Ultrasonic Log Response Evaluates Barrier Cells for Cementing Applications
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Borehole-cement-evaluation logs are used to verify the quality of the cement bond to the casing and formation and identify any defects that could compromise the quality of the annular seal. This study used ultrasonic logging to evaluate reference barrier cells constructed with known defects. The cell concept was developed with an emphasis on low cost and ease of use. Comparison of ultrasonic tool response against reference barrier cells validates the measurements, which are crucial in the decision-making process during well construction and abandonment.
Background: Barrier Verification
In 2013 and 2014, full-scale experiments were performed to investigate the sealing ability of annulus cement for a plug-and-abandonment design in which the tubing is left in hole (TLIH). The experimental configurations used 9⅝-in. casing with a 7-in. tubing, both with and without control lines clamped to the tubing surface. Experiments were run first using standard Class G cement. In a second phase, Class G cement with an expansion additive was used to compare the quality of the seal achieved. The barrier quality of the cement was evaluated by pressure-testing the casing cement sheath and the tubing plug. The test assemblies subsequently were cut into sections, enabling the cement displacement to be controlled visually. The results showed that good cement placement could be achieved with the tubing left in the well both with and without control lines. The pressure testing showed that the control lines did not have a detrimental effect on the barrier-seal quality compared with the test without control lines.
The choice of leaving the tubing in hole when abandoning a well is dependent on an assessment of the quality of the casing cement sheath. The barrier-verification project was initiated to build on the TLIH experiments by investigating the performance of annular-barrier evaluation technologies.
Materials and Fabrication
The project aim is to construct a barrier-verification reference facility with the objectives of
- Evaluating technologies for logging through multiple tubulars
- Investigating the performance of commercial logging technologies
- Performing experiments with alternative barrier-verification techniques
In the first project phase, the TLIH sections were reused as much as possible to enable a starting point with known specifications and quality. The cement plug in the tubing was first removed carefully by hydrojetting. Thereafter, the desired defects, such as long holes of different sizes drilled through the cement immediately adjacent to the 7-in. tubing, were introduced.
Reference cells were constructed with the aim of mimicking a range of potential annular-leakage scenarios such as gas channels, mud channels, and microannuli. A design requirement was to enable the required cells to be coupled easily using quick unions in the configuration required to meet the experimental objectives. The tubing-to-casing annulus was sealed such that both the tubing and annulus could be independently pressurized with the required fluid. The pressure rating for the tubing and annulus is 2,200 psi. Pressure ports machined into the annulus bulkheads allow controlled leakage rates to be applied using water or gas.
Barrier Reference Cells
Six barrier reference cells were defined for the validation experiment. Each cell’s construction and purpose are described in detail in the complete paper.
All cells are constructed from 9⅝-in., 53.5-lb/ft casing and 7-in., 32-lb/ft tubing. The eccentricity of the tubing in the casing is caused by the standoff created by the tubing collar. All cells are cemented with Class G cement with a density of 1.92 SG. Cells numbered 1, 2, and 4 were constructed from TLIH sections cemented with expanding cement, which provides a particularly good acoustic coupling between the cement and tubing. Cell 3 was constructed from a TLIH section that used conventional Class G cement with no expansion additives and in which shrinkage resulted in a degree of debonding between the cement and tubing.
Since the introduction of ultrasonic tools in the early 1990s, ultrasonic logging has been the preferred option for cement evaluation because the measurement provides information on the cement placement behind the casing and casing condition in the same run. The technique delivers high-resolution images that offer the possibility of resolving narrow azimuthal features such as drill wear or narrow channels in cement present behind the casing. The ultrasonic pulse-echo technique measures the acoustic impedance of the material in contact with the casing outer surface by analyzing the resonance decay of an exited compressional casing node. This acoustic impedance, expressed in units of megarayls, is a simple function of the material properties behind the casing.
In the mid-2000s, the well-established ultrasonic pulse-echo technique was complemented with the addition of an ultrasonic pitch/catch configuration. Placing the transmitter and receiver transducer at an angle enables generation of flexural mode in the casing, the attenuation of which is a function of the material properties behind the casing. The flexural attenuation is expressed in units of decibel/meter and has an excellent sensitivity to low-impedance materials, allowing the discrimination of light cements from heavy muds such as barite. By placing this transducer arrangement in a rotating sub, the high-resolution azimuthal images are preserved.
Combining the two modes (compressional and flexural) in a crossplot to probe the material behind the casing is like looking at the same object from two different angles; the method provides more information and increases confidence and understanding. Processing of the data using advanced interpretation software enables interactive zonation of log intervals, and the measurement can be compared with a modeled response. Specific physics-based models have been developed that enable proper classification and determination of the various conditions in the annulus (solid, liquid, or gas) on the basis of these two measurements. The classification of solid/liquid/gas (SLG) uses prior assumptions about the range of expected properties of the material both in the borehole and in the annulus. The SLG model works well for a wide range of situations to discriminate between solids and fluids and between liquid and gas.
To reproduce original logging conditions, rigup was performed through the derrick of the Ullrig drilling rig in Stavanger. The reference cells were installed vertically in the rig kelly bushing (Fig. 1). A 7-in.‑outer-diameter lubricator was attached to the top of the reference sections such that the logging tool could log the complete length of the section in addition to 1 m of the lubricator. A stuffing box was attached to the top of the lubricator and provided a seal around the cable when recording data under pressure. Reference cells were composed with the eccentricity lined up (i.e., with the wide side of the annulus on the same side).
Experimental results for the study are not detailed in this synopsis, but readers are encouraged to see the complete paper for a discussion of the experimental process, including analysis of the log-interpretation results for cement evaluation (including details of how each log response was recorded for each cell), a brief comparative analysis for the log responses of the microannulus and good cement cells, starting and ending crossplots for the good cement cell, and an acoustic impedance crossplot progression for the good cement cell.
During the cement evaluation of a tubular casing, interpretation is made on the basis of log response under certain well conditions. Inferences are based on fundamental sensor physics, laboratory experiments, common knowledge, and contemporary understanding of the technology. Comparison of ultrasonic tool response with reference barrier cells validates the measurements, which are crucial in the decision-making process during well construction and abandonment.
A selection of test cells with various cement qualities has been logged using an ultrasonic tool that simultaneously records signals from pulse-echo and flexural transducers. Logging-tool measurement and interpretation across the reference cell, along with experimental results, have shown that these measurements were found to correspond consistently with the known properties of the test cells.
Ultrasonic Log Response Evaluates Barrier Cells for Cementing Applications
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