In a typical well cementing operation, a cement slurry is pumped into the annulus between the well casing and the bore hole. As the cement slurry hydrates, it undergoes a volume change which translates into bulk expansion or shrinkage. Knowing the amount of expansion or shrinkage allows users to design cement systems which can achieve optimal bonding with the borehole and well casing without sacrificing the integrity of the cement matrix. The Circular Cement Expansion Mold is designed to measure the expansion or shrinkage of well cement slurries according to the guidelines in API Recommended Practice 10B-5.
The Circular Cement Expansion Mold has an external ring with a vertical slit on one side to allow it to expand. Before testing begins, the width of the gap is measured with the included micrometer. The expansion mold is filled with a cement slurry and cured in a water bath or curing chamber. As the cement cures, the external ring will expand or contract with the cement slurry. At the end of the test, the gap is measured again to determine the expansion factor.
The Atmospheric Consistometer is one of the staples of the cement lab. Every cement lab needs at least one for a critical step in preparing cement slurry for further testing.
Conditioning a cement slurry in an Atmospheric Consistometer simulates the action of pumping the slurry down the wellbore. The stirring motion and elevated temperature add enough energy to the slurry to appropriately mimic the pumping environment. And, by following the recommended practice published by the American Petroleum Institute (API), the Atmospheric Consistometer helps ensure your tests are consistent and repeatable.
Oil well cement must be strong enough to support the casing and to provide adequate zonal isolation. Before using a cement system in a well, developers must verify its compressive strength through direct measurement.
The OFITE CLF-40 Automated Compressive Load Frame performs destructed compressive strength testing on cement samples. It accepts either 2” cubes (cured in an HTHP Curing Chamber) or cylindrical samples (cured in an Ultrasonic Cement Analyzer). The sample is held between two platens while the instrument steadily increases the pressure. When the sample fails, the software automatically calculates the maximum pressure before failure.
The Constant Speed Blender is designed to prepare a cement slurry by mixing it at constant speeds according to API guidelines. The blender begins by mixing at 4000 rpm for 30 seconds. It then increases the speed to 12000 rpm for 60 seconds. As the slurry thickens, the onboard controller continuously adjusts the current to the motor in order to maintain the proper mixing speed.
The new Model 25 Constant Speed Blender from OFITE brings you the same functionality as the older Model 20 Blender in a more compact, economical package. The timer and speed controls are now integrated into the blending unit, eliminating the need for a separate control box. The result is a smaller, lighter unit that takes up less space in your lab and is easily portable.
Gel strength is a property of cement slurries that describes the attractive forces that exist between particles suspended within the mixture. Gel strength develops as the slurry remains under static conditions. Of the four rheological models that describe the flow behavior of fluids, the one that is used to best describe cement slurries is the Bingham-Plastic model. One of the particular characteristics of a Bingham-Plastic fluid is that it has a yield point. Below a fluid’s yield point, the fluid behaves as a solid. As force is applied to the slurry, it does not move. Once the force exceeds the fluid’s yield point, the fluid begins to flow. As long as the slurry remains in a static state, it has a chance to develop gel strength.
Cement service companies have a strong interest in cement gel strength. Gel strength describes the ability of a fluid to clean the walls of the hole and carry solids out of the well. After pumping operations have ceased and the slurry sits in a static state, gel strength describes the ability of a cement slurry to resist gas invasion. In the lab, technicians typically measure the rheological properties of a cement slurry using a concentric viscometer or rheometer. Either unit consists of a cylinder that rotates around a bob connected to a calibrated spring which measures the force that the rotating cylinder imparts on the fluid that is in between the cylinder and the bob. Rheometers or viscometers are complex or simple and have the capability to measure the rheology of fluids under both atmospheric conditions and conditions of temperature and pressure downhole.
When wet cement begins to set, the crystals that form the set cement matrix take up less volume than the initial reactants that make up the base cement blend mixture. As a result of the chemical reaction, the cement undergoes a net reduction in volume. This phenomenon is called bulk shrinkage. When the cement is in contact with water during the setting process, cement hydration consumes the water as a part of the chemical reaction. The influx of water, chemical shrinkage, into the cement matrix offsets the bulk shrinkage. The result is hardened cement that experiences significantly less volumetric shrinkage.
For cement that is placed in an oil well, the phenomenon of shrinkage is problematic. Volumetric shrinkage causes the cement to pull away from both the formation and the steel casing. The result is a loss of zonal isolation, which is one of the most basic functions of oil well cement. To overcome situations where shrinkage is a potential issue, cement service companies add certain additives to promote expansion of the set cement. The expansion of the cement promotes bond integrity between the cement and the formation and maintains zonal isolation throughout the life of the well.
Once dry cement is mixed with water, a complex chemical reaction begins. The chemical reactions have a tremendous effect on the physical performance properties of the cement slurry. The addition of other chemicals to the cement mixture further complicates the reaction process as these additives affect the slurry properties. Blends are modified in order to increase or decrease the thickening time, alter the rheologies, or change the fluid loss properties.
