Faced with high-cost failures using conventional slurries, an operator tests metallic microribbon cement for successful kickoff plugs.

An operator used a new, impact-resistant cement system to perform difficult, hard-formation sidetrack operations in a recent three-well Middle East field test conducted jointly by the Abu Dhabi Co. for Onshore Oil Operations (ADCO) and Schlumberger.
The cement system showed significant performance improvements when compared to conventional systems in terms of plug success rate and time required to sidetrack. This reduced well costs substantially.
The improved performance is attributed to recent developments in oil well cement systems that couple advanced metallic microribbon technology with particle size optimization techniques. This produces a set cement with increased durability, drilling toughness and load-bearing capacity. It has high impact and fracture-propagation resistance, as well.
No repeat plugs - a frequent problem in previous operations using conventional cements - were required, and average sidetracking time was reduced by more than half.
This, the world's first metallic-reinforced cement kickoff plug program, was so successful in meeting sidetracking objectives - operationally and economically - that 12 additional plugs were installed.
Metal-reinforced cement plugs
Sidetracking existing boreholes into ultrahard formations from balanced cement plugs historically has been problematic for the drilling industry. Extended time drilling and sometimes multiple plugs often are required to achieve sidetracking objectives. Such problems can lead to higher operating costs.
Traditionally, cement slurry systems developed for difficult well operations such as sidetracking were designed to have higher compressive strengths, achieved by lowering their water content. As a result, kickoff plugs formed with conventional, high-compressive-strength cement systems become brittle and display little resistance to impact and fracture propagation through the set cement. Thus, they can break and shatter when exposed to the high impacts and mechanical shocks of typical sidetracking operations.
New cement slurry technology focuses on optimizing the mechanical durability and impact resistance of the set cement. The Schlumberger DuraStone cement system's metallic microribbon provides a high-strength secondary matrix with increased surface area, improving upon the performance of fibers available in other slurries. This, along with techniques for optimizing particle size distribution, has resulted in more durable set-cement mechanical properties with greater impact and fracture propagation resistance than conventional systems. During challenging operations, increased durability enables the cement plugs to continue to support a substantial percentage of the maximum loads past the energy-to-rupture point.
Because of its improved set-cement performance, the new slurry can be applied to situations that anticipate extreme mechanical stresses. It has been used successfully as sidetrack kickoff plugs when repeated drillstring trips are required - an activity that historically has jeopardized the integrity of conventional behind-casing cement. The metallic-reinforced cement systems have three times more drilling resistance and are about 15 times more resistant to impact failure than conventional cements, thereby allowing faster kickoffs in shorter distances, even in hard formations. Also, they exhibit significantly higher tensile strengths than conventional cements, thus requiring three to four times more energy to cause flexural failure.
Laboratory analyses have shown these new systems to display significantly higher resistance to impact and fracture propagation at a substantially lower rate of penetration (ROP). ROP is used as an indicator of a cement plug's sidetracking effectiveness. Generally, the lower the ROP, the greater the chance of achieving a successful sidetrack. The system's metallic reinforcement and impact resistance work together to augment its integrity and increase its toughness. The increased strength improves sidetracking efficiency, which was demonstrated during drilling tests by an ROP three times less than that of conventional class G cement plugs.
Bottom line, the new cement systems are more effective in resisting impacts, vibrations and flexural stresses while still maintaining the desirable qualities of production cement. These factors were evaluated carefully when ADCO sought alternatives to the conventional kickoff plugs.
New slurry put to the test
Prior to using the new system, ADCO spent many hours time drilling while performing sidetracks using conventional cement plugs. Time drilling is defined as drilling with low ROP and low weight-on-bit. When attempting to sidetrack, the company's time-drilling modes averaged almost 26 hours per plug and spanned beyond 40 hours for several kickoff attempts. A survey of the 15 wells in which sidetracks were attempted with conventional cement techniques revealed 37% were unsatisfactory or entirely unsuccessful because of the excessive time drilling involved.
The basic problem was the makeup of conventional plugs, which lacked the hardness and toughness required to kick off and build angle in hard formations. In ADCO wells, traditional plugs were softer than the surrounding formation, even after long waiting-on-cement (WOC) periods. Provided a conventional cement could have been designed to be stronger than the formation, it would have required long WOC times - up to 40 hours - to set. Meanwhile, the rig remained idle.
In light of this unfavorable performance, ADCO worked alongside Schlumberger to achieve its sidetracking objectives. After studying alternatives carefully, they selected the DuraStone cement technology for kickoff plugs in upcoming sidetracks from new and re-entry wells.
