remove the cuttings from the borehole so the drill bit is free to dig further.)
In 1833 a French engineer named Flauville was watching a cable-tool-drilling operation in which the drilling apparatus struck water. He realized that the gushing water was very effective in lifting the cuttings out of the well. The principle of using moving fluid to remove cuttings from the well bore was established. He conceived of an assembly in which water would be pumped down the inside of a drilling rod and carry cuttings with it as it returned to the surface in the space between the drilling rod and the wall of the well bore. This procedure remains standard today.
Rotary drilling has largely replaced cable-tool drilling. With this technique the drill bit is at the tip of a rotating pipe. The process is similar to that used with a handheld electric drill or auger that you might use to drill into a piece of wood. But instead of drilling a few centimeters or inches into wood, modern oil wells may reach thousands of meters or feet into the ground. When wood is being drilled the cuttings are cleared out of the hole by way of spiral grooves along the shaft. This works for a shallow hole but not for a deep well. In the latter case, the cuttings are carried up to the surface along with the circulating mud.
As wells get deeper, drilling fluids have taken on increased importance, serving a number of purposes and solving a variety of problems that vary greatly from place to place.