Drilling is the first stage in the extraction of crude oil from the underground reservoir. Often many wells (called multilateral wells) are drilled into the same reservoir, to ensure that the extraction rate is economically viable. Also, some wells (secondary wells) may be used to pump water, steam, acids, or various gas mixtures into the reservoir to raise or maintain the reservoir pressure, and hence maintain an economic extraction rate.
There are two methods in drilling:
-
The older cable tool method used extensively until 1900, involves raising and dropping a heavy bit and drill stem attached by cable to a cantilever arm at the surface. It pulverizes the rock and earth, gradually forming a hole.
-
The cable tool system is generally preferred only for penetrating hard rock at shallow depths and when oil reservoirs are expected at shallow depths. The weight of the column is usually enough to attain penetration but can be augmented by a hydraulic pressure cylinder at the surface.
Preparing for Drilling:
Once the site has been selected, it must be inspected to determine its limits, and environmental impact studies may be required. Lease contracts, titles and access points for the land must be obtained and legally evaluated. For offshore sites, the legal jurisdiction must be determined. Once the legal problems have been resolved, the crew prepares to prepare the land; Preparation is essential and involves the following steps:
-
The land is cleared and levelled, and access roads may be built.
-
Because water is used in drilling, there must be a source of water nearby. If there is no natural source, a water well is necessary.
-
Reserve pit, which is used to dispose of rock cuttings and drilling mud during the drilling process and which is lined with plastic to protect the environment, is created.
Drilling Rig construction:
The anatomy of a drilling rig, although simple in a schematic representation is, in reality, quite complex and consists of the following systems:
Power system:
-
Large diesel engines to provide the main source of power
-
Electrical generators powered by diesel engines to provide electrical power
Mechanical system that is driven by electric motors:
-
Hoisting system that is used for lifting heavy loads and consists of a mechanical winch (draw works) with a large steel cable spool, a block-and-tackle pulley, and a receiving storage reel for the cable
-
Turntable that is part of the drilling apparatus
Rotating equipment (used for rotary drilling):
-
Swivel, i.e., a large handle, which holds the weight of the drill string; allows the string to rotate and makes a pressure-tight seal on the hole
-
Kelly, i.e., a four- or six-sided pipe that transfers rotary motion to the turntable and drill string.
-
Turntable or rotary table drives the rotating motion using power from electric motors
-
Drill string that consists of drill pipe (connected sections of about 30 ft=10 m) and drill collars larger diameter, heavier pipe that fits around the drill pipe and places weight on the drill bit)
-
Drill bit(s) at the end of the drill that actually cuts up the rock and comes in many shapes and materials (tungsten carbide steel, diamond) that are specialized for various drilling tasks and rock formations.
Casing:
which is a large-diameter concrete pipe that lines the drill hole, prevents the hole from collapsing, and allows drilling mud to circulate.
Circulation system:
pumps drilling mud (mixture of water, clay, weighting material and chemicals, used to lift rock cuttings from the drill bit to the surface) under pressure through the kelly, rotary table, drill pipes and drill collars:
- Pump—sucks mud from the mud pits and pumps it to the drilling apparatus
- Pipes and hoses—connect pump to drilling apparatus
- Mud-return line—returns mud from hole
- Shale shaker—shaker =sieve that separates rock cuttings from the mud
- Shale slide—conveys cuttings to the reserve pit
- Reserve pit—collects rock cuttings separated from the mud
- Mud pits—where drilling mud is mixed and recycled
- Mud-mixing hopper—where new mud is mixed and then sent to the mud pits
Derrick:
Support structure that holds the drilling apparatus; tall enough to allow new sections of drill pipe to be added to the drilling apparatus as drilling progresses
Blowout preventer
High-pressure valves (located under the land rig or on the seafloor) that seal the high-pressure drill lines and relieve pressure, when necessary, to prevent a blowout (uncontrolled gush of gas or oil to the surface, often associated with fire).
Drilling process:
Drilling an oil well is tedious and is often accompanied by difficulties. Some problems result from formation penetration and the occurrence of high-pressure gas, fissures, or unexpected high pressures in permeable rock.
Once the rig is set up, drilling operations commence. First, from the starter hole, a surface hole is drilled to a preset depth, which is somewhere above the location of the oil trap. There are five basic steps to drilling the surface hole:
-
Place the drill bit, collar, and drill pipe in the hole
-
Attach the kelly and turntable and begin drilling
-
As drilling progresses, circulate mud through the pipe and out of the bit to float the rock cuttings out of the hole
-
As the hole increases in depth, add new sections (joints) of drill pipes
-
Remove (trip out) the drill pipe, collar and bit when the preset depth (anywhere from a few hundred to a couple-thousand foot) is reached.
When the preset depth is reached, the casing pipe sections are run into the hole and cemented to prevent the hole from collapsing. The casing pipe has spacers around the outside to keep it centered in the hole. The cement is pumped down the casing pipe using a bottom plug, a cement slurry, a top plug, and drilling mud.
The pressure from the drilling mud causes the cement slurry to move through the casing and fill the space between the outside of the casing and the hole. Finally, the cement is allowed to harden and is then tested for such properties as hardness, alignment, and a proper seal.