Artificial Lift Methods

?Secondary Oil Recovery

In the form of pressure, in the compressed fluid itself and in the rock, all reservoirs contain energy, making it simple to recover. However in some reservoirs, oil and gas as easily and require special techniques to move fluids to the surface from the pore spaces in reservoir rock.

In a reservoir, the driving force is either water or gas. When there is a big underlying aquifer where the water is able to flow into the oil layer, a water drive reservoir takes place.

Pressure drop is created once production starts from oil layer. To replace the empty pore spaces in the reservoir aquifer expands and takes it place. The energy of a gas drive reservoir is derived from the expansion of a gas cap. The pressure will be sufficient to push the hydrocarbons to the surface early in a well's early life. Some methods of lifting the oil to the surface must be introduced when the pressure differential becomes insufficient for the oil to flow naturally. One can either use pressure maintenance or artificial lift methods. To keep the pressure at an acceptable level, pressure maintenance is about injecting water or gas into the reservoir.

By adding energy to the fluids produced in the well, artificial lift systems distinguish themselves from pressure maintenance; energy is not transferred to the reservoir. [1]

Artificial Lift Methods

Artificial lift is a method that is used to increase pressure within oil wells and encourage oil to reach the surface. Artificial lift is used to recover more production when the reservoir's natural drive power is not strong enough to push the oil to the surface.

This is usually accomplished by using a mechanical device inside the well (known as the pump or velocity string) or by reducing the weight of the hydrostatic column by injecting gas some distance down the well into the liquid. [1]

Purpose of Artificial Lift Methods

Reduction in bottom hole flowing pressure is the main purpose of artificial lift methods. By reducing Pwf fluids can be easily moved to the surface. With passage of time reduction in reservoir pressure because of produced fluids results in installation of artificial lift methods. The choice of artificial methods depends upon the type of produced fluid and other well parameters. [1]

There are several kinds of available artificial lift techniques such as sucker rod pumps, hydraulic oil well pumps, centrifugal electric submergible pumps, rotating rod pumps, plunger lift and gas lift. [1]

Artificial lift methods

·????????Gas Lift

·????????Electrical Submersible Pump (ESP)

·????????Hydraulic Submersible Pump (HSP)

·????????Jet Pump

·????????Progressive Cavity Pump

·????????Beam or Sucker Rod Pump

Gas Lift

In this method at specific depth in tubing string, high pressure gas is injected from the surface which results in reduction of fluid column density, ultimately reduces bottom hole flowing pressure and helps in movement of hydrocarbons to the surface.?

Gas lift consist of injecting high pressure gas from the surface to a predetermined tubing string depth to decrease fluid density in wellbore therefore reducing the hydrostatic load on formations which will allow reservoir energy to cause inflow and commercial hydrocarbon volumes can be boosted or displaced to the Surface. The gas injected into the tubing string through the operating valve makes it possible for the well to resume or increase production by:

·????????Reduction in density of fluids above injection point.

·????????As the fluid rises up the tubing string, some of the injected gas dissolves into the produced fluids (under saturated) and the remaining in the form of bubbles will expand as the fluid rises. [2]

Advantages

·????????This method is suitable for those wells which have high Gas to Oil Ratio, high productivity index and have higher bottom hole pressure.

·????????It is suitable for those wells having medium production rates.

·????????It is suitable for water drive reservoirs having high bottom hole pressure

·????????This method has low maintenance and operational cost.

·????????It is highly flexible method and can be used for wells having rates from 10 to 20000 bpd.

·????????This method can handle solids in production as well.

·????????This method provides full bore tubing string access. [3]

Disadvantages

·????????For applicability of this method gas should be available.

·????????This method has higher installation costs such as installation of compressors.

·????????Crudes having API of less than 15 degree are difficult to lift with this method.

·????????This method is difficult to restart after shut down.

·?????????Due to Joule Thompson Effect after injection of gas increases the chances of wax precipitation.

·????????This method applicability is limited by reservoir pressure and bottom hole flowing pressure. [3]

Electrical Submersible Pump

The Electric Submersible Pump (ESP) is a down-hole electric motor-driven multi-stage centrifugal pump. The pump unit consists of a stacked series of centrifugal rotating impellers running inside a stack of stationary diffusers on a central drive shaft. Depending on the back pressure held on the system, the flow rate through the pump will thus vary.

The type and number of pump stages are determined by the necessary rate and pressure to lift liquids to the surface. [4]

Advantages

·????????This method is suitable for those wells which have low Gas to Oil Ratio, and have high productivity index.

·????????This method is suitable for wells having high water cut.

·????????One of this method biggest advantage is that it can lift higher volume of fluids to the surface.

·????????It is highly flexible method and can be used for wells having rates from 50 to 60000 bpd.

·????????Well performance monitoring and real time pump performance can be monitored through ESP.

·????????Electrical submersible pumps can be quickly restarted after shut down.

·????????Electrical submersible pumps have a long life.[3]

Disadvantages

·????????This method is not suitable for wells having high GOR and also for wells that have sand production.

·????????In order to replace the pump tubing string has to be pulled out.

·????????This method has high maintenance costs especially for offshore fields.

·????????Electrical submersible pumps require high voltage of around 1000 V.

·????????This method cannot be used for oil wells having very low production. (<150 BPD).

·????????Viscous crude oil reduces the efficiency of Electrical Submersible pumps.

