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eciprocated) along the axis of the wellbore, Fig. 6. For
example, the completion string can get stuck in filter cake that
was previously deposited on a permeable zone. Differential
sticking typically occurs when high contact forces caused by
low reservoir pressures, high wellbore pressures, or both, are
exerted over a sufficiently large area of the downhole assembly.
The pipe is held in the cake by a difference in pressures
between the hydrostatic pressure of the mud and the pore
pressure in the permeable zone. A relatively low-differential
pressure (delta p) applied over a large working area can be just
as effective in sticking the pipe as can a high-differential
pressure applied over a small area. The force required to pull
the pipe free can exceed the strength of the pipe.
Mitigation of Differential Sticking with Centralizers
With the knowledge that differential sticking was the most
likely cause of deployment issues for the MSS, the task was to
determine how to overcome these challenges. The most readily
available tool to combat differential sticking is centralization
of the pipe so that it stands off the wellbore. Although it is
widely recognized that centralized pipe is much less prone to
differential sticking, many operations personnel strongly
oppose the use of centralizers because of the increased risk of
mechanical sticking. They cite occasions where they have
attempted to run centralized liners, but could not get them to
the bottom. In these cases, they pulled the liners back to
surface, removed the centralizers, and then were able to force
the liner to TD.
It was immediately realized that the ledge phenomenon that
had been identified early in the deployment of MSS was
possibly the same issue that had caused the problems with
deployment of centralized liners. If so, the gauge/reamer tool
that had been successfully used to address the mechanical
sticking issues for early MSS could also solve the problem
with mechanical sticking when using centralizers.
It was also noted that the centralizers had not been run in
both previous cases where the MSSs had become stuck.
Therefore, the lack of centralization was considered to be the
most significant factor contributing to the MSS becoming stuck.
Other Steps Taken to Reduce Sticking Potential
Several other steps were taken to improve the chances for
successful deployment of MSS assemblies.
First, it was noted that on the wells where the MSSs had
become stuck, that the 9 5 ⁄8” casing had been set above the
lower-pressure zones, Fig. 7. The MSSs had to be conveyed
through these zones before reaching the target depth. To
mitigate this issue, candidate wells were selected where the
open hole would not go through zones with widely ranging
formation pressures, thereby minimizing the potential for
differential sticking, Fig. 8.
Second, the mud weights in some previous MSS wells had
been excessively high, up to 2,000 psi overbalanced compared
to the pore pressures in the lower pressurized zones, resulting
in differential sticking conditions. Because of the variation in
formation pressures, when the mud weight was increased to
provide adequate hydrostatic pressure for the high-pressure
zone, Fig. 7 – Zone 4), the low-pressure zones, Fig. 7 – Zones
1 and 2, were over pressured, creating conditions for
differential sticking. The mud quality itself is also influential
in the probability for differential sticking. A thick, solids laden
mud cake is more likely to cause sticking issues than a thin
Fig. 7. Differential sticking of completion string due to open hole going through
low-pressure zones.
Fig. 6. Differential sticking as viewed in a cross-section of the wellbore 8 .
Fig. 8. Candidate wellbores where the open hole does not go through low-pressure
zones.
SAUDI ARAMCO JOURNAL OF TECHNOLOGY SUMMER 2010 37