Laboratories need the capacity to gauge the performance of the cement slurry in order to accurately predict how that slurry will perform under conditions that are found in an oil well. One of the tools that technicians have at their disposal to prepare slurries for additional testing is the atmospheric consistometer. Originally conceived as an instrument to test the thickening time of cement slurries, the atmospheric consistometer is often used to condition the slurry for additional testing.
***UPDATE: The September 21 lunch and learn is full. We will be scheduling more in the near future. If you are interested, let us know and we will keep you updated about upcoming events. ***
If your company or organization has an interest in measuring the mechanical properties of cement and other materials, then we have an opportunity for you. On Thursday, September 21st, OFITE is offering a free seminar at our main headquarters in Houston, TX. Come have lunch with us as we talk about our new TLF-112 Triaxial Mechanical Properties Testing System designed specifically for the testing of well cement and other materials.
The mechanical properties of a material describe how that material behaves when subjected to an applied force. When placed in an oil well, cement is subjected to numerous stresses which impact its ability to both protect the casing and maintain zonal isolation. Lateral stresses within the formation threaten to warp and crush the casing. Expansion and shrinkage caused by temperature and pressure cycles cause brittle cement to develop cracks, which place well integrity at risk as the cement debonds from the casing or formation.
There are four primary mechanical properties of cement that oilfield service companies need to understand to predict the behavior of the set cement under downhole stresses:
Cement slurries that are placed in gas bearing formations as part of the well completion process are at risk of gas migration under the right conditions. Gas migration refers to the annular flow of natural gas from the formation up through the cement column. As the gas moves through the cement matrix, it carves a channel that becomes permanent once the cement sets. This permanent channel becomes a conduit that allows for natural gas to continuously bleed from the formation.
After cement placement operations have stopped, the cement slurry begins to develop gel strength under static conditions. The amount of gel strength development and the speed at which it occurs are two of the main factors that describe a cement slurry’s ability to resist gas intrusion. Generally, gas intrusion begins when cement slurries develop 100 lbf/100 ft2 (48 Pa). Gas intrusion ends when the cement develops 500 lbf/100 ft2 (240 Pa). The transition time is defined as the time it takes the slurry to go from 100 lbf/100 ft2 (48 Pa) to 500 lbf/100 ft2 (240 Pa). The shorter the transition time is, the less risk there is for the cement sheath to lose well control.
For oilfield service companies trying to cement a well, the thickening time of the cement slurry is the critical slurry property to control. A typical cement job requires the slurry to be mixed at the surface and pumped downhole. Cement slurry that sets up too soon leads to a Cement Left In Pipe (CLIP) situation. A CLIP occurs when cement fluids set up inside the steel casing during placement. Costs associated with both the well and nonproductive time is significant. Slurries with extended thickening times lead to costly extended time on station as the drilling rig is required to remain on station in order to provide support for the casing. In a worst case scenario, the well is at risk of complete loss of control as the cement is unable to maintain both zonal isolation and well integrity.
Laboratories need the capacity to gauge the performance of the cement slurry in order to accurately predict how that slurry will perform under conditions that are found in an oil well. The most fundamental tool to measure the thickening time of a cement slurry is the pressurized consistometer.
Cement in an oil well needs to be able to resist the forces found in the formation in order to protect the steel casing around which it is placed. The cement sheath is also required to support the weight of the casing as well as resist perforation operations conducted by wireline operators in preparation for fracturing and stimulation activities. Cement needs to develop an adequate level of compressive strength in order to meet these requirements.
Cement compressive strength describes the ability of hardened cement to resist crush force. Laboratories have multiple options to measure the compressive strength development of a cement design. The most practical and efficient method to accurately calculate the compressive strength is to use an Ultrasonic Cement Analyzer (UCA). The UCA works by sending a continuous sound pulse into the cement slurry. As the cement hardens, the sound pulse travels faster through the cement matrix. The transit time is correlated to the compressive strength of the hardened cement.
Cement slurries used in the oil field are dynamic fluids with complex chemistries. Laboratories tasked with the development of slurries used for cement pumping operations strive to prepare these mixtures under conditions that reflect those found out in the field. With the replication of conditions that are as close as possible to those found out at the wellsite, technicians are able to make accurate predictions the behavior of cement slurries under those same conditions.
The most basic way to ensure that lab-prepared slurries match the performance of field-prepared counterparts begins with a properly mixed slurry. Cement laboratories use a constant speed mixer to ensure that mixed cement slurries are a homogeneous blend. The proper amount of mixing energy ensures that individual cement grains are completely surrounded by water which kicks off the cement hydration process. Laboratory technicians are then able to perform the remainder of their evaluation tests confident with the knowledge that any results produced properly predict the performance of the cement slurry under field conditions.
To define the physical properties of a cement slurry, laboratory personnel need to run a complete suite of performance tests. To produce consistent test results that inform how the slurry will perform under field conditions, technicians need to treat the slurry in a manner that is similar to those conditions found downhole. Atmospheric Consistometers are the ideal tool that enables technicians to condition the target slurry for further testing.
