Case History

Horizontal Well Drilled and Sampled Through the Shuaiba Limestone Formation

Qatar
Challenge

The Shuaiba Limestone Formation can be problematic when drilling horizontally
in the Al Shaheen Field in Qatar. This particular drilling campaign included drilling horizontally through the Shuaiba Formation with a total length of 1500-ft. The Shuaiba Limestone had an expected matrix permeability of 10 – 30mD and was
prone to induced mud losses. The challenge for this particular well was to drill
the horizontal section with the programmed mud weight of 9.6 – 9.7 lb/gal
without mud losses. At the programmed mud weight, overbalance was expected
(0.5 – 1.0 lb/gal) in static conditions. The expected equivalent circulating density
(ECD) values of 10.5ppg and higher meant the overbalance would be 1.5 – 1.7 lb/
gal.

Prior to landing the liner, a sampling run was planned, leaving the bottom-hole
assembly (BHA) across the open formation for a considerable period
(programmed 36 hours). On completion of drilling the section, the fluid and
pressure measurements were taken. This method of sampling meant that the
BHA would be held static in overbalance conditions for several hours. The entire
process of measurement and sampling lasted 24 hours.

Solution

The fluid designed to drill the section was a low solids non dispersed (LSND) mud,
with the mud weight range of 9.6 – 9.7 lb/gal. FLC 2000® ultra-low fluid invasion
additive was added to the LSND mud system to “shield” the formation while
drilling, and at the same time, for a long period when the BHA was held static to
perform sampling. The addition of FLC 2000® was held at an average 8 lb/bbl in
the fluid, and the addition of high concentration pre-mixes approximately 53 lb/
bbl of FLC 2000 continually bled into the active system to help maintain the mud
properties and prevent losses and differential sticking.

Results

The operator successfully drilled the 8 ½-in. section without mud losses or
differential sticking in the Shuaiba Limestone Formation. Fluid sampling was
completed while the BHA was held static for a prolonged period of time across
the open hole of the Shuaiba Limestone Formation. The formation did not break
down with overbalance of the mud weight or high ECDs. FLC 2000 successfully
protected the frac gradient in this scenario to allow the operator to complete the
challenging drilling conditions.