Above: The cement unit’s 600-horsepower triplex pumps When drilling a well, numerous, critical steps must be followed to ensure its successful completion and to protect the drilling crew and other valuable assets. In addition to people and processes, the equipment needed for successful well completion is equally important. One critical step in the drilling and completion process is wellbore cementing. This mandatory process involves pumping specially-formulated cement slurry into the wellbore. The cement provides structural support to the well casing and zonal isolation. Whether preparing a well for additional drilling, production or abandonment, well cementing is required. Cement pumping units are the heart of the well cementing operation and must feature rugged and reliable construction to handle the challenges associated with this task. Cement pumpers are manufactured in many configurations including skid, chassis and trailer-mounted units. Skid-mounted units are primarily used offshore. Chassis and trailer-mounted units are built with mobility in mind and are used for land drilling operations. Regardless of location, cement pumping units must mix and pump a precise density of cement slurry into the wellbore. To accomplish this task, cement pumping units are comprised of six key components that work together to ensure the successful completion of well cementing projects. These critical components and subsystems include:
- Mixing head
- Circulation system
- Control panel
For redundancy purposes, most common skid configurations will consist of twin engines, transmissions and triplex pumps.
Engines & Transmissions
Essential for providing power to drive the pumps, cement pumping skids use twin diesel engines that are rated at 450 to 600 horsepower each. Depending on the manufacturer, the engine configuration could be an inline or v-style design featuring six to eight cylinders. In an effort to reduce harm to the environment, new cement skids use low-emission engines that are designed to minimize nitrogen oxides to meet current Environmental Protection Agency regulations. With a fan-cooled radiator, these liquid-cooled engines must be capable of operating continuously for extended periods of time, in extreme temperatures—such as arctic or desert climates. To convert the power provided by the twin engines into a useful energy source, each engine is coupled to a transmission. The transmission, commonly referred to as a gearbox, uses a series of gears and gear trains to provide controlled application of the power provided by the engine. A typical transmission package for a cement unit will feature five gears and neutral. When operating a cement pumping unit, gear selection determines the pump’s speed, torque and flow rate. The transmission and engine make up the drive end of the cement pumping unit. For reliable, peak performance, the engine and transmission must be compatible to mitigate mechanical issues caused by excess wear and tear. For example, a transmission that is not manufactured to handle the engine’s torque parameters will likely fail prematurely because of the additional stress placed on the transmission’s components during pumping operations. Conversely, an underpowered engine must operate at peak power if the transmission is oversized, which would result in poor fuel economy and additional wear and tear on the engine.
Operators must consider flow, pressure and volume when selecting pumps for a cement pumping unit. Similar to engine and transmission manufacturers, many pump suppliers are available in the market. The most common pump configurations for downhole cementing are triplex or quintuplex design. Both pump types are capable of reaching pressures of approximately 15,000 psi and can be used on cement pumping units. The main determining factor is user preference. The key difference between triplex and quintuplex designs is the number of cylinders. A triplex pump features three cylinders, and a quintuplex pump has five. The additional cylinders on a quintuplex pump allow for larger pumping volume. In addition to increased volume, the extra cylinders also increase the horsepower rating of the pump. On the smaller triplex designs, each cylinder is rated to handle 200 horsepower or 600 horsepower per unit. On the quintuplex designs, the horsepower rating is 1,000. Whether operators choose a 600-horsepower triplex or 1,000-horsepower quintuplex pump, both have a compact design that allows them to be mounted on the cement pumping unit in a back-to-back configuration.
Moreover, many of the manufacturers in the industry have standardized the pump components to allow for the interchangeability of critical pump equipment from one manufacturer to the next. Critical wear items—such as fluid-ends, valves, seats, springs, plungers and plunger packing—are interchangeable among different manufacturers’ models, which eases maintenance and increases aftermarket parts availability for end users.
The Mixing Head & Circulation System
With the engines to provide power, transmissions to convert that power to usable drive force and pumps for pumping, a unit essentially has all the necessary equipment to pump a slurry of cement downhole. However, a cement unit is not complete without the mixing head and associated circulation system. The mixing head and mixing tub, which make up the mixing system, are the most critical components of a quality cement pumping unit. The mixing head controls the amount of bulk cement and water that enters the mixing tub. The mixing system can be fully automated or manually controlled, requiring the operator to throttle the cement valve and water valve to achieve the desired density. The circulation system typically consists of three centrifugal pumps—one for water supply, one for recirculation and one for boosting the cement slurry into the twin pumps. Once the cement is in the tub, the recirculation pump sends slurry back into the recirculation nozzle of the mixing head. This allows the operator to increase or decrease the density of the slurry to meet the job design. A densometer located in the recirculation line determines the cement density of the mixture. In addition, a port in the line allows for a slurry sample to be taken and manually weighed prior to downhole pumping.
Some operators may choose manual operation of the cementing system. However, to ensure that the proper cement density is mixed and that it is pumped at the desired pressure and flow rate, many operators rely on a control system that can be monitored using a control panel. The control system features instrumentation and gauges. These components allow the operator to monitor and adjust many aspects of the equipment, such as:
- Flow rate
- Engagement and disengagement with other equipment (pumps and agitators)
Data recording is a critical function of the control system. This data is incorporated into the well report developed for every well that is drilled and completed.
Reliability & Maintenance
As with all mechanical equipment, reliability is critical to the unit’s operation. Depending on the configuration, a new cement pump unit can range from $600,000 to more than $1 million. The unit should be thoroughly inspected after each project to identify potential issues. It should be maintained and cleaned as well. Operators should ensure that all the cement is removed from the unit and perform preventive maintenance in preparation for the next job. Maintaining this equipment with good startup and shutdown procedures is essential to protecting this investment and keeping it operational for years.