An energy supplier in Hannover had initially used a deeply located geological horizon for gas storage, but then decided to extract crude oil from the same deposit in 2016. But first, a suitable extraction system was required for transporting the multi-phase mixture with a high gas content to the surface from 1,200 m below. Due to the extreme extraction conditions, this required special pump technology: centrifugal pumps with submersible drive were not suitable due to the high risk of failure from gas lock, while the progressing cavity pumps with above-ground drive – as commonly used in Central Europe – posed the risk of a blowout on the surface. NETZSCH Pumpen & Systeme GmbH from Waldkraiburg finally designed a progressing cavity pump (PCP) system with submersible drive, permanent magnet motor (PMM) and a special control which prevents these problems: thanks to the special rotor-stator principle, on which all progressing cavity pumps (PCP) from this manufacturer are based, the consistency of the medium is irrelevant for function and capacity of the pump. As all moving parts are positioned deep down in the well, there is also no risk of environmental impact from leaks. In addition to this, the system allows flow rates of 1:5 and therefore a flexible adaptation to changing inflow conditions.
“Around 30 years ago, 5P Energy had started to use an underground deposit for gas storage. After a few years, however, it became evident during pumping of the medium that the geological horizon in which it was stored had emitted substantial oil quantities into the gas,” states Jörg Eitler, Global Head of Business Field Oil & Gas Upstream at NETZSCH Pumpen & Systeme GmbH. This prompted a detailed production and profitability analysis which showed that a conversion from gas to oil production for this bed was economically viable.
Previously installed pump system unsuitable due to extreme general conditions
A suitable pump system had to be acquired in order to reliably transport the crude oil to the surface from a depth of more than 1,200 m. The extreme conditions on site, however, presented very high requirements for the pump design: as the medium was a multi-phase mixture with a very high gas content, the usual centrifugal pumps with underground drive were not an option. “With a very high free gas content, this type of pump always presents the risk of all impellers filling up with gas,” Eitler explains. If this occurs, the centrifugal submersible pump can no longer move the medium from the impellers towards the pressure end. The entire system becomes unable to generate pressure. “It was extremely likely that a centrifugal submersible pump would have failed in this application at the 5P Energy deposit due to this so-called gas lock after a very short period of use,” Eitler continues.
The alternative progressing cavity pump technology is therefore normally used for the draining of gas wells, for example, where this risk is also very high. All pumps of this type used in Central Europe until now, however, have been PCP systems with above-ground drive. These units have a dynamic seal on the surface which has a significant disadvantage in case of extreme pressure increase on the intake side: at 100 bar overpressure on the seal, it can become overloaded and trigger a blowout. “Because the deposit was used for gas storage in the past, a very high level of pressure is still stored in the respective bed today,” Eitler explains. “The reaction of the free gas cannot be predicted, though – that means a massive pressure increase can happen unexpectedly and suddenly. This phenomenon can also occur in new wells and is then referred to as ‘swapping well’.”
Suitability for multi-phase mixtures
NETZSCH therefore faced the challenge of designing a pump system for the application that was suitable for the difficult medium and the extreme conditions. “Due to the special displacement principle, which ensures a high level of robustness towards the composition of the medium, we only considered a solution based on the progressing cavity pump technology from the outset”, Eitler recalls. The main parts which define a PCP system are a rotating component, the “rotor”, and a fixed one, the “stator”, in which the rotor turns. The rotor is designed as a kind of round threaded screw with an extremely large pitch, large thread depth and small core diameter. The stator has one extra thread and twice the pitch length of the rotor. The precise geometrical mating means that conveying chambers are maintained between the stator and the rotor, which rotates inside it and also moves radially. These chambers continuously move from the intake to the outlet side and transport the medium.
The volume of these chambers remains constant and the chambers themselves are self-sealing in the process. This not only prevents backflow, but also ensures that the conveyed medium is transported at stable volume and pressure, so that no shear forces and hardly any pulsation occur. Conventional pumping systems soon reach their limits if consistencies vary, which results in interruptions, loss of pressure and material damage. For progressing cavity pumps, on the other hand, consistency and viscosity of the medium are not relevant to the flow. Multi-phase pumps from NETZSCH can therefore also handle mixtures of oil and water with sand or gas, reaching flow rates of 300 to 400 m³/day.
Underground progressing cavity pump with PMM motor
To exclude the possibility of overloading the dynamic seal on the surface and therefore a blowout, NETZSCH suggested the use of a progressing cavity pump which is driven underground – the NETZSCH ESPCP. “The special feature of this pump is that the rotor is not driven via a very long shaft or linkage from a drive head on the surface, but rather the rotor-stator combination and the motor are sunk into the well,” Eitler explains. The motor or bearing unit is connected directly to the rotor via a flexible rod. All radial and axial forces of the rotor are absorbed by a special underground bearing housing. In addition to this, the dynamic seal is also located in the 1,200 m deep well. This moved all critical components of the pumping solution below ground, precluding environmental impact from leaks above ground.
The motor for the system is a permanent magnet model (PMM) with special SPMM control which can ensure flexible flow rates. “In old wells, the flow rates fluctuate in a very limited range. In these cases, we can assume that the value levels out in a range from 10 to 20 per cent. For this project, however, we do not yet have any current inflow data – that means no productivity index – so that extreme fluctuations of up to 500 per cent are also possible,” Eitler says. “On an installed pump, this type of variance can only be covered with a very large speed range of 1:5. This factor cannot be achieved with systems with above-ground drive – that was another reason to move the complete system into the well.” For the NETZSCH ESPCP, a special 10-pole PMM has been used, as this can provide the slow speed required for the PCP, in contrast to the usual submersible motors without submersible gearbox. “The submersible motors normally used for centrifugal submersible pumps have a speed of 3,000 or 1,500 rpm at 50 Hz – that is much too high for use with progressing cavity pumps,” Eitler states. “A realistic speed is in the range between 100 and 400 rpm.”
Thanks to the PMM motor and the precisely adapted SPSS control, the solution designed by NETZSCH reliably achieves speeds from 100 to 500 rpm. That can ensure the desired flow rate of 1:5 and therefore achieve maximum flexibility. The system has been successfully in use since May 2017.