Improving operations through circulation subs

Daniel Perez says that the growing trend among oilfield technology companies to create tools that feature advanced functionality is exemplified by National Oilwell Varco's second-generation circulation sub tool, MOCS G2

Daniel Perez is the Eastern Hemisphere's product line director for Drilling Technologies at NOV Wellbore Technologies.
Daniel Perez is the Eastern Hemisphere's product line director for Drilling Technologies at NOV Wellbore Technologies.

Recent improvements to circulation subs have led to greater success in high inclination wells, MPD environments and aggressive fluid regimes where conventional technology has failed. Operations in these environments require versatile tools that can effectively conquer multiple mud and fluid-loss situations and hole cleaning applications in the same interval.

Oilfield technology companies have created tools with advanced functionality, such as the second-generation circulation sub tool from National Oilwell Varco (NOV), which improves drilling efficiency by providing unlimited, on-demand cycling between the ID of the string and into the annulus, in just 10 seconds. The tool has an enhanced hydrostatic pressure-balancing mechanism and automatically closes ports when incoming flow drops, maintaining well control safety. Through a single activation drop ball that can be preloaded on the surface, the circulation sub can be placed anywhere in the string, and operators can change the flow path despite an obstructed ID above the tool.

This advanced circulation sub technology has been proven in operations in the Middle East.

Case studies: Middle East

One operator in the Middle East had a challenge curing losses in a 16in carbonate section without damaging the directional BHA tools when pumping lost circulation material. Using an 8in circulating sub tool, in conjunction with a 16in HLB directional assembly, the operator directionally drilled the 16in section with 9ppg WBM from vertical to an inclination of 47° from 282ft to TD at 5,010ft, through Dammam and Ahmadi formations. The tool functioned fully through 151 circulating hours, successfully cycling 84 times and meeting the client’s requirements. This many cycles would not be physically possible with a conventional multiple drop-ball sub that comes with limited cycling and a delayed response. The next generation circulation sub saves trips to empty an otherwise exhausted tool, and reduces loss curing time.

Another operator in the Middle East activated the circulation sub at a depth of 2,396ft to pump 140bbls of 9ppg super sweep to the annulus. While tripping out to adjust the BHA, the operator pumped another 100bbls of super sweep at a depth of 550ft. The ball was retrieved from the tool and it was picked up again after adjusting the BHA. The operator activated the circulation sub a second time, at a depth of 3,595ft, to pump 300bbls of super sweep to the annulus. In total, the tool successfully cycled 32 times, helping pump a total of 540bbls of super sweep to the annulus, and achieving 252 circulating hours with responsive functionality. The tool protected the directional BHA tools and exceeded expectations when it came to the amount of time that had to be spent bypassing flow.

A third operator in the Middle East picked up and made up a 6½in circulating sub to a 6in clean-out BHA. The tool was activated at a bit depth of 14,453ft MD to circulate the hole clean. A total of 1,010bbls of 1.22sg water-based mud (WBM) was displaced through the tool using a flow rate of 600gpm. The tool was then tripped out after circulation was complete. The tool was shifted three times in this assembly, and used for 36 circulating hours. The same tool was later picked up and made up to the 7in by 9⅝in tandem scraper assembly, and activated at a depth of 14,453ft MD to circulate the hole clean. A total of 656bbls of 1.18sg WBM was displaced through the tool, using a flow rate of 700gpm. The tool was shifted six times in this assembly and was used for 10.5 circulating hours.

Case studies have demonstrated how new circulation sub technology can help operators to improve drilling efficiency and reduce drilling costs. Unlimited cycling alleviates the need for multiple balls, allowing operators to use the tool with one ball. The flexibility of cycling the tool quickly allows drilling operations to resume and, ultimately, reduces the cost of drilling a well. Whether it is quickly curing losses, allowing pumping LCM multiple times and in multiple zones as needed, or circulating out debris from the hole without needing dedicated hole-cleaning runs, the circulation sub tool allows the operator to safely and efficiently improve the performance of the operation. NOV’s new second-generation Multiple Opening Circulating Sub (MOCS G2) eliminates the need for trips to remove flasks full of activation/deactivation balls, increasing safety around the rig floor. NOV’s new technologies promise to achieve lower drilling costs and deliver efficiency more safely.

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