By Kelli Ainsworth, Editorial Coordinator
Since acquiring its first five blocks in Kenya in 2012, Tullow Oil has reduced its well costs from $55 million to approximately $10 million to $15 million per well, Alexandra Reddaway, Tullow Drilling Engineer, said at IADC World Drilling 2015 in Rome on 17 June. Challenges that have arisen during the exploration campaign in this East African country range from a limited infrastructure to a unique subsurface geology to a lack of offset data. Further, one of the biggest challenges the company had to confront was implementing the right drilling fluids solution to address the wide variation in bottomhole temperatures, with some wells reaching as high as 190°C.
From the start of Tullow’s exploration campaign in Kenya, the company knew it would face significant logistical challenges. The blocks were located in remote areas and separated by vast stretches where road infrastructure was very limited. Subsurface, the challenges were not any easier. “All our blocks lie upon the East Africa rift system,” Ms Reddaway said. “All our blocks are prone to significant faulting… The faults in themselves cause potential massive losses and stability issues. It also means that we have a lot of compartmentalization, so each well is completely unique.” Due to this compartmentalization, wells that are located only 5 km apart can be radically different, she added.
Because environmental protection is critical in the region, Tullow decided to always utilize water-based muds (WBM) in the surface hole. Initially, the company used a polymer spud mud. Although this was a low-cost option, it did not provide the level of inhibition and wellbore stability that was needed, Ms Reddaway said. “In some of our fields, we had shales quite high up so we were having balling from the moment we entered the well,” she said.
To increase inhibition, Tullow then deployed a K-silicate mud, a high-performance WBM. While this fluid did improve inhibition and wellbore stability, it was relatively high cost and interfered with logs. Next, Tullow switched to a lower-cost synthetic-based K-sulfate spud mud. The operator found that this mud delivered the same level of wellbore stability and inhibition, but with added hole cleaning ability, higher ROP, improved tripping times and better data acquisition. This is the spud mud Tullow is currently using in its Kenyan wells.
The operator went through a similar evolutionary process with its fluids selection for the drilling of the production interval. Early wells using a WBM were unstable and suffered from slack-offs and cave-ins. “You name it, it happened,” Ms Reddaway recalled. “It’s part of the reason they were such high-cost wells.” Further, the poor hole conditions in these wells sometimes led to failed wireline logging programs and inadequate data acquisition – a significant obstacle to any exploration and appraisal programs.
Tullow then switched to the K-silicate mud that was being used for the surface hole, and amped up the mud weight, going from 9-9.5 ppg with the previous fluid to 9.5-10 ppg with the K-silicate. Although this solution provided better inhibition, formation damage occurred while this mud was used. In addition, the bottomhole static temperatures exceeded 190°C in some wells, and the K-silicate fluid is not stable over 120°C.
Finally, Tullow introduced a synthetic mud for the production hole drilling and increased the mud weight even further to 10.5-12 ppg. The higher mud weight was found to improve wellbore stability, Ms Reddaway said. “This is actually the best solution for us,” she said. “It provides better inhibition and really good wellbore conditions. We have great directional control in our deviated wells. It’s reusable, and it’s stable at higher temperatures.”
The evolution of Tullow’s fluid system in Kenya, in addition to changes to well design, has helped to significantly decrease its drilling time per well. The company’s first wells in the region were delivered in 30 days; the most recent wells have been delivered in 12. Tullow’s ultimate goal is to deliver a 2,500-m well in just 10 days, Ms Reddaway said, and the operator is within 10% of that target. “Our timings have been reduced by 300% from a year ago,” she said. “All in all, we’re a hair’s breadth away from being able to deliver a feasible and affordable development campaign.”