Adapting cavity pumps to meet new, unconventional challenges

Global demand for crude oil and natural gas continues, unabated. Prices are rising, and interest in opening up new reserves, including those from unconventional sources, is growing. Methane from coal seams and highly viscous oil, at more than 10,000 millipascal/sec (mPas), are already of commercial interest today, but they are just as complicated to extract as oil-sand mixtures from the Sudanese desert.

Conveying systems must, above all, be adapted to the media, to the environment and to one another, if production is to be efficient. To this end, Netzsch, a German family firm, has developed a range of specialized progressive cavity and rotary lobe pumps, for various fields of application in upstream, midstream and downstream areas.

Traditional oil production, using pumpjacks with their distinctive “nodding donkeys,” can still be seen today in many fields around the world. However, they are being replaced increasingly by a less-spectacular, but more efficient system—progressive cavity oil pumps.

The main characteristic of this technology is that it can convey highly viscous oils at above 50,000 mPas, while maintaining stable pressure. It can also do this with hardly any pulsation or shear forces occurring, thereby avoiding emulsion effects with oil-water mixtures, which would significantly hinder subsequent separation of these mixtures at the surface. This is achieved through the progressive cavity pump’s characteristic conveyance principle, which is based on a rotor that turns in an oscillating motion within a fixed stator. The geometrical mating means conveying chambers are formed between the rotor and the stator. As the rotor turns in the stator, the medium is transported from the inlet to the discharge side of these chambers. This is achieved with stable volumes and pressure, irrespective of the consistency or viscosity of the medium. The volumetric efficiency of the downhole progressive cavity pump is between 75% and 90%.

The robustness of progressive cavity pump technology comes into play in various areas, especially where oils with a high content of other substances have to be transported. Netzsch has developed a multi-phase pump for use in this area of application. The pump handles mixtures of oil, water and gas, along with sand content, and, in doing so, achieves conveyance rates of up to 600 cm/hr.

WIDE RANGE OF REQUIREMENTS

As the pump body is comparatively slim, it is perfectly suited to use inside the bore shaft, Fig. 1. To generate torque above ground, Netzsch has developed various drive heads for different performance and speed requirements. The vertical drive comes with 20 to 100 hp, and 3,400 newton-meter (Nm) torque, and is specially designed for large loads, as well as enabling a turndown ratio of 1:4.

Fig. 1. Progressive cavity pumps are particularly well-suited for use in boreholes, thanks to their slim shape and ability to convey hydrocarbons with low pulsation and stable volumes.

A right-angled version is used for very low piston-rod speeds, and can be connected to both combustion and hydraulic motors, thanks to its geometry. The economical direct drive is an additional option for higher rotation speeds, up to 500 rpm. They all have hydraulic brakes, and can be fitted with glands or lip seals. One version is also ATEX-compliant, in accordance with the European Parliament’s Directive 94/9/EC, and is authorized for use in Level 1 (potentially explosive) zones, having integrated overload protection. Drive heads are available for 1¼- and 1½-in. polished rods, and, with these, the rod speed can be changed, either by using different belt pulleys or a variable-speed motor.

The pump body, itself, is connected to the sucker rods and is available in various designs. For instance, the standard model, NTZ, only generates low life-cycle costs, thanks to its small number of wearing parts, and is suitable for oil mixtures with up to 40% gas or sand content, and temperatures up to 160°C. This means it can achieve conveyance rates of between 0.5 and 300 cmd, at a maximum 300 bar, depending on the size. This type of pump is used, for example, to extract methane in coal seams in China’s Qinshui basin. Even though this means not only getting a valuable energy source, but also reducing, considerably, the firedamp risk during subsequent coal mining, there is still little gas produced from these kinds of sources. One reason for this is that the methane, to some extent, adheres to the seam due to the water pressure.

The NTU model of the progressive cavity pump, which has an elastomer wall, has been designed with a focus on low start and operating torques, and low susceptibility to influence from temperature or chemicals.

