When to consider a jet pump?
A jet pump is a solution to fluids with corrosion, solids (sand) or moderate gas such as in the case of frac fluid recovery. Other successful applications are active aquifers, or reservoirs with pressure maintenance programs. In Frac fluid recovery, the same high-volume jet pump configuration can continue to provide satisfactory production until mixing losses decrease efficiency. This occurs when reservoir pressures or gas create conditions that increase horsepower requirements exponentially; however, at that point the completion can be modified to continue producing within the fluid circulation limits of the well bore. The following provides an overview of the energy transfer mechanism to provide insight to limiting factors.
High pressure fluid (normally produced water) is injected from the surface to a nozzle located in the well. The energy from the high-pressure fluid converts to a high velocity stream that mixes with incoming well fluid, transfer sufficient energy for the combined fluids to reach the surface. Understanding a little more about how this works in an oil well may help recognize where it can be applied.
The venturi principle models this energy transferring process where well fluids and gas are drawn to the mixing tube entrance by the static pressure created by the high velocity power fluid stream. If producing reservoir pressure is around 100 psi per thousand feet of lift, the well fluid enters at sufficient speed to minimize mixing losses as the energy is transferred. This is the case with frac fluid recovery, active aquifers or reservoirs with pressure maintenance programs and minimum gas.
As the pressure of well fluids decreases, the high velocity jet stream has a greater tendency to shear through the incoming well fluid that results in increased mixing losses and increased horsepower required to produce a barrel.
On the other hand, small amounts of gas increase the mobility and reduce the density of the produced fluid that lead to decreased mixing losses. The gas also reduces the weight of the combined fluids coming up the well, so horsepower required to lift a barrel decreases further; however, a larger amount of gas inflow requires a larger mixing tube opening where in most cases the ratio of nozzle area to mixing tube area is reduced while the density of the incoming well fluid is decreased where the smaller proportion of high velocity stream has a greater tendency to shearing through the low density mass leading to an increase in horsepower per barrel produced. These losses are greater than the effect of reducing the weight of the fluid column with the greater volume of gas. In these cases gas is not harming the jet pump but the greater mixing losses can exponentially increase horsepower per barrel produced.
Cavitation is said to occur when the high velocity stream from the jet pump creates a static pressure less than the vapor pressure of the incoming produced fluid causing vapor cavities. The result is momentary choking while pump intake pressure increases with the choking and damage to the mixing tube when the vapor cavities collapse as it encounters the high discharge pressure. Note that vapor pressure is estimated because it is difficult to predict the exact vapor pressure of the multiphase fluid at the conditions at the entrance of the throat; however, field observations indicate that the estimate is very close to results. It is still dependent on actual well characteristics.
The efficiency explained above can be applied in lower pressure gassy reservoir by applying the same technique used in beam pumping systems where gas flows up the casing annulus and the pump is set below perforations to increase downhole gas separation while increasing pump intake pressure. A concentric coil tubing or reduced diameter tubing jet pump can be powered by injected down the small diameter tubing while flowing up the production tubing and allowing gas to flow up the casing annulus. This is not to say that the landscape should go from pumping jacks to concentric jet pumps, but wells with frequent rod failures that strain economics should consider the concentric jet pump.
In conclusion, the jet pump is efficient in frac fluid recovery, active aquifers and fields with reservoir pressure maintenance. It can continue to produce the well while density and pressures promote low mixing losses. Fortunately, the jet pump can be adapted to all other lift methods, so its temporary use does not have to required workover cost.
The well completion of the jet pump can also be modified to allow for gas separation and separate flow to surface. Gas production in these completions is unlimited as proven by gas wells using the concentric tubing jet pump for dewatering.
An important part of any hydraulic lift application is the specific engineering and equipment selection for surface power and process systems. All have proven to be reliable, low cost and maintenance free in the most remote areas of the world.