Challenge: Downhole loss of drilling mud is
one of the biggest economic challenges while drilling, especially in the Gulf
of Mexico deepwater. Losses occur when the drilling fluid weight requirement to
overcome the pore pressure needed to stabilize the wellbore exceeds the
fracture gradient of the formations. During one such drilling project, mud
losses and wellbore stability while drilling offset wells were identified as
major challenges to drill a sand section above salt, as well as the salt
itself, the “rubble zone” below the salt, and perhaps zones deeper
Solution: To ensure success in this high-risk
drilling scenario, M-I SWACO engineers recommended wellbore strengthening via
I-BOSS in an interval above the salt section, an interval within the salt, and
three intervals below the base of the salt. The technique of choice in all five
intervals was circumferential stress enhancement, or stress caging, which is
attained by inducing shallow fractures in a formation using elevated wellbore
pressure and simultaneously forcing Wellbore Strengthening Materials (WSM) into
the fractures to keep them propped and in a stressed state. Wet sieve analysis
was used to maintain the correct concentration and sizes of the WSM. For all
five intervals treated, blends of sized SAFE-CARB and G-SEAL PLUS products were
used as the WSM to treat the whole mud system. Each interval required a
different WSM blend, which was determined through application of software that
determined the widths to which fractures needed to be opened in order to
achieve the required wellbore stress and degree of strengthening and also the
optimum PSD of the WSM blend.
Results: The 2,600+ ft (792+ m), 18 1/8 in. x 21 in. interval above the salt
section was drilled with no mud losses or downtime related to hole stability.
Screens of 14 mesh over 20 mesh were used on six shakers with one shaker
dressed with 175 mesh screens on the top deck in order to catch cuttings
samples. The intention was to omit the last screen on the top deck of one
shaker to allow the fluid and Wellbore Strengthening Materials to pass through
the 20 mesh screens and be retained in the system. Using this method, cuttings
integrity was improved compared to two offset wells using conventional
110–140 mesh screens. An 18 in. liner was set, and the shakers were
dressed with 120–140 mesh screens while drilling 7,000+ ft (2,134+ m) of
salt without incident, and 13 in. casing was run. The 12 1/4 in. x 15 in. salt
exit was drilled (4,300+ ft / 1,310+ m) without incident and an 11 7/8 in.
liner was run, though the liner hanger failed to set. A second hanger failure
required a clamp tool be run below, and a successful cement squeeze was
performed to seal the 300 ft liner lap.
The low-pressure sand section below the salt was drilled without
losses, followed by a shale section with an expected pressure regression. No
Wellbore Strengthening Materials were used in the shale section.