See also: Petroleum Industry
Oil drilling is part of a very long and complicated process used to locate, extract, and pump oil from wells. It involves the exploration for oil, setting up a drilling rig, drilling exploratory oil wells, testing the productive capacity of a well, and then extracting the oil from the well. Rotary drilling is the primary method used to drill oil wells today. Wells are either drilled straight down from the surface towards a reservoir trap or at a slant or angle using a method called directional drilling.
Before oil wells were even drilled, geologists and engineers had attempted drilling wells for water. The technology used to drill these water wells was eventually adapted for use in the drilling of oil wells.
The first type of oil drilling ever employed was the “percussive method” practiced by the Ancient Chinese. Percussive drilling, also called cable-tool drilling, entailed the raising and lowering of a heavy tool repeatedly down onto the same spot of overlying earth. This constant pounding would result in the fracturing of rock and dirt. The loosened rock and dirt were then removed from the hole.
A method called impact drilling was used to drill the very first oil well in Titusville, Pennsylvania in 1859. A chisel bit was suspended from a cable to a lever at the surface. The raising and lowering of the lever resulted in the pounding of the bit in the bottom of the hole, which slowly chipped away at the rock. Impact drilling was a slow process, having to be stopped periodically to clear away loosened debris from the hole. It was also important to make sure the hole was free of other liquids. One of the drawbacks was that dry drilling often led to large oil gushers.
Today the majority of oil wells are drilled using the rotary drilling method. Rotary drilling was first developed in Europe in the 1930s. It quickly was replaced by cable-tool drilling. Rotary drilling derives from the drill bit being attached to the end of the drill string, rotating into the earth with a motion similar to that of a screwdriver.
After a drill site has been prepared, an oil rig consisting of multiple systems is used to drill a starter hole. These systems include a primary power system that consists of large diesel engines or electrical generators powered by diesel engines. Every oil rig also features a mechanical system driven by electric motors and a hoisting system for lifting heavy loads. The hoisting system consists of a heavy winch called a drawworks, also known as a derrick. The drawworks includes a large, steel cable spool, a block-and-tackle pulley, and a receiving storage for the cable. The drawworks or derrick must be tall and strong enough to hold up the drilling apparatus and allow for new sections of pipe to be added to the drill string as drilling progresses.
When rotary drilling is the method used to drill an oil well, the oil rig is outfitted with rotating equipment. The equipment used in rotary drilling is very complex and involves the following components:swivel, kelly, drill string, drill bit, casing, circulation system, and blowout preventer.
When the oil rig is finally set up drilling operations begin. Drilling crews use a starter hole to drill a surface hole down to a preset depth, usually directly above where an oil reservoir is predicted to be. Drilling the surface hole occurs in five steps. The first step involves placing the drilling bit, collar, and drill string into the hole. The kelly and turntable are attached and the crew begins drilling. As the drilling progresses, drilling mud is circulated down through the drill string and out through the drill bit to float the rock cuttings up to the surface and out of the hole. As the borehole deepens, new sections of drill pipe called joints are added. When the preset depth is reached, the drill string, collar, and drill bit are removed from out of the hole. Casing is inserted into the hole. Cement is pumped down into the casing pipe using drilling mud. The pressure of the drilling mud moves the cement through the casing so it fills the space between the outside of the casing and the hole. The cement hardens and provides a seal. A bottom plug and top plug are also used.
The drilling continues in stages in this fashion. Eventually the rock cuttings will reveal that oil has been struck. When this occurs, the final depth of the oil well has been reached. The entire drilling apparatus is removed from the hole and several tests are conducted to verify that an oil reservoir has been tapped. For example, mud logging involves lowering electrical and gas sensors down into the hole to take measurements of rock formations. Drill-stem testing is another method of testing for oil, whereby a device is placed down into the hole to measure pressure. Sometimes core samples of rock are taken to determine whether or not oil is present.
When the well is finished and oil flows into the casing, a perforating gun is lowered into the well to begin production. The gun is filled with explosive charges, which are detonated to create holes in the casing for the oil to flow through. Production tubing is then placed down into the hole to serve as a conduit for oil and gas to flow up through the well to the surface. An apparatus known as a packer is run down the side of the tubing to the level of production. It is expanded to form a seal that surrounds the outside of the tubing. A multi-valved structure known as a Christmas tree is connected to the top of the tubing and cemented to the top of casing to form the wellhead. The structure is a configuration of valves, blocks, and control systems. The Christmas tree is used to control the flow of oil from well. Once the flow of oil is started into the well the drilling rig is removed and a permanent pump and production equipment is set up to support the continuous extraction of oil from the well.