Atmospheric Consistometers are a fundamental piece of laboratory equipment. Designed to conform to API Spec 10A/10B2 standards, the Consistometers utilize a heated mineral oil bath to maintain the temperature of the slurry within 2°F (1°C) of the set point. The slurry is conditioned at temperature by rotating at 150 rpm for the duration of the conditioning period.
Advanced oil well cement testing goes beyond compressive strength. The elastic properties (Young’s Modulus, Poisson’s Ratio, and Tensile Strength) also play a major role in well integrity. But measuring these properties has always been a challenge. The equipment is often large, complex, and expensive. What is the solution?
The OFITE TLF-112 Triaxial Mechanical Properties Testing System is the first instrument specifically designed to physically measure the four major mechanical properties of oil well cement:
The Model 800 Viscometer has been around for years. It’s the workhorse of laboratory and field rheological testing. But why is it so popular? Why do so many people prefer it to the competition?
A lighted dial. It may not seem like much, but the tiny light inside the case illuminates the dial and makes it much easier to read. It is especially helpful in situations where ambient lighting is less than optimal. Whether it’s the glare from the sun or the gloom of a dark lab, the dial is always easy to read.
Universal voltage. All of the electrical components of the Model 800 operate exclusively on DC power. What does that mean to you? It means you can plug it in almost anywhere. The included power supply accepts between 100 and 240 volts of AC power. But the best part is the frequency. Whether you are operating on 50 Hz or 60 Hz, the power supply converts it to DC. We even offer a special 12 volt adapter for use in the cigarette lighter in your car or truck.
Threaded rotor. Have you ever run a test with a wobbly rotor? The rotor on the Model 800 is threaded. It mechanically attaches to the unit the same way every time. And it stays tight throughout the entire test.
No gearbox. Gears are complicated and messy. They wear out easily and they are prone to jamming and locking. The model 800 has no gear box. It’s just a motor and a belt. There’s no need to switch gears to change speeds; simply turn the knob. And it’s much easier to maintain and repair. Fewer moving parts means fewer problems to fix.
Speed control. The Model 800 uses a stepper motor to drive the rotor. Stepper motors have very precise speed control. While you are testing, the motor control board is constantly monitoring the speed and making adjustments. So you always know that your rotor is turning at the right speed.
Extra speeds. They say less is more. But at OFITE, we disagree. We believe more is more. So instead of offering only 6 speeds, our viscometer offers 8. The standard speeds of 600, 300, 200, 100, 6, and 3 RPM are suitable for most testing on drilling fluids. But for testing cement you need two extra speeds: 60 and 30 RPM. By including all 8 speeds, the Model 800 becomes a more versatile instrument.
Have you tried the Model 800 Viscometer yet? If not, you are missing out. Check the product page for more information or to place an order.
Foam cement is a versatile, low-density cementing system used in many oil-well applications. It is commonly used for low fracture gradient or lost circulation problem wells. The formulation is unique compared to other oil-well fluids because it uses three phases of matter. The solid phase consists of cement and other dry additives. The liquid phase is mix water and any liquid additives. And the gaseous phase is, in most cases, nitrogen (N2) gas.
Adding nitrogen gas to a cement slurry dramatically reduces the density while using the same amount of mix water. Because no extra water is used, foam cement develops sufficient mechanical strength for well-bore application while improving the bonding characteristics. As the temperature increases down hole, the nitrogen gas expands, pushing the foam cement against the formation and casing and creating a stronger bond. Despite the added benefits, expansion also creates challenges for laboratory testing.
Times are tough in the oil patch. Staffing is reduced, budgets are cut, and approval processes are tightened. It is harder than ever to keep your lab equipped with the latest technology. But now is when it’s most important. We can help.
OFITE is now offering a new equipment lease program. Our goal is to offer a selection of instruments at monthly rates that fall below your capital expenditure limit.
The all new Model 4020-SG Automated UCA/SGSM is a revolutionary instrument that combines two powerful cement measurement methods into a single, easy-to-use device. First, the UCA provides a non-destructive method of testing the compressive strength of a well cement. Second, the SGSM directly measures the static gel strength of the slurry. And both are combined to create the only self-contained Ultrasonic Cement Analyzer / Static Gel Strength Measurement (UCA/SGSM) device on the market.
The OFITE Circular Cement Expansion Mold measures the expansion or shrinkage of a well cement. It has an external ring with a vertical slit on one side to allow it to expand. Before testing begins, the width of the gap is measured with a micrometer. Then, after curing, the gap is measured again.
The complete kit includes the mold, micrometer, micrometer stand, and carrying case. Click here to learn more.
Any instrument that operates at elevated temperatures and pressures creates a safety concern in the lab. HTHP filter press cells are no different. The traditional design, with its six locking screws around the cap, has been known to trap pressure and become a safety risk.
OFITE has designed a new cell with safety in mind. This modular design is much safer and more convenient. The two-piece cap is threaded, and cannot be opened while the cell is pressurized. And interchangeable caps make it easy to reconfigure the cell for testing with different filter media (filter paper, ceramic disks, or cement screens) with a single cell body.