When 3D seismic investigation has been completed, it is time to drill the well. Normally, dedicated drilling rigs either on mobile onshore units or offshore floating rigs are used. Larger production platforms may also have their own production drilling
The main components of the drilling rig are the derrick, floor, drawworks, drive and mud handling. Control and power can be hydraulic or electric.
Earlier pictures of drillers and roughnecks working with rotary tables (bottom drives) are now replaced with top drive and semiautomated pipe handling on larger installations. The hydraulic or electric top drive hangs from the derrick crown and gives
pressure and rotational torque to the drill string. The whole assembly is controlled by the drawworks.
The drill string is assembled from pipe segments about 30 meters (100 feet) long, normally with conical inside threads at one end and outside at the other. As each 30 meter segment is drilled, the drive is disconnected and a new pipe segment inserted
in the string. A cone bit is used to dig into the rock. Different cones are used for different types of rock and at different stages of the well. The picture above shows roller cones with inserts (on the left). Other bits are PDC (polycrystalline
diamond compact, on the right) and diamond impregnated.
As the well is sunk into the ground, the weight of the drill string increases and might reach 500 metric tons or more for a 3,000 meter deep well. The drawwork and top drive must be precisely controlled so as not to overload and break the drill string
or the cone. Typical values are 50kN force on the bit and a torque of 1–1.5 kNm at 40–80 RPM for an 8-inch cone. Rate of penetration (ROP) is very dependent on depth and could be as much as 20m per hour for shallow sandstone and dolomite
(chalk), and as low as 1m per hour on deep shale rock and granite.
Directional drilling is intentional deviation of a well bore from the vertical. It is often necessary to drill at an angle from the vertical to reach different parts of the formation. Controlled directional drilling makes it possible to reach subsurface
areas laterally remote from the point where the bit enters the earth. It often involves the use of a drill motor driven by mud pressure mounted directly on the cone (mud motor, turbo drill, and dyna-drill), whipstocks — a steel casing that
bends between the drill pipe and cone, or other deflecting rods, also used for horizontal wells and multiple completions, where one well may split into several bores. A well that has sections of more than 80 degrees from the vertical is called
a horizontal well. Modern wells are drilled with large horizontal offsets to reach different parts of the structure and achieve higher production. The world record is more than 15 km. Multiple completions allow production from several locations.
Wells can be of any depth from near the surface to a depth of more than 6,000 meters. Oil and gas are typically formed at 3,000–4,000 meters depth, but part of the overlying rock may have since eroded away. The pressure and temperature generally
increase with increasing depth, so that deep wells can have more than 200 °C temperature and 90 MPa pressure (900 times atmospheric pressure), equivalent to the hydrostatic pressure set by the distance to the surface. The weight of the oil
in the production string reduces wellhead pressure. Crude oil has a specific weight of 790 to 970 kg per cubic meter. For a 3,000 meter deep well with 30 MPa downhole pressure and normal crude oil at 850 kg/m3,
the wellhead static pressure will only be around 4.5 MPa. During production, the pressure will drop further due to resistance to flow in the reservoir and well.
The mud enters though the drill pipe, passes through the cone and rises in the uncompleted well. Mud serves several purposes:
- It brings rock shales (fragments of rock) up to the surface
- It cleans and cools the cone
- It lubricates the drill pipe string and cone
- Fibrous particles attach to the well surface to bind solids
- Mud weight should balance the downhole pressure to avoid leakage of gas and oil. Often, the well will drill though smaller pockets of hydrocarbons, which may cause a "blow-out" if the mud weight cannot balance the pressure. The same
might happen when drilling into the main reservoir.
To prevent an uncontrolled blow-out, a subsurface safety valve is often installed. This valve has enough closing force to seal off the well and cut the drill string in an uncontrollable blow-out situation. However, unless casing is already also in place,
hydrocarbons may also leave though other cracks inside the well and rise to the surface through porous or cracked rock. In addition to fire and pollution hazards, dissolved gas in seawater rising under a floating structure significantly reduces
The mud mix is a special brew designed to match the desired flow thickness, lubrication properties and specific gravity. Mud is a common name used for all kinds of fluids used in drilling completion and workover and can be oil-based, water-based or synthetic,
and consists of powdered clays such as bentonite, oil, water and various additives and chemicals such as caustic soda, barite (sulfurous mineral), lignite (brown coal), polymers and emulsifiers.
A special high-density mud called "kill fluid" is used to shut down a well for workover. Mud is recirculated. Coarse rock shales are separated in a shale shaker before it is passed though finer filters and recalibrated with new additives before
returning to the mud holding tanks.