Mixing and pumping cement into a drilled borehole is one of the most critical steps in oil and gas well production. The injected slurry hardens to form a strong, protective sheath around the drilling string and isolates it from surrounding geologic features. Oil well cementing is typically among the first operations to prepare a well for production and one of the last operations to plug a well prior to its abandonment.
The cement sheath creates a smooth internal bore for operating well-drilling equipment. It fortifies drill pipe strength, protects the casing from shock loads during operation, and wards off contamination and corrosion. Through zonal isolation, cementing segregates the various zones that may have different pressures or fluids. By sealing off high-pressure zones from the surface, cementing curbs blowout potential.
Cementing also stabilizes surrounding geology and prevents unstable formations from caving-in and bogging down the drill string to bring production to a halt. Cement’s impermeable seal prevents water, soil, and sand from contaminating the well flow.
Level Control Considerations for Oil Well Cementing
Mixing and injecting the cement into a well is accomplished by truck-mounted, trailer-mounted and skid-mounted equipment configurations. The mobile cementing truck, with its integrated mixing, pumping, and control systems, is the preferred choice among oilfield professionals. Trucks feature bulk transport or batch mixing units and an accompanying high-pressure pump to force the slurry down the casing. The mixing system blends Portland cement, water, and various additives that affect the weight, density, behavior, and setting time of the slurry. Control systems typically include automatic density control and data recording systems with user-friendly interfaces, serial data output, and manual backup controls. An essential consideration for the performance of the control system is effective level control.
Accurate mixing and storage tank levels are key parameters. Measurement must be precise and responsive since sluggish level response can lead to delayed control reactions that damage cementing systems and shut down operations when tanks exceed high or low level limits. The level control’s ability to deliver consistently uniform, blended slurries that meet performance criteria is vital throughout the life of the equipment. In addition to accuracy, responsiveness, user-friendliness, and reliability, the mobile level controls must be extremely robust to tolerate the day-in, day-out concussions of oilfield travel.
A Measurement Success Story
A leading manufacturer of cementing equipment in China wanted the above attributes in level controls for the cementing trucks it manufactures. The measured medium is a complex mix: an agitated slurry with foam, solids, and changing density. Operation is under atmospheric pressure with a typical temperature of around +95oF (+35o C). Liquid additives mixed into the slurry reduce viscosity to about 60 centipoise (cP), which is similar in viscosity to corn oil.
Given the nature of this application, the petroleum machinery manufacturing company selected the Magnetrol® Eclipse® Guided Wave Radar (GWR) level transmitter with an overfill-capable probe installed in an external cage with a 31 inch (800 mm) center-to-center measurement range. The closed coaxial design tends to reject the false target that foam is known to produce, and the probe’s high accuracy ensures safe and efficient operation. The convenient user interface makes field adjustment and configuration quick and easy.
The ECLIPSE guided wave radar unit was tested against competitive GWR instrumentation. Two advantages gave ECLIPSE transmitters the edge. First, the response time was extremely quick – within one second – and thus showed operators real-time level. Competitors’ units required a full 10 seconds to respond. Second, the overfill safe probe delivered outstanding performance, giving accurate level readings to the top of the probe and close to the very top of the tank. By demonstrating best-in-class transmitter response time and full probe measurement range, over 100 ECLIPSE GWR transmitters were used for upgrades to the customer’s oil well cementing trucks.