The steel housing of the spiral form, of the inner elastomer stator, was adapted for this purpose—the rubber walling is of equal strength throughout, and it swells or shrinks evenly. In addition, there is a special pump for particular application areas, which can be assembled and disassembled inside the pipe, and is fixed using an anchor module. This reduces the downtime, when work has to be done on the pump, and lowers replacement costs by up to 60%.

Finally, there is a deep-well submersible pump for directional and horizontal bores that does not require an above-ground drive. It operates, instead, with an upstream submersible motor. This obtains its energy via a power cable, which means long transmission rods are not needed.

ROBUST UNDER DIFFICULT CONDITIONS

The metal and elastomer materials used are selected to match the anticipated operating conditions for all types of pumps, to keep wear from the medium to a minimum, while simultaneously creating optimal conveyance conditions. Robustness is all the more crucial, as an increasing number of oil fields are now developed in remote regions, which means a lot of expense is involved, every time a repair is carried out or a spare part is procured. Temperature and density are among the aspects taken into account here, as well as water, natural gas and H₂S content. This approach not only increases the performance of the systems, but also ensures a long service life and long maintenance intervals.

In one example, highly viscous, heavy crude oil is extracted from one of the world's deepest deposits in China’s Tuha oil field. Even at an ambient temperature of 50°C, it still has a viscosity of 10,000 mPas. This type of oil would normally be heated, to make it more free-flowing, but that is not possible with a borehole depth of over 2,500 m. Instead, light oil is injected to dilute the raw material for pumping out. The temperature at the bottom of the borehole is 80°C to 100°C, and the oil is still warm, at 40°C, after mixing.

Given this situation, a resilient nitrile butadiene rubber was fitted, which only exhibited a swelling rate of 0.41% in special tests. A direct drive, which has a nominal speed of 180 rpm and can be controlled via a frequency converter, supplies the power. This enables flexible adjustment of the system to the source output, which varies from 7 to 40 cmd.

ROTARY LOBE TECHNOLOGY

In addition to the progressive cavity technology, the company’s portfolio also includes a series of rotary lobe pumps, Fig. 2. The special feature of the self-priming, valve-less, positive displacement pump is its synchronized gear with belt drive, which, in contrast to standard timing gears, is extremely resilient and very easy to maintain. A belt with teeth on both sides transmits the power from the motor to both rotary lobes, and synchronizes these at the same time.

Fig. 2.The rotary lobe pump, which can convey maximum volumes in a limited space, offers an alternative to progressive cavity technology.

There are two models of the pump. The first has a specially shaped elastomer insert for the inside of the housing, which improves conveyance, while the second is all metal for more demanding media.

The rotary lobes are manufactured from stainless, hastelloy or duplex steel, depending on requirements. The advantage of this choice of material, in contrast to elastomer bodies used commonly, is that the steel lobes are less affected by the strong dynamic forces during rotation, and have a correspondingly longer service life. They also expand less than rubber, which allows the components, generally, to be made with lower tolerances and enables higher performance.

Because this design means that there only soft surfaces running on hard ones, the drive runs quietly and generates very little vibration, noise or friction heat. The rotating components are, therefore, preserved and last longer. Furthermore, oil changes are eliminated, because synchronization works completely without oil.

The pump's direction of conveyance can be reversed easily, so that the same system can be used both for filling and emptying. Possible conveyance rates over the various sizes range from 3 cm/h to over 900 cm/h. The system also can be installed in any orientation and has a space-saving design, because the motor is attached directly above the pump chamber.

This has proved essential for application in the ex-zone 1 of a preparation unit with a very narrow design, into which the pump was directly integrated. There, the conveying system ensures the inflow of the pumped oil-sand mixture, with a kinematic viscosity of 30 to 35 sq mm/sec, and a temperature of 60°C, into a separator, where the remaining sand and water are separated.