A case history of an underbalanced drilling project in the Danish sector of the North Sea was presented by Geoff Gough, DONG Energy senior process engineer, at the 2011 IADC/SPE Managed Pressure Drilling & Underbalanced Operations Conference & Exhibition last week in Denver, Colo. “Why underbalanced drilling? Essentially because we have to. It’s the only way you can reach these reserve targets,” Mr Gough said.
The project also verified UBD as a viable technology for unlocking reserves in the north of the field, where pore pressure is hard to predict, he said.
Two flank wells were drilled in this project, using UBD for the 6-in. horizontal sections. The first well targeted a high-pressure zone, and the second well targeted a lower-pressure zone in a slightly different formation. “We used a closed-loop circulating system so we completely bypassed the rig’s pits and shakers,” Mr Gough said. Hydrocarbons were produced at surface while drilling.
Crude oil was initially selected as the drilling fluid because it was expected to lower the friction factor and therefore generate smoother drilling conditions. “Unfortunately, while it worked in terms of separation and we maintained underbalanced conditions, the composition of the crude was detrimental to the downhole tools,” Mr Gough said. The team then changed to water and actually found lower friction with water and lubricants than with oil.
One key piece of equipment in this drilling campaign was the disc-stack centrifuge, thought to be the first deployment on such a project. Because 80% of the solids were less than 50 microns, they were incredibly difficult to separate, Mr Gough said. Using the disc-stack centrifuge allowed the team to remove the fine solids and allow for recirculation of the fluid. The centrifuge also acted as a three-phase separator for solids, water and oil.
The first well stayed underbalanced throughout, with both crude oil and the treated seawater. It was very productive, and all the oil and gas produced were sent to the production platform. “On the second well, we weren’t underbalanced at the toe. We couldn’t achieve production without a high drawdown, and we had no desire to do this. We would’ve required gas injection, which would give us increased complexity and a risk of hole enlargement,” Mr Gough said. Overall, the team was able to send 68,000 bbls of oil and 45 million standard cu ft of gas to the production platform while drilling.
Looking back at the success of the project, Mr Gough said that he believes the pre-job onshore commissioning of surface separation and circulating equipment was invaluable. “We had numerous commissioning sessions and recommissioning sessions, but it was so worthwhile getting that lack of NPT we had offshore. I just can’t stress how important it is to have onshore commissioning for new equipment like this,” he said.
The success of this UBD drilling campaign has led the company to sanction a new underbalanced drilling project, and two new platforms are being built. “We’re hoping to drill 11 new wells in one or two campaigns … 10 of which will be UBD wells,” he said. The project is due to start in August 2012.
Further details about this innovative UBD project can be found in the March/April issue of Drilling Contractor. The article, based on IADC/SPE 143096, can be read online here.