Static gel strength testing of cement slurries in today’s world

Testing equipment used to evaluate the gel strength development of cement slurries.

The oil industry has long recognized the need to mitigate and/or control gas and fluid migration tendencies in a well. After a job is completed and the cement slurry is properly placed in a well, the cement slurry undergoes phase changes from liquid to gel and finally to a solid cement. During the gel phase, the slurry becomes self-supporting and can no longer transfer the hydrostatic head of the fluid column above it. It is critical to understand when this gel phase occurs after cement placement, and for how long this phase lasts. The longer the transition period from liquid to solid, the more likely the possibility of gas or fluid migration.

Numerous cement slurry designs have been developed to try and control gas migration. Some of these include slurries that generate in situ pressures, ultra-low fluid loss systems, latex additives, expanding cements to improve bonding, and slurry designs that reduce the ability of water to move within the cement pore spaces during strength development. The purpose of this article is not to evaluate these approaches, but to review the evolution and current state of laboratory test methods for gel strength development of cement slurries, with the objective of minimizing these gas migration tendencies.

The American Petroleum Institute (API) Sub-Committee 10 for Oil Well Cements has an active group dedicated to the testing of Gel Strength Development of Cement Slurries (TG03–Static Gel Strength Measurements). The charge of the work group includes the evaluation of different measurement geometries and calibration methodology.

Several studies also have been conducted over the years on this subject. These studies included the earliest designs and models of laboratory testing apparatus. Each method is discussed below.

MECHANICAL METHODS

The gel strength of a cement slurry is described as the shear stress of the sample along a cylindrical boundary, created by axial pressure. The simplest method to measure the shear stress involves filling a pipe with the slurry, combined with displacing the slurry, using a syringe pump at an infinitesimal rate while measuring the differential pressure across the slurry. In this manner, the data are used to determine the shear stress in accordance with the following equation:

Shear stress = D*ΔP/4L, dyne/sq.cm. —usually converted to oilfield units of lbf/100 ft² or Pascals

where,

D = Diameter of the pipe, cm

ΔP = Differential pressure, dyne/cm2

L = Length of the sample, cm

Fig. 1. Mechanical gel strength analyzer with high-precision syringe pump.

For practical reasons, along with a need to study the gel strength development characteristics under downhole conditions, alternate methods were devised. These include a means to condition the slurry prior to the gel strength measurement phase, followed by evaluating the gel strength development.

During the 1980s, a cement thickening time consistometer was adapted to mechanically measure the development of gel strength.¹

The consistometer was fitted with a low-friction magnetic drive at the top of the pressure vessel, which provided a means to rotate a paddle at a slow speed in a sample container filled with the sample. As the paddle is rotated, the reaction torque on the paddle is measured while exposing the sample to elevated temperatures and pressures in the well.

Fig. 2. Motor and paddle assembly without pressure vessel.

The reaction torque was related to the gel strength of the slurry through continuous measurements, and an experimentally determined relationship between the gel strength and reaction torque. A drive motor rotated the paddle, to condition the sample at high speed (150 rpm, est.) to simulate pumping and displacement of the slurry. Once the placement time was reached, the paddle was rotated at a speed from 0.20° to 2.0°/min, minimizing shearing of the sample to approach static measurement conditions. The reaction torque, indicative of the gel strength, was measured, using a capstan drive, variable speed gear motor, and force transducer. The infinitesimal motion of the paddle allowed gel strength to be measured without inhibiting development.

A smaller apparatus (Figs. 1 and 2) was developed, which operates by using the previously described method. A 250-mL pressure vessel was equipped with a top-mounted magnetic drive and stepper motor to rotate a flat, vane-style paddle at speeds ranging from 0.20° to 2.0°/min while measuring the reaction torque induced on the paddle. An offset load cell is used to measure torque. The torque is related to shear stress on the sample, indicating gel strength. The motor and magnetic drive can condition the sample at 150 rpm before the gel measurements are started. The motor speed durations are user-programmable, as needed, to model well conditions. Additionally, the apparatus resists damage, if a cement slurry under test becomes a solid.

