22 Oct 08 21:24
An ESP will not separate gas and liquids. The turbulence within the pump is so high and the centrifugal traverse is so short that gas bubbles (not disolved gas or phase changes) will just get smaller as the pressure increases through the pump. If you have too much gas then you'll either overheat the pump from lack of lubrication or overheat the motor from lack of cooling.
The Net Positive Suction Head required (NPSH-r) for an ESP is a function of several pump parameters, but you can say for certain that the higher the capacity of the pump (in volume per unit time), the more pressure you need at the pump inlet (the higher NPSH-r). A mid-range pump will require about a hundred meters or so of head above the pump inlet. As casing pressure increases, the apparent head increases (3 bar increase in casing pressure raises the NPSH available by 30 meters), but the actual liquid level in the casing will tend to go down as this "extra" pressure forces liquid back into the formation. In moderately extreme cases, the casing pressure can push all the liquid back into the formation and introduce a very large quantity of gas into the pump which won't run for long.
In general, it is best not to rely on "artificial" head created by casing pressure and to keep casing pressure as low as practical.
David Simpson, PE
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