English Edition (6 MB pdf) - Saudi Aramco

mud cake. Low gravity solids should be kept as low as
possible. Lubricity of the mud also plays an important role in
minimizing the sticking effect. Therefore, the mud weights and
solids content in subsequent candidate wells were reviewed to
keep the overbalance as low as practicable without
compromising safety.
Third, use of the reamer BHA previously mentioned, will
also help to reduce the thickness of the filter cake. The thicker
the filter cake, the bigger is the cross-sectional area that the
formation pressure acts on. Therefore, the differential sticking
force is higher when the mud cake is thicker.
Additionally, several run in hole procedural changes were
implemented to minimize the potential for differential
sticking. Documentation on the subject notes that the
potential for differential sticking increases with the length of
time that pipe is stationary across lower-pressure zones. It was
therefore resolved that steps would be taken to reduce the
time that the pipe would be stationary when in the open hole.
Once the running string was set in the slips, the elevators were
quickly moved up to pick up the next stand. This stand was
immediately connected and the string was pulled out of the
slips and the MSS run in was continued. There were no stops
for filling the running string; instead, a fill up hose was used
to add mud to the running string while the elevators were
picking up the next stand. As soon as the stand was ready to
be connected, the fill up hose was removed. There were no
pauses during crew change; the focus was on keeping the pipe
moving at all times while in the open hole section.
Steps to Take if a Liner Becomes Differentially Stuck
As noted above, the force holding the pipe against the
wellbore can exceed the tensile strength of the liner or tools in
the liner. When drilling a well or when tripping, if differential
sticking is encountered, the immediate response by drilling
personnel is to push, pull and twist – whatever is required – to
get the pipe free. Drilling assemblies can take a lot of
punishment, but liner assemblies cannot. Experience has
shown that if an MSS liner becomes differentially stuck, the
only way to free it is to incrementally reduce the mud weight
(and therefore overbalanced pressure) until the wellbore
releases the pipe.
PROBLEM SOLVED
The next MSS that was deployed had rigid, spiral centralizers
on the middle of each joint of pipe that was run into the open
hole, Fig. 9. All of the other differential sticking precautions
mentioned above were also conducted during the installation
of this system. The system was run to TD without any
sticking incidents. In fact, after the assembly was on bottom
for 30 minutes it was necessary to move the pipe to space out
– and it continued to move freely.
Since then, three more MSSs have been successfully run
into Saudi Aramco deep gas wells. All systems have been
deployed to TD without any sticking incidents.
Fig. 9. Rigid, spiral centralizers installed on the middle of each joint of pipe to
mitigate differential sticking.
CONCLUSIONS
Any liner or casing string is susceptible to differential sticking
if the wellbore has one or more low-pressure zones, a high
mud weight and excessive overbalance relative to the lowpressure
zone(s). The MSSs are particularly susceptible to
differential sticking because the large diameter tools can
scrape filter cake off the wellbore exposing the low-pressure
formation to the high wellbore hydrostatic pressure. The
following are liner running practices incorporated by Saudi
Aramco to help ensure their MSSs are deployed to TD.
1. Where possible, drill the well so that the liner does not
need to be run through nonproductive higher up zones
with low formation pressure. Ideally, the casing shoe
should be in the production interval.
2. Reduce the mud weight to a safe, yet manageable, level to
avoid over-pressuring the weaker zones.
3. Proper centralization of the pipe can reduce, if not
eliminate, the potential for differential sticking.
4. The stiff gauge/reamer assembly that is run prior to
running a MSS will identify and remove all ledges in the
wellbore. This will dramatically reduce, if not eliminate,
the potential for mechanical sticking of the MSS
components and the centralizers.
5. Minimize the time that the liner is stationary in the open
hole. The potential for differential sticking increases with
the amount of time the pipe is not moving due to the filter
cake tending to buildup around the pipe and then
increasing the differential sticking force. This includes
minimizing connection time, no circulation breaks until the
liner is at TD, and no pauses during crew change.
6. If the MSS assembly does become differentially stuck, do
not use excessive force to attempt to free it. Incrementally
reduce the mud weight until the well releases the pipe and
38 SUMMER 2010 SAUDI ARAMCO JOURNAL OF TECHNOLOGY