Drilling of onshore oil wells

Before becoming petrol and plastic, oil needs to undergo a very complex artificial production process which starts with the search for oil fields and, through the extraction, processing and transportation stages (often taking place in countries very far away from each other), it ends with the petrol available at the local gas station or the rubber hose at the shop round the corner. Drilling wells is the only way to assess the value of a field, i.e. the type and quantity of the hydrocarbons contained therein. Drilling a well is a long and expensive, albeit simple, operation.
Rocks are drilled with a rotating bit mounted at the end of a series (battery) of mutually screwed steel pipes (rods), which is extended as the well becomes deeper. The rods are supported by a 50-meter tall tower (derrick) and their rotation is ensured by a rotating plate operated by means of a dedicated electric engine. The bit is made of very hard material and, in some cases, equipped with components of synthetic diamond.
The rod battery is as long as the well is deep. In some cases an extension of 6/7,000 metres is reached, whereas the weight supported by the derrick can reach 500 tonnes. The rods are empty to ensure the circulation of an ad hoc mud which greases and cools the bit supports the well walls and, when it goes back to the surface, removes the debris resulting from drilling the rocks. At pre-defined depth, the hole is covered with steel pipes (casting) to reduce its diameter gradually from 75 to 15/20 centimetres.
Modular drilling plants transported by truck are used on the mainland. A 1,000 metre well is drilled in less than a month, but in the case of wells exceeding 6,000 metres, over 45 million Euro and one year of constant round-the-clock drilling are needed.
Off shore drilling techniques are the same although the plant features vary. Up to a 100-metre depth, mobile self-lifting platforms (jack-up) including a hull supported by a sliding scaffolding (legs) are used. The legs of the structure stand on the sea bottom and keep the hull 15-20 metres above the sea surface to avoid the impact of waves and tides. In the case of 6/700 metre wells, floating platforms are used. After they are fixed to the sea bottom, they float above undersea hulls. If the well is deeper (up to 2,500 metres), drilling ships are necessary, featuring a hole in the hull to operate the telescopic pipes (riser).
During the drilling, the debris produced is continuously analysed, in order to evaluate if the quantity and quality of extractable hydrocarbons is sufficient to pay the production costs back. During this stage, before going on with the actual well development and production stages, other “perimeter wells” are drilled.
The drilling stage is one of the most critical and delicate of the petroleum life cycle and can cause strong environmental impacts. In fact, during drilling a lot of rock fragments are produced which are covered with the so-called “drilling mud”. Drilling mud is a complex mixture made up of water-based and oil-based chemical additives, utilised to prevent the borehole from collapsing during drilling. In the past, drilling mud was accumulated and abandoned on the site. Nowadays, disposal techniques have changed and the mud is treated and then properly disposed of in order to reduce the environmental impact to zero. First, depending on the composition of the mud, the water- or oil-based fluid is separated from the mud and all the potentially harmful substances are eliminated. Both the base oil and water are recovered and recycled, while the solid decontaminated component can follow three different courses: it can be taken to the dump, it can be reused as building material, for roads or bricks for example, or lastly it can be incorporated into the soil.

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