Electrical Submersible Pumps Manual
Usually, oil wells in the early stages of their lives flow naturally to the surface and are called flowing wells. Flowing production means that the pressure at the well bottom is sufficient to overcome the sum of pressure losses occurring along the flow path to the separator. When this criterion is not met, natural flow ends and the well dies. The two main reasons for a well’s death are as follows:
(1) their flowing bottom-hole pressure drops below the total pressure losses in the well, or (2) pressure losses in the well become greater than the bottom-hole pressure needed for moving the wellstream to the surface.
The first case occurs because of the removal of fluids from the underground reservoir; the second case involves an increasing flow resistance in the well. This can be caused by (1) an increase in the density of the flowing fluid as a result of decreased gas production or (2) various mechanical problems such as a small tubing size and downhole restrictions.
Artificial lifting methods are used to produce fluids from wells already dead or to increase the production rate from flowing wells; several lifting mechanisms are available to choose from. The importance of artificial lifting is clearly seen from the total number of installations: according to one estimate there are w2 million oil wells worldwide of which about 50% are placed on some kind of artificial lift [1].
One widely used type of artificial lift method uses a pump set below the liquid level in the well to increase the pressure so as to overcome the pressure losses occurring along the flow path. Other lifting methods use compressed gas, injected from the surface into the well tubing to help lifting of well fluids to the surface. Although all artificial lift methods can be distinguished based on the previous basic mechanisms, the customary classification is somewhat different as discussed below.
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