Continuous circulation systems build healthier wells, reduce risk in difficult environments

By Linda Hsieh, managing editor, and Katherine Scott, editorial coordinator

Just as humans can be injured even by a short interruption of the blood supply through our arteries and veins, wellbores also need continuous flow through the pipe and annulus to prevent potential harm. That was the analogy used by Angelo Ligrone, vice president logistics for Eni, at the 2012 SPE/IADC Managed Pressure Drilling and Underbalanced Operations Conference and Exhibition on 20 March in Milan, Italy. He presented Eni’s work on two proprietary technologies – a circulation device called the e-CD and a near-balance drilling technology called e-NBD. The company has used both to significantly reduce risks during the drilling process, particularly in difficult deepwater drilling environments.

“If we can get performance but without safety, this is not performance. Safety and performance have to come hand in hand,” said Mr Ligrone, who until recently served as vice president of drilling technology for Eni.

The e-CD, introduced in 2005, is a system that allows for continuous mud circulation to maintain constant bottomhole pressure while making up or breaking out drill pipe connections during drilling operations. “We have controlled the ECD (equivalent circulating density) throughout the drilling process, thus eliminating mud pressure fluctuations,” Mr Ligrone explained. This in turn prevents problems such as wellbore instability, as well as reduces nonproductive time.

In 2010, Eni introduced e-NBD, a technology born out of the company’s success with the e-CD. “By adding a rotating BOP and bringing it down on top of the conventional BOP stack and the active choke system, the e-CD becomes the e-NBD system,” he said. The technology allows for the maintenance of constant bottomhole annular pressure at all times while circulating and to manage the annular dynamic hydraulic pressure profile. “The benefits indeed are well control and safety first of all … and improving hole conditions.”

He added that e-NBD is an enabling technology helping Eni get to targets where conventional technologies cannot, particularly for operations within narrow pore/fracture pressure gradients, as well as for HPHT and underbalanced operations. In March 2008, for example, Eni completed its first e-NBD HPHT well in Egpyt at 5,450 meters TD; this “nightmare” scenario involved 2.25 sg mud, 2.23 pore gradient and 2.26 equivalent mud weight fracture gradient.

In another example offshore Libya, the e-NBD system was deployed from a floating rig to reenter a temporarily abandoned exploration well where drilling activities had been stopped due to conventional drilling limits. The bottomhole target was reached without problems using e-NBD, Mr Ligrone said.

Onshore as well, the technology has been used in difficult drilling environments. A vertical land well in Pakistan used e-NBD to drill the 10 5/8-in. and 8 ½-in. sections to reach the gas targets. “The decision to drill the section with the e-NBD system was taken based on the fact that such phases were explorative, and the only data available from one reference well in the area was showing a high formation pressure with an incremental trend of the pore pressure gradient,” he said. The reference weight had been suspended due to continuous formation pressure increase and strong gas showed a high percentage of gas present and well construction limitations, he continued.

“I will stress again that safety and performance have to go hand in hand. We cannot have performance without safety or safety without performance,” Mr Ligrone emphasized. “They are essential in ensuring a sustainable drilling business. Continuous circulation is recognized as a key factor for safer and faster drilling.”

e-CD and e-NBD are trademarks of Eni.