IDENTIFICATION OF HOLE Problem
An event which causes the drilling operation to stop is described as a Non-Productive Time(NPT) event. Pipe sticking and lost circulation are the two main events which cause NPT in
the drilling industry. Well kicks, of course, require operations to stop and when they occur
can result in a large NPT. At the time of writing this book, the average NPT in the drilling
industry is 20%.
There are many events which cause NPT in the drilling industry: see Chapter 15 for
details.Hence rather than detail every minor hole problem that has ever been recorded, this
chapter will deal with the main problems normally encountered while drilling. These
problems are: differential sticking, mechanical sticking and lost circulation. There will also
be a discussion of other miscellaneous problems.
1.1 PIPE STICKING
When the pipe becomes stuck, there are two key actions that will best influence the chance
of freeing the pipe:
• Determination of the cause of the stuck pipe incident.
• The initial response of the Driller and subsequent actions taken.
During the earliest stages of trying to free the pipe, the Drilling Supervisor should collate all
the relevant information and determine what caused the pipe to stick. This may well be
obvious from the well conditions that existed before the pipe became stuck. An incorrect
assessment of the cause of pipe sticking problem will reduce the chance of freeing the stuck
pipe.
There are basically two mechanisms for pipe sticking:
1. Differential Sticking
2. Mechanical Sticking
Mechanical sticking can be caused by:
• Hole pack off or bridging, or
• Formation and BHA (wellbore geometry)
Table 12.1 gives a summary of the pipe sticking mechanisms and their most common
causes.
2 - D. . I.F . F. E. .R . E. N. .T . I.A . L. . S. T. .I C. .K . I. N. .G
2.1 CAUSES OF DIFFERENTIAL STICKING
During all drilling operations the drilling fluid hydrostatic pressure is designed and
maintained at a level which exceeds the formation pore pressure by usually 200 psi. In a
permeable formation, this pressure differential (overbalance) results in the flow of drilling
fluid filtrates from the well to the formation. As the filtrate enters the formation the solids in
the mud are screened out and a filter cake is deposited on the walls of the hole. The pressure
differential across the filter cake will be equal to the overbalance.
When the drillstring comes into contact
with the filter cake, the portion of the
pipe which becomes embedded in the
filter cake is subjected to a lower
pressure than the part which remains in
contact with the drilling fluid. As a
result, further embedding into the filter
cake is induced.
The drillstring will become
differentially stuck if the overbalance
and therefore the side loading on the
pipe is high enough and acts over a
large area of the drillstring. This is
shown diagrammatically in Figure
12.1.
The signs of differential sticking are the clearest in the field. A pipe is differentially stuck if:
1. drillstring can not be moved at all, i.e. up or down or rotated
2. circulation is unaffected
Mathematically, the differential sticking force depends on the magnitude of the overbalance
and the area of contact between the drillpipe and the porous zone.Hence
Differential force = (mud hydrostatic – formation pressure) x area of contact
Hence for the data shown in Figure 12.2, and assuming the formation contacts only 4" of the
drillpipe perimeter, then the differential force is given by:
Differential Force = (5000-4000) psi x 4 x 00 = 1,200,000 lb
A more accurate form of the above equation contains a term for the friction factor between
the drillstring (steel) and the filter cake is given in Equation (12.1).
The force required to free a differentially
stuck pipe depends upon several factors,
namely:
1. The magnitude of the
overbalance. This adds to any side
forces which already exist due to
hole deviation.
2. The coefficient of friction
between the pipe and the filter
cake. The coefficient of friction
increases with time, resulting in
increasing forces being required
to free the pipe with time. Hence, when differentially stuck, procedures to free the
pipe must be adopted immediately. Figure 12.3 shows the coefficient of friction vs.
time for a bentonite filter cake which shows a 10 fold increase in under 3 hours
The surface area of the pipe embedded in
the filter cake is another significant factor.
The greater the surface area, the greater
the force required to free the pipe.
Thickness of filter cake and pipe diameter
will obviously have a great effect on the
surface area. It is for reasons of reducing
available surface area that spiral drill
collars are often specified
when drilling sections which exhibit the
potential for differential sticking
problems.
Statistically, differential sticking is found
to be the major cause of stuck pipe
incidents, hence great care should be taken
in the planning phase to minimise the overbalance wherever possible. However, in certain
circumstances, drilling with minimum overbalance is not be possible, as is the case for large
gas reservoirs (e.g. the Morecambe Field in the UK) where the pressure differential across
the reservoir starts at the minimum overbalance (200 psi) and increases substantially with
depth to a maximum of 1300 psi. In these cases, strict adherence to precautionary drilling
practices and good communication between personnel will help reduce the incidence of
stuck pipe.
