Cement plugs are widely used to achieve zonal isolation in well intervention or plug-and-abandonment applications. Plug cementing operations can be performed during operations to restore or increase production, such as by altering the gas-oil ratio or by controlling water production. Cement plugs isolate a certain well zone (e.g., depleted zone), providing options to perforate a new well zone or temporarily or permanently abandon a well or a specific well zone. One of the most commonly used techniques to place cement plugs is to use a dump bailer run by wireline.
In some well intervention applications, the cementing process can consume more than 40% of the total operation time. In the oil and gas market, there is an increasing demand to improve well intervention operations through lower costs, shorter operation times or by eliminating nonproductive time (NPT). Also, cement plug failures result in more than 1,000 hours of lost time and $20 million in NPT. The typical cement plug failures include
- Loss of cement plug due to insufficient bond strength between casing and cement;
- Insufficient hydraulic and pressure seal due to weak or even hardened cement; and
- The cement plug not set at the designed depth.
There are multiple causes for these failures, such as cement slurry contamination, inadequate mixing and displacement methods, and incorrect downhole temperature used for slurry design.
Understanding the key challenges associated with wireline- run cement plugs is essential for improvement on a system level. Several factors control the quality of the cement plugs and the total amount of time necessary for cementing. A few of the critical parameters are listed below.
Wait on cement time
Wait on cement (WOC) time is the amount of time necessary to suspend operations to allow the cement slurry to solidify, harden and develop compressive strength. Any interruption to the cement slurry during WOC time can lead to cracks, voids and pathways for gas and/or fluid, which can compromise zonal isolation. Existing cement mixes usually have a recommended WOC time of 24 hours to achieve sufficient strength for the cement plug. The WOC time can be shortened by reducing the retarder quantity added to the slurry.
Reducing WOC time also can shorten the dump time, which is usually estimated by the gelling or thickening time after mixing. After the dump time, the slurry becomes difficult to dump from the bailer, where it could become hardened. For example, typical wireline has a run-in-hole speed of 30.5 m/min (100 ft/min). A cement mix with a 90-minute dump time can only be set up to 2,743 m (9,000 ft) in the well.
Dump bailer operation time
The amount of time to run the dump bailer for cement plugs is mainly determined by the number of runs and the volume of the dump bailer. The number of runs depends on the length of the cement plug required as well as the volume of the bailer. The typical capacity of a 3-m (10-ft) bailer varies from 0.64 gal (13⁄8-in. bailer) to 9.24 gal (5-in. bailer), depending on the diameter. Multiple bailers can be stacked into a longer bailer string. Because of the diameter of the pipe and the maximum height provided by the crane, the total length of the bailer string is limited. In reality, increasing the volume of dump bailer per run may not be a practical option.
The minimum required cement plug length depends on the shear bond strength between the casing and the hardened cement to achieve a long-term hydraulic and pressure seal. The actual bond strength is significantly affected by the multiple dumping processes, temperature fluctuations and chemical shrinkage of cement during hydration. The presence of materials like crude oil or heavy brine in or near the setting depth can negatively affect the quality of cementing operations and substantially reduce the integrity of cement plugs. Early studies show that shear bond strength under ideal conditions can be estimated to be about 10% of the compressive strength. The improvement in compressive strength provides a margin to either reduce the WOC time or decrease the cement column length and therefore the number of bailer runs, depending on operator preference.
Cement mix temperature sensitivity
The properties of cement mixes are sensitive to wellbore temperature and pressure conditions. Lower than expected downhole temperature can delay the cement hydration reaction and compressive strength development, leading to longer setting time and lower strength at the same amount of WOC time. When the temperature is higher than the cement mix was designed for, the setting time can be accelerated, but compressive strength might be negatively impacted. In the high-temperature environment, the strength retrogression caused by insufficient silica sources (e.g., sand and quartz) might occur, leading to the breakdown of the set cement matrix.
Figure 1 shows the compressive strength development for two different cement mixes, C1 and C2. Both cement mixes were formulated for 121 C (250 F) bottomhole temperature but tested at 115.5 C (240 F), 121 C and 126.6 C (260 F). From the measurement data, it can be observed that key performance parameters, such as WOC time and dump time, are significantly different, even for a temperature difference of 12 C (10 F).
It also can be seen that by tailoring the recipes of cement mixes, higher compressive strength can be achieved with a minimal impact on setting time. Cement mix C1 is a commercially available cement mix, whereas cement mix C2 was developed in-house. Cement mix C2 exhibits almost twice the compressive strength than cement mix C1 over the same time. This can help reduce the WOC time by a factor of two without sacrificing compressive strength.
In many cases, the plug length and WOC time are regulated by the local government or company policies. However, this does provide the operator with added flexibility to tailor the service according to its specific needs (Figure 2).
For example, if WOC time remains the same at 24 hours, to achieve a compressive strength of 3,000 psi, the length of the cement column can be reduced by half, which means fewer bailer runs. If the length of the cement column is unchanged, the WOC time can be reduced from 20 hours to 9.5 hours to achieve a compressive strength of 3,000 psi. Both the WOC time and cement column length can be optimized to help improve plug cementing. As a result, the operator can realize a 15% to 25% savings in the overall time needed to operate.
Accurate knowledge of the well temperature profile enables precise selection of the best additives, which is necessary to achieve a cement plug as designed and optimize the service.
References available. Contact Brian Walzel at email@example.com for more information.
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