Schlumberger

Performance Report: BREAKDOWN Breaker Enhances Production on Southern North Sea Gas Well

Challenge: A well in the UK sector of the North Sea came on stream as a Rotliegendes gas producer in September 1998. A production logging tool run indicated water ingress at the base of the perforations with a rising water interface. The proposed sidetrack aimed to enter the Rotliegendes at an angle of 90°, with a 500‐ to 2000‐ft (152‐ to 610‐m) reservoir section to increase wellbore contact with the reservoir to maximize reserves recovery. The well would be completed with a 230‐micron standalone premium screen. Formation or completion impairment would need to be avoided to maximize productivity, minimize drawdown and discourage water encroachment.

Solution: A 9.0‐lb/gal (1.08‐SG) FLOPRO NT fluid would be designed to seal the formation face and minimize filtrate invasion while delivering optimum drilling performance. 1‐2% KLA-STOP NS additive would be used to minimize low‐gravity‐solids (LGS) buildup and help maintain a pristine filter cake. The BREAKDOWN chelant/enzyme‐breaker system was chosen for its ability to fully remove the FLOPRO NT filter cake from the sandface, while controlling breakthrough time. This would ensure that the well did not take losses prior to pulling out of the hole with the washpipe and closing the Fluid Loss Control Valve (FLCV). The FLOPRO NT reservoir drilling fluid/ BREAKDOWN breaker yielded positive results in an independent return permeability study.

Result: An 8½‐in. hole was drilled with VERSACLEAN LT oil‐base mud through the Haisborough group and Zechstein sequence. The 7‐in. liner was set, and a 6‐in. hole was drilled through the Leman Sandstone reservoir with the FLOPRO NT system. A solids‐free FLOPRO NT fluid was displaced into the well prior to running screens. The 9.0‐lb/gal (1.08‐SG) BREAKDOWN breaker was spotted after the screens and packer had been run. The BREAKDOWN breaker allowed the 2⅞‐in. washpipe to be pulled from inside the screens and the FLCV to be closed without losses to the formation.
An average permeability of 2 mD was expected, but the logging‐while‐drilling (LWD) resistivity tools showed a better reservoir section in terms of porosity. As a result, permeability values were expected to increase. The well test analysis reported an average value of 10 mD. The field well test analysis revealed no skin damage.

Well Information
Location UK Southern North Sea
Well Type Development Gas Producer
Date February 2006
BHST 162˚F (72˚C)
Filter cake destroyed 9.0‐lb/gal (1.08‐SG) FLOPRO NT
Completion method 1846 ft (563 m) 230 micron 6x4‐in. (152‐ x 102‐mm) premium screens

After trouble‐free drilling of the interval, the 230‐micron, 4‐in. screens were run to TD without encountering any problems. Then, a 2⅞‐in. washpipe was run inside the sand screens on 3½‐/5‐in. drill pipe. After setting the packer, 54 bbl of BREAKDOWN breaker was pumped into the string. The breaker was displaced with brine, which placed 27 bbl of breaker between the sandface and the screens. The string was pulled back so that a washpipe‐perforated pup joint could be placed abovea seal bore. Next, additional brine was pumped to displace the remaining breaker inside the screens. The washpipe was pulled out and the FLCV closed. No losses were observed while spotting the breaker or pulling back the washpipe.

The breaker was premixed, shipped offshore in tote tanks and pumped via the cement unit. This approach minimized waste and avoided personal contact with the fluid, emphasizing safety and ease of use.

The breaker was designed for an 8‐ to 24‐hr breakthrough time, assuming a drilling overbalance pressure of 1387 psi and bottomhole temperature of 150°F (66°C). In fact a slightly higher overbalance pressure (1656 psi) was encountered in the actual well.


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