Field personnel install a 10,000 b/day ESTSP. (Images courtesy of Canada’s Can-K Group of Companies Inc.)

Twin-screw technology has been around for years, but until now it has never been used downhole. The technology was not applied for several reasons. If the pump is going to be used downhole, it has to be designed small enough to fit into the casing, which presents a challenge. Reducing the size, though it presents a hurdle to designers, is not the only consideration. A pump that is deployed downhole must be capable of handling high differential pressures to work in deep wells and handle solids encountered without any filtering systems.

Facing downhole challenges

Investment in technology development over the past eight years has led Can-K to the creation of a twin-screw pump that has the potential to replace electrical submersible pump (ESP) and gas lift technology.

There are three types of subsurface, twin-screw multiphase pumps that are identified by the way they are installed. The electric submersible twin-screw pump (ESTSP) is installed exactly like an ESP, which is a primary appeal. Placing the ESTSP does not require any additional equipment or expertise. The topdrive twin-screw pump (TDTSP) is sucker-rod driven or coiled-tube driven, and the rigless top-drive twin-screw pump (R-TDTSP) can be installed using coiled tubing, slick line, or a sucker rod.

Each type of pump is commercial for different applications and provides advantages when compared with ESP and gas lift systems.

The pumps come in various dimensions, with several models capable of directly replacing ESP systems.

Additional custom-made models also are available. Rates vary from 150 to 56,000 b/d for subsurface pumps, depending on the casing size. In surface applications, the pump can manage rates as high as 600,000 b/d using screws as large as 485 mm in diameter.

Advantages

The ESTSP has a number of significant advantages over a traditional ESP. Because it is a positive displacement pump, it is always volumetric, as pressures remain the same with varying speed. Speed is the only parameter to be adjusted on this pump, and it can be changed to alter the pump rate while keeping pressure constant. An additional advantage in using the ESTSP is that there is no up or down thrust. The design also incorporates balancing pistons (patent pending). The pump does not distinguish between gas and liquid because it interprets volume only, so it can handle very high gas at intake conditions. Unlike a centrifugal pump, the twin-screw unit has no best efficiency point, and it can draw down to 0 psig (vacuum conditions).

Another feature is that it is excellent for asphaltene wells because asphaltene will not break out due to minimal shear. Residual asphaltene improves efficiency because it increases viscosity, a fact that was clearly noted during field applications.

Though the twin-screw pump is not a sand pump, it can handle more sand than an ESP. The twin-screw design only conveys solids; because it does not have a centrifugal component, it does not throw the sand around.

Field results

Additional benefits surfaced when the ESTSP underwent field trials.

In a high gas application, the ESTSP did not heat the motor to the same levels as an ESP. This is possibly due to the fact that mist forms on the motor as the gas is continuously pumped. The temperature rise was only a few degrees even when moving approximately 96% gas volume. Further studies are required to understand this phenomenon.

Because a downhole pump is often run at very low and very high rpm, the motor for this design is high-powered so the pump can be run at higher maximum rpm to perform greater gas displacement. Nothing restricts gas from moving through the pump or through the casing. It is always recommended to ensure that approximately 3% of the liquid passes through the motor to keep it cool. In a sour well all of the gas can be moved through the pump if required, but a larger pump size would be required.

The TDTSP system is designed such that the installation is exactly the same as that for progressive cavity pump. The system can be run at temperatures to 536ºF (280ºC) ambient and has no minimum speed. The fluid actually goes through the patented thrust bearings. When used with the patented Dass rotary system, the axial load created by the pump can be eliminated.

The rods are centralized, and if a downhole gearbox is used, the sucker rod rpm can be reduced. Adding a gearbox adds more components downhole, and the potential chance of failure is increased. However, the certification process has eliminated conceivable risks in the system.

Ease of use

The subsurface twin screw pumps’ capacity to handle gas directly tackles the issues posed by inefficient gas lift operations, especially in areas where gas is scarce and when no other major operational issues need to be taken into account.

The pumps can also be installed with a Y tool, and all of the other methods used to install ESP systems can be used for ESTSP systems. All of the bellows used in the system are made of various grades of stainless steel. There are no elastomer systems.

Any existing ESP protector can be used since the thrust is taken care by the pump itself. The rotation can be clockwise or counter-clockwise but must be determined prior to pump assembly.

When using ESTSP gas separators, gas-handling systems are not required. The pump comes with built-in thrust sections.

The pumps are made to order, and delivery is available within 120 to 180 days. However, the company is using various casting models to reduce the cost of manufacturing and expediting delivery using Lean Manufacturing techniques.

To size the ESTSP, the system was simplified in such a way that once the parameters for the chosen ESP software have been established, the installation depth is the only remaining variable. Once this is done, production reported in barrels per day at intake conditions completes the selection process. The pump can generate up to 3,500 psi differential pressure using multiple stages.

In most fields, one or two models will suffice to service the whole field because the rates can be modified by varying the speed. The minimum and maximum rates are used to fine-tune the pump.

The pump can be landed vertically or horizontally, or it can be slanted. The same principles of dog leg for landing ESPs applies to landing twin-screw multiphase pumps. If there are no other well-related issues or sand issues, drawdown is close to the intake because the pump can work at very low intake pressures, unlike an ESP.

It is anticipated that subsurface twin-screw multiphase pumps will capture areas with high gas volumes or high temperatures related to steam-assisted gravity drainage, wells with asphaltene problems or emulsion issues, or complicated operations.

One area of caution for operators that have previously used ESPs — it is important not to choke this pump since it is a positive displacement pump. If the pump must be choked, a downhole relief valve is required. However, a downhole relief valve is not recommended if there is sand in the well since the valve may not close and seal properly.

The technology is now used for onshore applications, and marinization plans are being studied for use of this technology subsea.