Wellbore instability, differential sticking, and lost circulation are significant challenges while drilling build-up sections through stressed shale and formations with varying pore pressures. Conventional drilling fluid systems have not eliminated these drilling challenges, and thus it was necessary to identify a fluids solution which could enhance drilling performance and meet these challenges.
Based on extensive laboratory tests, an ultra-low fluid invasion (ULFI) additive was selected. The ULFI technology is designed to form a very-low permeability seal that limits the transmission of destabilizing wellbore pressure into the geologic formations. This pressure barrier effectively minimizes formation breakdown and prevents fractures from propagating, creates a wellbore seal which effectively minimizes fluid invasion into micro-fractures, and thus stabilizes the weak shales.
A customized drilling fluid system was designed using the ULFI additive in conjunction with sized CaCO3 and synthetic resilient graphite. These particles effectively plugged the pore throats and minimized the fluid invasion, which was confirmed by sand-bed tests as well as permeability plugging tests (PPT) under downhole conditions. The customized drilling fluid utilizing ULFI additive proved its success by providing:
Achieved zero non-productive time (NPT) related to fluids
Improved wellbore stability in stressed shale formations
Eliminated the risk of differentially stuck pipe across low-pore-pressure formations
Mitigated induced losses by utilizing customized bridging additives
Enhanced hole cleaning at critical build angles
All challenging build-up sections were successfully completed without any issues. There were no wellbore instability issues including differential sticking tendencies or drilling fluid losses reported. Wellbore instability was mitigated even when drilling in the minimum stress direction, which is typically more challenging compared to the maximum stress direction.
This paper will present the success of the ultra-low fluid invasion (ULFI) additive in an invert-emulsion fluid (IEF) with case histories for reference. The molecular weights of the polymer components in the ULFI additive are low, which allowed for easy mixing and did not contribute negatively to rheological changes in the drilling fluid system.