Earlier, however, Schlumberger performed for ADCO a surface mixing demonstration to test the enhanced slurry's mixability, pumpability and stability throughout its application. In the demonstration, 10 bbl of 130-pcf base slurry was batch mixed. Metallic microribbons were then added to reach a specified blend having a 135-pcf density. When the slurry was recirculated, it flowed easily through triplex and batch mixer pumps. Also, it flowed freely through restrictions, ensuring its free flow through similar restrictions in downhole tubulars. Additionally, no separation or settlement problems occurred.
A short while later, the first DuraStone kickoff plug was installed for ADCO in an 8,398-ft (2,561-m) vertical well. The well had 95/8-in. casing set to 7,700 ft (2,349 m) in a 121/4-in. hole, with 700 ft (214 m) of 81/2-in. pilot hole below. After coring, the pilot hole was plugged back using conventional cement at bottom, above which 46 bbl of the new slurry was placed as a kickoff plug at 7,920 ft to 7,600 ft (2,416 m to 2,318 m). Plans were to sidetrack into the target formation 50 ft (15 m) below the casing shoe.
After placement and just 16 hours of WOC time, the plug was drilled using a build-up assembly with 2.38° bent housing. At 7,750 ft (2,364 m), kickoff began from the plug to 7,767 ft (2,369 m). The sidetrack was achieved after just 17 ft (5 m) of plug drilled and 5 hours of time drilling spent. At this time, shale shaker returns were mainly formation cuttings. Following the sidetracking operation, a gyro survey run on measurement-while-drilling confirmed that a successful kickoff was achieved.
A month later, the second microribbon- enhanced cement plug was installed, this time in a deviated well. Previously, a lengthy sidetrack had been attempted into the target formation using a 14-bbl class G cement plug placed at 13,678 ft (4,172 m) MD and 8,790 ft (2,681 m) TVD. The sidetrack began after 26 hours WOC, and time drilling continued for 33 hours to a 13,737-ft (4,190-m) depth. At this time, 70 ft (21 m) of cement had been drilled out without building new angle, confirmed by 100% cement returns to the shale shakers. The bottomhole assembly (BHA) was pulled to surface, and the kickoff attempt repeated, this time using the new metallic-reinforced cement system.
An 18-bbl kickoff plug was placed at the same depth as the previous one. After a 16-hour WOC period and a bit change, a new BHA was run that included a 1.83° bent housing. Within the first 4ft (1 m) of drilling, formation cuttings were returning to the shale shaker. After only 5 hours time drilling to 13,705 ft (4,180 m) - 51ft (16 m) of cement drilled - a survey indicated the angle had increased from 64° to 70°, confirming a successful sidetrack.
The next month, the new system was considered for another well - a 22-year-old vertical injector. A sturdy, rugged cement plug was needed to sidetrack from a milled section in order to place a horizontal lateral in the well, which had 95/8-in. casing set to 5,880 ft (1,793 m) and a 7-in. vertical liner set at 8,950 ft (2,730 m). After milling the top 7-in. liner to 120 ft (37 m) below the casing, a 30-bbl kickoff plug was placed at 6,100 ft (1,861 m). Kickoff was initiated after 20 hours of WOC and successfully achieved in the targeted formation within only 5 hours of time drilling.
The three-well field test proved the successful application of the new cement system. Its use substantially improved the kickoff success rate and reduced the time required to achieve sidetracking objectives by minimizing time drilling and eliminating plug repeats. While numerous plug repeats were required in previous sidetracking operations using conventional cement plugs, no repeats were needed with the 12 DuraStone plugs set to date.
Additionally, during the 6 months of the system's use, the average time required to achieve sidetracking objectives dropped to 11 hours per plug from the previous 25.5-hour average.
What's ahead
The DuraStone cement system provided ADCO with significant efficiency improvements and cost reductions. Originally, this new slurry was developed to provide support and isolation in level 5 and 6 multilateral junctions. However, optimizing well sidetracking operations has proven an equally important application. Other potential applications yet to be tested are maintaining isolation across weak production intervals and high-density perforated zones. An additional application is reinforcing casing shoes in weak formations or highly deviated hole sections.
As more level 5 and 6 multilateral wells are drilled, the system will be used increasingly to ensure zonal isolation integrity across junctions. But the system's metallic reinforcement and impact resistance combine to provide solutions not only to many difficult well scenarios currently faced in the industry, but to those to be addressed increasingly in the future, particularly as well configurations become more challenging.
Acknowledgments
The authors thank ADCO for providing the field data. The authors also thank ADCO and Schlumberger for permission to publish this article. In addition, the authors thank ADCO lead cement engineer Hassan El-Hassan and lab supervisor Basim Jamous and Schlumberger service supervisor Haitham Jarouj, lab engineer Abdul Hameed Mohsen, field service manager Abdul Latif Hasni and cementing well services business development manager Roger Keese, keese@slb.com, for their contributions to this article.

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