Before drilling commences, onshore, the well site is supposed to be sealed. The surface around the well head is covered with concrete and tarmac, around which a drain is dug and a bank built up. The drain is lined with a water-proof impermeable membrane to avoid the contamination of surrounding land from liquids either used in the drilling or escaping from the well. Within the bank, the drain and the concrete surface should be able to contain the equivalent of a few days production.
Boring a well involves using a giant drill. Onshore, the drill equipment is housed within an open steel girder tower called a Derrick, which is about 50m high and mounted upon a platform set about 11m above ground. Within the derrick, a system of pulleys and a wire rope, called a Drilling Line, acts like a crane, from the hook of which is suspended the Drill String.
The Drill String consists of 9m lengths of 25cm diameter, hollow, steel pipes, the lower lengths, called Drill Collars, being the heavier in order to keep the bore hole straight.
In order to drill, a drilling Bit (that 'bites' into the rock or mud strata through which the well is being bored) is screwed into the lowest drill collar and lowered through a Rotary Table mounted on the platform. The rotary table provides the necessary rotation for the drilling string and bit to bore the hole.
Necessary drilling fluids are passed under high pressure down inside the hollow tubes of the drilling string and through jets in the drill bit. The fluids are required to cool and lubricate the bit and to bring debris up the Annulus, which is the space between the drill string and the sides of the borehole. The drilling fluids also prevent the sides of the hole from caving in, and are used to control the pressures within the hole. Drilling fluids can be a simple mixture of water and clay (mud) or combinations of oil and chemicals. Usually, the fluid is stored and mixed on site, and having returned up the annulus, it can be cleaned and used again.
As the well is deepened, more sections of pipe are added to the string. When the well is deep enough sections of protective steel casing pipes are let down as a lining. Down to about 100m the diameter of the well is about 60cms diameter. When drilling has reached this depth the drilling string and bit are withdrawn, a narrower casing put in place and a cement slurry pumped down under high pressure into the annulus between the casing and the well wall to create a lining. Drilling then continues using a smaller bit; the process continues so that the well gets progressively narrower. In the Niger Delta, a typical exploration well might be drilled to 4000 metres below the ground, at which stage it is about 18cms in diameter.
A diagram of an exploration well, figure 7., shows how the progressively narrower Casings fit into one another and are cemented together to create the well.
Finally, a comparatively narrow pipe, known as the Production String, is run below the hydrocarbon producing rock (the sandstone sponge). The last casing and the adjacent part of the production lining is perforated to allow fluids to flow up to the surface. The decision is taken whether to complete the well as a Producer, oil having been struck, or to abandon it as a Dry hole, in which case, the drilling rig and the associated equipment is removed. For a total depth of 4000m, onshore, the operation takes sixty to seventy days.
Where the well is capable of producing, but is to be held for later production, the well head is capped and pressure control valves left in place. This ensures that the formation of any pressures that may build up are contained within the well and that leaks do not occur. The site is regularly monitored.
