PCP systems for gassy wells

In most operations, dissolved gas begins to evolve as free gas when the pressure drops as the fluid moves toward and then enters the well. Depending on the fluid properties and gas volumes, the free gas may coalesce and flow as a separate phase, or, as in the case of many heavy oil wells, it may remain trapped as discrete bubbles within the liquid phase (foamy oil). Gas entering the pump causes an apparent decrease in pump efficiency because the gas occupying a portion of the pump cavities is normally not accounted for in the fluid volume calculations. The pump must then compress the gas until it either becomes solution gas again or it reaches the required pump discharge pressure.

Handling gas interference

The best way to reduce gas interference is to keep any free gas from entering the pump intake. When possible, the intake should be located below the perforations to facilitate natural gas separation. Even if the pump can be sumped below the perforations, small casing/tubing annuli can lead to high flow velocities that can “trap” free gas and carry it to the pump intake, thereby reducing the effectiveness of the natural gravity-based separation. Thus, seating of the stator, which typically has a larger diameter than the tubing, either within or above the perforation interval should be avoided if possible. Another option is the use of slimhole PC pumps in such circumstances.

In gassy wells in which the pump must be seated above the perforations, passive gas separators that divert free gas up the casing/tubing annulus can be effective. In such cases, assemblies that centralize the pump intake in the center of the casing should be avoided because free-gas bypass tends to be more efficient in a skewed annular space. In directional or horizontal wells, it is best to have the pump intake positioned on the low side of the wellbore away from any free-gas flow, which naturally tends to be along the high side of the casing. With the gravity assistance available in such wells, a short tail joint can be used to locate the pipe intake on the low side of the well casing. Special intake devices are also available that incorporate a swivel assembly to ensure that the fluid intake port remains on the low side of the wellbore. Small-diameter or long tail joints should be avoided since flow losses within the tail joint can result in increased gas volumes entering the pump.

Gas production through the pump can lead to substantial fluctuations in rod-string loading, as illustrated by the field data shown in Fig. 1. Loading variations can be attributed to discharge pressure fluctuations associated with changes in the fluid-column density and to the lifting effects of the gas produced up the tubing. Pump friction may also vary because of changes in fluid lubricity. The load fluctuations can be significant, particularly when substantial percentages or slugs of gas enter the pump. Large, continuous changes in load may accelerate rod fatigue problems or damage surface power transmission equipment.

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  Fig. 1—Fluctuating polish rod loads in a gassy well application.

When attempts are made to maximize fluid rates in gassy wells, the pump speed should be increased in relatively small increments, with subsequent monitoring of the resulting effects on production rates in order to identify the onset of gas interference problems.

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See also

Progressing cavity pump (PCP) systems

PEH:Progressing_Cavity_Pumping_Systems