In the North Field in Qatar, operators typically drill through the Sudair and Khuff
Formations. The Sudair Formation is frequently isolated in a casing run while the
Khuff Formation is drilled as a separate interval. The operator wanted to drill
through both formations in one interval using a new well design. The novel well
path was designed to manage the high differential pressures associated when
drilling the Sudair and Khuff reservoir in the same interval. The new well design
isolated the Sudair formation, which required a higher mud density for controlling
salt-water inflow and maintain wellbore stability. However, the required mud
density in the Sudair could cause mud losses in the Khuff formation. Excessive
overbalance could lead to stuck pipe issues through differential sticking. The
challenge was to design a fluid that would protect the Khuff formation and
prevent differential sticking and limit mud losses.
All varieties of drilling fluids have been used in this field. In this case, the
operator selected a sodium chloride polymer water-based mud, with mud
weights ranging from 12.4 – 12.5 lb/gal. The drilling fluid was designed to give a
low-end rheological profile to maintain a steady equivalent circulating density
(ECD). Laboratory tests demonstrated FLC 2000® ultra-low fluid invasion
properties would provide the wellbore stability needed for the operator.
FLC 2000® has been used succesffully in reservoirs to prevent differential
sticking and wellbore instability.
The operator successfully drilled the 8 ½-in. section without mud losses or
differential sticking in the Khuff formation. The addition of FLC 2000 at 6 lb/bbl
into sodium chloride polymer water-based mud helped to prevent wellbore
instability issues through the both Sudair and the Khuff formations. The 7-inch
liner was successfully run after a clean-up trip and cemented in place. A 6-inch
hole was also drilled on this well using FLC 2000® in the formulation to maintain
wellbore stability and prevent mud losses.