When the sample is loaded into the pressure vessel, and the HPHT test conditions are defined, the sample is conditioned at 150 rpm prior to starting the gel strength measurement phase. This eliminates a separate slurry conditioning requirement prior to loading the sample in the pressure vessel. Pressure ramping control is provided, using a precision syringe pump system to simulate well conditions, and to ensure that the gel development is not compromised with sudden changes of pressure, due to the use of an air-over-liquid pump.

Once the gel strength test is complete, the development times and transition time are provided by the controlling software, reported as the time to reach 100, 200, 300, 400, 500 lbf/100 ft² (or other engineering units). The transition time is the time difference between the 100 and 500 results. In Fig. 3, a typical test uses the mechanical gel strength analyzer, noting the automatic conditioning and gel measurement phases.

Fig. 3. Typical gel strength—mechanical method.

Calibration of the torque measurement vs shear stress (gel strength) involves loading the torque transducer with masses used to create known torques. This method provides optimal accuracy and provides an easy-to-use means to verify the performance of the apparatus and the linearity of the torque measurements.

Determining the relationship between paddle torque and gel strength is challenging, due to variation of paddle geometries used in the industry. Unlike the thickening-time test and standard API slurry cup geometry, an industry standard defining gel strength measurement paddle geometry does not exist. As the paddle rotates slowly, the sample shears along the swept cylindrical area; the swept diameter of the paddle, length, and other aspects of the paddle geometry affect the calibration. Each manufacturer will provide a means to calibrate the instrument.

The basis of the mechanical method for evaluating the gel strength is comparable to the ASTM D2573 “Standard Test Method for Field Vane Shear Test in Saturated Fine-Grained Soils.” This standard provides a means to evaluate the shear stress of soils, using a flat blade paddle and torque measurements.

Advantages to this approach include:

• Testing under HPHT conditions that simulate the temperature and pressure profile
• In-situ sample conditioning with automatic transition to the gel strength measurement
• Useful in cases where the acoustic properties of the cement sample are incompatible with the acoustic method
• Ease of operation
• Disadvantages include:
• Not a true static measurement, due to the motion of the paddle
• A need to maintain the apparatus to minimize measurement friction that degrades the sensitivity of the reaction torque measurements.

ACOUSTIC METHODS

An ultrasonic analyzer (UCA) was developed during the early 1980s that relates sonic velocity of the sample to the compressive strength of the cement.²

Fig. 4. Acoustic gel strength analyzer.

A heated high-pressure vessel is used to contain the cement sample, as sound pulses are used to determine the sonic velocity through the slurry as the cement becomes a solid. As a part of this development, numerous cured cement cubes (2-in. x 2-in.) were destructively tested after measurement of the sonic velocity. The resulting data set was used to relate sonic velocity to compressive strength in the form of three algorithms for low-, mid-range, and high-density samples. The resulting apparatus is used to continuously measure the development of compressive strength under downhole conditions.

Calibration of the UCA is performed, using a steel bar for which the sonic velocity is known, serving as a reference. Alternately, the calibration can be achieved using distilled water at a known temperature and pressure. The sonic velocity is determined, using the REFPROP7 database from NIST.

Approximately 16 years after development of the UCA, Sabins and Maki discovered that the ultrasonic analyzer technique could be adapted to measure a change in the acoustic signal characteristics during the gel formation phase before the start of the compressive strength development. (Refer to “Acoustic method for determining the static gel strength of a cement slurry,” U.S. Patent 5992223, 1999.) Reference Fig. 4.

They discovered that the transient acoustic signal attenuation through the slurry can be used to predict the development of static gel strength. A high-energy acoustic pulse is transmitted through the slurry that is contained in a heated HPHT pressure vessel. The signal attenuation that is indicative of the gel development is measured at 30-sec intervals while also measuring the signal velocity indicative of the cement compressive strength development, creating a dual-function instrument capable of compressive strength and gel strength development measurements.

Fig. 5. Typical gel strength—acoustic method.

The cement slurry is prepared, as per API Specification 10 procedures, including conditioning. When the sample is poured into the acoustic pressure vessel and HPHT test conditions are defined, the initial static gel strength of the slurry is provided to the operating software. The signal attenuation transient is used to determine development of static gel strength, detecting inflection points in the transient that indicate the start and end of the gel formation phase. As a post-processing step, the rate of change of signal attenuation is used to calculate the development of static gel strength.