·????????Wells having high temperatures can damage the electrical motors .[3]

Hydraulic Submersible Pump

A high pressure power fluid pumped from the surface is used by hydraulic pumps which powers downhole installed positive displacement pumps. Pressure energy of fluid is converted into velocity when it passes through nozzle or venture. In throat of pump this high velocity/ low pressure fluid combines with the production fluid. [4]

Next fluid moves into diffuser which reduced the velocity and helps in increasing fluid pressure which allows the fluid to travel to the surface.

Hydraulic Submersible Pump

A high pressure power fluid pumped from the surface is used by hydraulic pumps which powers downhole installed positive displacement pumps. Pressure energy of fluid is converted into velocity when it passes through nozzle or venture. In throat of pump this high velocity/ low pressure fluid combines with the production fluid. [4]

Next fluid moves into diffuser which reduced the velocity and helps in increasing fluid pressure which allows the fluid to travel to the surface.

Advantages

·????????This method can be applied on wells having small tubing diameters.

·????????Hydraulic Submersible pumps require few stages than Electrical Submersible pumps because they have three to four times higher speed than ESP.

·????????These pumps don’t need any down-hole electrical connections.

·????????These pumps can handle fluid rates of around 59 to 2000 bpd.

·????????Hydraulic Submersible pumps can be controlled by the supplied power fluid.

·????????Power fluid used in this method can be returned to the surface with oil production. [3]

Disadvantages

·????????These pumps require higher maintenance because they have moving parts installed in it.

·????????Clean power fluid (solid free) should be used for safe operation.

·????????These pumps cannot be used for wells having high Gas Oil Ratio.

·????????Viscous crude oil reduces the efficiency of Electrical Submersible pumps. ?[3]

Jet Pump

The only form of artificial lift that does not require down-hole moving components is jet pumps. The jet pump is a dynamic-displacement ejector-type pump operated by a high-pressure power fluid stream that converges into a jet in the pump nozzle. The high-velocity, low-pressure jet is mixed with the well's fluid downstream from the nozzle.

In a diffuser, the stream of the mixture is then expanded and pressure is built up as the flow velocity drops. [4]

Advantages

·????????One of the biggest advantage of Jet Pump is that it doesn’t involve any moving parts.

·????????It can be easily installed and retrieved through wire line.

·????????A jet pump doesn’t need any type of down hole electrical connections.

·????????This pump is ideally for remote areas because it can be easily operate.

·????????Power fluid used for jet pumps doesn’t need to be clean like in Hydraulic piston pumps. [3]

Disadvantages

·????????Jet pumps are less efficient than other pump systems.

·????????Jet pumps require large volume of power fluid.

·????????Appropriate power fluid selection is important for jet pumps.

·????????These pumps are very sensitive to change in back pressures.

·????????In order to gain proper efficiency from jet pumps they require at least 20% submergence. [3]

Progressive Cavity Pump

In low to moderate rate wells, especially onshore and for heavy (and solid laden) fluids, progressive cavity pumps (PCPs) are a common form of artificial lift. PCPs, unlike Jet pumps, ESPs and HSPs, are positive displacement pumps.

Their operation includes either a metal or an elastomeric spiral stator to rotate a metal spiral rotor inside. Rotation causes a constant volume cavity formed by the rotor and stator to be displaced. [4]?????????

Advantages

·????????This pump is quickly repaired by replacing its rotor and stator as whole unit because of its simple design.

·????????In absence of gas it has high volumetric efficiency above 80 percent.

·????????This type of pump can be useful for viscous crude oils.

·????????When compared with ESP and HSP these pumps cannot formed emulsion due to its low pumping speed.

·????????It can pump oil upto 6000 BPD.

·????????These pumps have long life.

·????????Because of their simple design and easy to operate they are best for remote areas. [3]

Disadvantages

·????????These pumps require high starting torque.

·????????Their life is short in presence of abrasive fluid.[3]

Sucker Rod Pump

The Sucker Rod Pump is the world's oldest and most popular artificial lift process, simple in design and still widely used. In low production wells in shallow to mid-deep oil fields, it is very economical.

In this system there is a vertical displacement pump which is combination of barrel having a standing valve at its bottom and a travelling valve connected to plunger. [2]

The downhole Plunger is connected by the Sucker rods string mechanically to a surface walking beam. The pump is rocked up and down by an electric or reciprocating motor driven by the movement of the walking beam.

The standing valve opens during the plunger's upstroke, the travelling valve closes and fluid fills the barrel. The travelling valve opens during the down stroke, the standing valve closes and the fluid in the barrel is shifted into the tubing. [2]

Advantages

·????????It is most widely used artificial lift system in the world because of its simplicity and cheapness.

·????????These types of pumps can be used in oil wells having rates of less than 100 bpd.

·????????They can pump viscous crude oil and requires low operating expenditure.

·????????Their maintenance cost is low and they are easy to operate.

·????????They require low intake pressure as compared to other pumps.[3]

Disadvantages

·????????Its capacity is decreased with increased in depth.

·????????Presence of gas halts its operation so they are highly sensitive.

·????????They cannot be installed in offshore because equipment is too heavy.

·????????Sucker rod pumps cannot be useful in highly deviated wells.

·????????They are highly suspected to corrosion.

·????????Presence of scale and wax impacts sucker rod operation. [3]

References

1.?????The Technology of Artificial Lift Kermit.E Brown (The University of Tulsa) Volume (2a).

2.?????Baker Hughes Centrilift.2008., "Electrical submersible Pumping System Handbook”. Edition v2.

3.?????MASTER’S THESIS (Artificial Lift – Electrical Submerged Pump, best practice and future demands within subsea applications) university of Stavanger ?present by Bernt St?le Hollund.2010

4.?????Basic Artificial Lift. Available (Canadian Oil well Systems Company Ltd-2010).