Production consists of bringing the hydrocarbons contained in the substratum to the surface. This requires the use of a large number of
wells. A field spreads over a vast area, at least several km² and sometimes more than 100 km². A traditional well (vertical or slightly deviated) only draws oil or gas from a radius of a few tens of metres. Moreover, such wells only cross the reservoir over the limited height of a vertical or near vertical cross section. A large number of vertical wells would therefore be necessary to completely extract the contents of a reservoir. The horizontal well technique has totally revolutionised the industry, because such wells have a much greater length of contact with the reservoir. Thus, the technique enables a significant reduction in the number of wells necessary for a given development.
Production drilling presents a number of challenges.
What is the principle of extraction?
The basic principle is to generate pressure at the bottom of oil or gas wells, inferior to the pressure in the reservoir. As a consequence of this pressure difference, the hydrocarbons will move towards the well and thence to the surface. In practical terms, the well is totally lined with tubing right down to the reservoir. This tubing, difficult to move once it is fixed into position, guarantees the operational effectiveness of the well throughout its working life. The oil and gas are brought to the surface via another tube, in oil field jargon the (extraction) tubing, placed in the lining. This tubing is detachable and can be changed whenever corrosion or
deposition
problems appear.
Sometimes, the oil field pressure is sufficient for the hydrocarbons to make their own way to the surface; in this case the well is said to be “eruptive”. In other cases wells are never eruptive. And in all cases, the oil field pressure diminishes gradually as production continues. After a certain time, it is no longer sufficient for eruptive extraction and it becomes necessary to stimulate production, what is called assisted recovery.
On arrival at the surface, the output from the extraction wells begins its journey through the surface installations. |