Once the static gel strength test is complete, the development times and transition time are provided by the controlling software, becoming a part of the final graphical summary. An example of a typical test using the acoustic gel strength analyzer is shown in Fig. 5. Once the tests (gel strength and compressive strength) are complete, and the sample is cooled, the cement sample is removed from the pressure vessel as a solid.

Advantages to this approach include:

• No sample shearing, a true static measurement
• Testing under HPHT conditions that simulate the temperature and pressure profile in the well
• Combined measurement of static gel strength and compressive strength, a dual-function instrument
• Ease of operation and maintenance

Disadvantages include:

• No in-situ sample conditioning, prior to the gel strength measurement
• Not a physical (direct) measurement of gel strength
• Some specialized slurries designs, including additives, may not exhibit the acoustic properties necessary for use with the acoustic method.

DATA COMPARISONS

Fig. 6. Comparison of results—mechanical vs acoustic measurements.

Due to differences in methods for determining gel strength characteristics, a study compared results from acoustic and mechanical methods. Figure 6 provides data sets from separate instruments used to evaluate identical cement samples. In each case, the sample was identically conditioned, heated and pressurized. Results from the two methods are in agreement.

LABORATORY MODELING FOR GAS MIGRATION

As a separate development, an apparatus creates a means to evaluate the gas migration mitigation performance of a slurry under downhole conditions, essentially used as a laboratory model of conditions in the wellbore. The gel strength result obtained from the acoustic or mechanical instruments may be used with establishing the testing parameters.

The slurry under test is placed in a cylindrical pressure vessel and placed in a convection oven for heating to match well conditions. The disposable vessel is equipped with a flexible diaphragm, located at the bottom of the vessel along with temperature and pressure transducers. Nitrogen is injected to pressurize the sample. Once stable pressure and temperature conditions are achieved, differential pressure is created across the sample.

The start of the sample evaluation is indicated by a change in sample temperature, due to an exothermic reaction. A nitrogen flowmeter is then used to evaluate whether gas migration occurs, due to the presence of the differential pressure across the sample. Knowing well and pressure vessel geometries, one can calculate the experimental differential pressure across the cement slurry under test, to create comparable shear stress conditions, known as the “scale-down” method.

PRECISION PULSE-FREE PUMP

For the methods mentioned, it is important to ensure that integrity of the gel structure of the cement is not disturbed during testing, due to sudden pressure changes. Pressure disturbances that occur with use of a typical air-over-water style of pump interfere with acoustic measurements, and are likely to affect mechanical method measurements. To avoid this possibility, and to improve accuracy of the measurements, a pulse-free syringe pump is used with the acoustic and mechanical gel strength apparatus. This removes the possibility of damage to the gel structure under test by supplying static pressure control. The pump also provides a means to ramp the sample pressure to model well conditions, Fig. 1.

Advantages of a pulse-free syringe pump system include:

• Greater accuracy and precision of the pressure control, including programmed pressure ramping
• No pulses or pressure spikes
• Extremely low maintenance and reduced associated costs.

Disadvantages of a pulse-free syringe pump system include:

• Higher initial costs
• Possibly an increased instrument footprint.

CONCLUSIONS

• Gel strength development is a critical component in the evaluation of completion cement slurry design. Mechanical and acoustic methods each have certain advantages and disadvantages.
• Calibration of the torque measurement vs shear stress (gel strength) is critical to verify the performance of the apparatus and linearity of the torque measurements used to determine gel strength values.
• Gas migration instruments may incorporate gel strength test results into the test procedure. A scaled-down method to simulate downhole conditions is used with this technology.
• Precision pulse-free pumps are recommended with gel strength tests to eliminate pressure spikes, which can jeopardize gel structure integrity during a test. These pumps also provide much better pressure control and accuracy while greatly reducing maintenance.

REFERENCES

1. Sabins, F. L., and D. L. Sutton, “The relationship of thickening time, gel strength, and compressive strength of oilwell cements,” Society of Petroleum Engineers, March 1, 1986.
2. Rao, P. P., D. L. Sutton, J. D. Childs and W. C. Cunningham, “An ultrasonic device for nondestructive testing of oilwell cements at elevated temperatures and pressures,” Society of Petroleum Engineers, Nov. 1, 1982.