Petroleum Department
Ass. Lect. Waseem Ali
Fourth Stage
Drilling Problems
Drilling Problems
1.3- REDUCTION OF HYDROSTATIC PRESSURE
The reduction of hydrostatic pressure is the obvious and most successful method of freeing
a differentially stuck pipe. The lowering of the hydrostatic pressure reduces the side loading
forces on the pipe and therefore reduces the force required to free the pipe from the filter
cake. There are several methods by which this may be achieved. However prior to
implementing this action the following factors should be seriously considered:
1. Are there other pressured zones in the open hole section?
2. Will these exposed zones kick if the hydrostatic pressure is reduced?
3. The confidence level in the accuracy of pore pressure estimates made while drilling and
the pressure control equipment.
4. The effects of a reduction in hydrostatic pressure on the mechanical stability of all exposed
formations.
5. The volumes of base oil or water required to achieve the required reduction in hydrostatic
pressure. (This may well influence the method chosen).
All the above factors need to be carefully considered prior to reducing the hydrostatic
pressure as the potential for inducing a well control problem or formation instability are
considerably increased. The following methods for reducing hydrostatic pressure can be
used:
• circulation & reducing mud weight
• displacing the choke
• the ‘U’ tube method
1.3.1- CIRCULATION & REDUCING MUD WEIGHT
In this method, the drilling mud is circulated and its weight is gradually reduced. The
minimum mud weight required to balance the highest pore pressure in open hole should be
determined and the mud weight cut back in small stages. Close attention must be made to
all kick indicators whilst circulating down (reducing) the mud weight, frequent flow checks
should also be made. Whilst reducing the mud weight, tension should be held on the pipe.
Petroleum Department
Ass. Lect. Waseem Ali
Fourth Stage
Drilling Problems
Disadvantages of this methods are:
• It is slow, and remember the force required to free pipe is time dependent.
• The volume increase required may overload the surface pit handling capability.
This may be a serious problem when OBM is used.
• The active volume will be increasing during the reduction in mud weight, making kick
detection difficult.
1.3.2- DISPLACING THE CHOKE
This method is applicable to floating rigs where BOPS are placed on the seabed. The
hydrostatic pressure can be quickly and effectively reduced by displacing the choke line to
base oil or water. The well is shut in using the annular preventer and the displaced choke.
The advantage of this method is that if any influx is taken, the well can be immediately killed
by closing the choke and opening the annular. This action again exposes the well to the active
hydrostatic pressure from rig floor to TD. The disadvantage of this method is that the amount
of reduction in hydrostatic pressure is limited to the water depth. This may well result in a
limited reduction in shallow water, or in the case of deep water, an excessive reduction in
hydrostatic pressure. line opened thereby reducing the overbalance. Note that the annular
preventer isolates the wellbore from the hydrostatic head of mud in the riser from rig floor
to the annular preventer.
1.3.3- THE ‘U’ TUBE METHOD
The U-tube method is used to reduce the hydrostatic pressure of mud to a level equal or
slightly higher than the formation pressure of the zone across which the pipe got differentially
stuck.Clearly, the objective is to free the differentially stuck pipe safely without losing control
of the well by inadvertently inducing underbalanced conditions. A pipe free agent should be
spotted across the permeable zone prior to adopting the ‘U’ tube method. The mathematics
required for the full method is laborious.
Problem (1-2): Calculate the volume of oil required to reduce the hydrostatic pressure in a
well by 500 psi, using the following data:
Mud weight = 10 ppg
Hole depth = 9843 ft
Drillpipe = OD/ID = 5"/4.276"
Petroleum Department
Ass. Lect. Waseem Ali
Fourth Stage
Drilling Problems
Hole size = 12.25"
Specific gravity oil = 0.8 (6.7 ppg)
Solution:
Initial hydrostatic pressure = 0.052 x10x 9843 = 5118 psi
Required hydrostatic pressure = 5118 - 500 = 4618 psi
Thus,
New hydrostatic pressure = pressure due to (mud and oil) in drillpipe
4618 = 0.052x 10xY (mud) +0.052x (6.7) x (9843-Y) (oil)
Where:
Y = height of mud in drillpipe.
Therefore = 6927 ft
Hence,
Height of oil = 9843 - 6927 = 2916 ft
π
1
4
144
Volume of oil = capacity of drillpipe x height = × 4.2762 ×
× 2916 = 290.79 ft 3 =
51.7 bbl
Note that when the required volume of diesel oil is pumped inside the drillpipe, the
hydrostatic pressure at the drillpipe shoe becomes 4618 psi, while the hydrostatic pressure
in the annulus is still 5118 psi. This difference in the pressure of the two limbs of the well
causes a back-pressure on the drillpipe which is the driving force for removing the diesel oil
from the drillpipe and reducing the level of mud in the annulus. It is only when the annulus
level decreases that the hydrostatic pressure against the formation is reduced and the stuck
pipe may be freed.
When the formation pressure is unknown, it is customary to reduce the hydrostatic pressure
of mud in small increments by the U-tube technique until the pipe is free.
A variation of the U-tube method is to pump water into both the annulus and the drillpipe to
reduce hydrostatic pressure to a value equal to or just greater than the formation pressure.
1.4.- SPOTTING PIPE RELEASE AGENTS
The severity of differentially stuck pipe can be reduced by the spotting of pipe release agents.
Pipe release agents are basically a blend of surfactants and emulsifiers mixed with base oil or
diesel oil and water to form a stable emulsion. They function by penetrating the filter cake,
therefore making it easier to remove and at the same time, reduce the surface tension
between the pipe and the filter cake. Due to the time dependency of the severity of
Petroleum Department
Ass. Lect. Waseem Ali
Fourth Stage
Drilling Problems
differential sticking, the pipe release agent should be spotted as soon as possible after
differential sticking is diagnosed. Typically the pill will be prepared whilst initially attempting
to mechanically free the pipe; ie by pulling and rotating.