Cement mixing equipment has to be robust and fail-safe. It has to cope with a wide range of slurry densities and slurry types. It has to be capable of mixing continuously at relatively high rates (minimum 4 to 5 bbl/minute, or greater than 1 MT/minute) and at a constant density. It must be able to cope with variations in bulk cement supply.
These are very demanding requirements and in the early days required skilled and experienced operators. Even today mixing can, and does, create problems. It is, unfortunately, not that uncommon for the slurry to go downhole at a somewhat different density from that designed in the lab.
Early mixing consisted of a jet mixer, hopper, small slurry tub and a cutting table for sacks. The cementer operating the unit would vary the water going to the jets in an attempt to cope with variations in cement supply or blockages in the hopper.
The slurry tub would contain only about a bbl of slurry so little homogenisation took place. This type of mixing set-up is still the default, fail-safe, method on many sophisticated modern units.
Figure 1: Early Mixing Setup
Problems with delivery of a uniform, consistent slurry at constant density lead to the development of recirculating mixers with a much larger holding tub and the ability to get better control of density before pumping downhole. These mixers still deliver the mix water through jets but the cement is fed through a knife gate which can be controlled by the unit operator. The cement and water meet with high energy and and the resulting slurry is ejected with some force into recirculating tank. Here the slurry is further mixed by paddles and there is provision to recirculate some of the slurry back to the mixing point in order to raise the density.
Once density matches the target value the slurry is fed to to triplex pumps and downhole. While pumping, further water and cement are constantly being mixed and fed to the recirculating tank which now acts as an averaging tank, smoothing out fluctuations in slurry density.
Figure 2: Recirculating Mixer
In place of sacks most cement now comes to the well as bulk powder. In most land operations this bulk will have been pre-blended (dry-blended) with additive powders at the service company base and brought to the well in a bulk trailer.
Offshore, it is usual for neat cement stocks to be held in rig silos (P-Tanks) and additives (usually liquid additives) added at the time of mixing.
Both systems will usually involve a ‘surge tank’ which is essentially a small silo or bulk tank. This is used to provide a steady, near constant feed to the mixer and it is usually situated directly over it.
Figure 3: Surge Tanks
Figure 4: Halliburton Skid Unit
Batch Mixing
Batch mixers come in many forms and sizes. All are basically tanks which will hold the slurry before it is pumped downhole. Some hold only 50 bbl; others are 100 bbl or more. They will usually be equipped with some form of agitation. This may be paddles, centrifugal pumps or compressed air.
The main use is in liner and plug cementing; the advantage being that the slurry can be brought up to weight with all the additives, homogenised and even tested before being pumped. There is thus no variation in density or properties and there is more assurance for critical slurries that everything is optimised.
Two aspects to be aware of:
• Lab testing (particularly pumping time) should take account of the holding time on surface. Testing in the consistometer should include a period without application of temperature and pressure.
• Centrifugal pumps, recirculating small volumes of slurry over a long time, can lead to heating and shear effects which can alter the slurry properties.
The slurry may then behave differently to the lab design. This has been a problem with Coiled Tubing (CT) jobs where slurry volumes are often small (<25 bbl).
Automated Mixing
Increasingly, computer automation is being used to control mixing rather than relying on the cementer’s skill and experience with the knife gate and water valves. These systems can work well and reliably but a fall back system is good insurance.
Computer systems are essential for foamed cement slurry mixing with nitrogen.
Pumping
High pressure pumping units, like the mixer, have to be robust and capable of highly reliable performance. The industry has standardised on triplex (three plunger) pumps. Most cementing units will use two banks of triplex pumps – 2 x 3 cylinders. These are fed by centrifugal charge pumps through spring loaded valves. The pumps are very ruggedly constructed, can easily be stripped and repaired and are generally very reliable. They can pump highly abrasive slurries across a full range of densities and rheologies. The pluger stroke can vary from 5 to 10 inches and the plunger diameter from 3 to 6 inches. By altering the sizes, the delivery capability of the pumps can be varied to cope with different circumstances but using the same horsepower of the unit – higher pressures/lower flow rate or higher flow rate/lower pressure.
A unit will usually deliver between 200 and 500 hydraulic horsepower (hhp).
Rates of up to 8 bbl/minute at 500 to 1,000 psi are typical cementing parameters when pumping downhole using 2” treating lines.
Both diesel and electric powered units are available. The diagrams below show the guts of the fluid ends of a Halliburton HT 400 unit.
Recording
The variables of interest are:
• Slurry density
• Flow rate
• Pressure
• Cumulative volume
These variables are rquired throughout the job – mixing, pumping and displacement. Where displacement is done by the rig pumps, either extra sensors have to be installed and hooked up to the cement unit recording device or the data has to be downloaded from the mud loggers.
Any cement job should have the above parameters recorded to provide:
• Assurance that what was pumped was what was designed
• Enable comparison between planned execution and actual – for example, the final pressure just before bumping the plug can give a rough check on the height of cement in the annulus and confirm the estimated TOC.
• Allow analysis of a job to determine root cause of failure – pressure can be particularly decisive in identifying problems
• Aid planning and optimisation of subsequent jobs – transfer of learning
Slurry Density
The main methods are:
• Standard mud balance
• Pressurised mud balance
• Radioactive densitometer
• U-Tube densitometer
The first two are non-continuous devices, measuring only a sample. The Pressurised balance is much preferred although if is considered awkward to use.
The main reason is the considerable amount of air entrained in the slurry from the mixing process. There is little time for this to break out and a normal mud balance will give a reading which is ‘lighter’ than the downhole (compressed) density. A pressurised balance will give a true, downhole density.
The latter two devices will give continuous readings and are, therefore, preferred.
However, it is still good practice to use a pressurised balance during the job to check the reading being given by the densitometer.
Figure 5: Pressurised balance (top) and normal mud balance
Jobs have gone seriously awry because the cementer worked from a densitometer which has drifted off calibration. In any case, calibration of these devices should be routinely checked.
Flow rate
If the cementing unit is being used for displacement, the flow rate will be taken from the downhole pump stroke counter, corrected for pump efficiency.
If rig pumps are being used for displacement, then a separate flow meter is needed or the data has to be obtained from the mud loggers and tied-back to the job timescale.
Flow rate is particularly useful when compared with pressure.
Pressure
Pressure can be recorded from a tranducer at the the downhole pump discharge and is also usually measured by a Martin-Decker gauge which records on a circular chart. Since the Martin-Decker is an old-fashioned clockwork device it often provides a record when the more sophisticated devices, or the recording media, fail.
Cumulative volume pumped
This can be calculated from pump rate and time and also from strokes and pump efficiency.
If displacement is with the cementing unit, then the number of displacement tanks can be counted to give a good estiamte. In fact, where displacement volume is important – pumping plugs, spotting fluids, picking up plugs, seating darts, etc – the cement unit displacement tanks are the most accurate since they do not rely on assumptions about pump efficiencies. Displacement tanks do need to be calibrated like all other devices. Slight errors in the markings are cumulative.
Although the markings are fixed at the time of manufacture, the unit may not be quite level.