Marginal Column

Endurance Test

Simply flexible: The Wintershall company, Germany’s largest producer of petroleum and natural gas, tests a Rexroth electro-hydraulic drive for down hole pumps.

 
Photo | Bosch Rexroth AG

Test setup of the R7 unit

Pumping petroleum and natural gas imposes great demands on the technology used there. Safe, clean, energy-efficient, reliable, and economical solutions are needed. If, for example, the pressure in the field is not sufficient to move the crude oil or gas to the surface, then down hole pumps will have to be used. In the past, “nodding” pumpjacks were predominant at the wellheads. But when wells reach depths of 3,000 meters and more, the string of sucker rods that drives the pump can weigh as much as 20 tons. That means massive mechanical drives at the surface. Wintershall Holding GmbH decided to carry out testing for the model R7 electro-hydraulic unit, made by Rexroth to drive down hole pumps. The first phase of the trials, lasting 18 months, took place at Wintershall’s location in the town of Landau, Germany – in a well 1,400 meters deep.

Flexibility for greater productivity

The R7 can flexibly adapt cycle speed and the cylinder stroke to suit current pumping conditions. The raising and lowering speeds can be set separately, making for greater efficiency and productivity. “Simple and quick adjustment to suit varying production and inflow conditions makes for perfect pumping performance at all times,” emphasizes Dmitry Kuznetsov, who participated in the Wintershall project.

The down hole pump protects the production system against overloads and in that way boosts productivity. If, for example, sand should enter the borehole, then the R7 will carefully clear the rods by running at slow speed for a while. Once the resistance has been overcome, the hydraulic drive will automatically return to normal operation. This reduces downtimes to a minimum. In mechanical systems, by contrast, such situations often result in the string breaking, followed by complex and expensive repairs.

Easy to use

When compared with the mechanical pump drive, the R7 is as much as two tons lighter and occupies considerably less space, too. That simplifies shipping and installation. What’s more, it requires no foundation, since the hydraulic cylinder is attached directly to the casing and the hydraulic power unit can be installed outside the area subject to explosion hazard.

Not only is on-site handling simple; the controls in the command center are far more user-friendly. The electro-hydraulic drive system automatically prepares a chart of tensile forces and stroke paths. Based on this chart, the operators can quickly detect changes in the pumping situation and optimize the work without delay.

After analyzing these intermediate results, Wintershall is continuing the series of testing for the R7 concept. Long-term observations are to examine reliability and the maintenance requirements. In addition, the company will also be testing the system’s flexibility at other sites exhibiting different production parameters. This field testing project is scheduled to run for five years.

 

How the R7 works

 
Illustration | Bosch Rexroth AG

The system comprises a hydraulic cylinder and a hydraulic power unit. Its main components are the axial piston pump unit, the electric motor with attached flywheel, and the system’s controls. To effect the lifting stroke, the pump unit goes into the pumping mode, moves hydraulic fluid to the cylinder, and raises the cylinder rod, the attached sucker rod string, and the piston in the down hole pump.

The axial piston pump (or the motor in this phase) accelerates the electric motor and the flywheel so that they are again synchronous with the power supply grid. Energy is stored as rotary motion in the flywheel. This concept reduces the amount of power consumed by the electric motor.