The Drilling Programme shall indicate any permeable formations which may lead to differential sticking, particularly if the risk of differential sticking is considered to be high.
Data Acquisition
An estimate of the problem formation pressure shall be made using the best offset data available. If good, and recent, RFT, or producing well information is available, it should be used to estimate the expected formation pressure.
On an appraisal wells where differential sticking has been identified as a potential problem, taking an RFT measurement should be considered. The risk of sticking the logging tool must be taken into consideration, but this could be outweighed by the value of the information obtained.
Careful consideration must be given to the number of pad type logging tools employed in holes where differential pressures are known to be high. Any tools which have high borehole contact areas should be considered very carefully prior to programming in these hole sections.
Stabilisers and Drill Collars
Stabilisation and spiral drill collars should be used to centralise and minimise wall contact.
Differential Pressure
The differential pressure across permeable formations shall be continuously tracked as accurately as possible. The trends of overpull on connections, trip gas levels, and connection gas levels shall be followed to anticipate changing pressures.
Differential pressures across permeable formations should be minimised by keeping the mud weight at the lowest safe level. A guide figure of 200 psi static overbalance may be considered although conditions will often dictate a different figure.
Drilling Fluid
All mud parameters must be maintained within the specifications of the Drilling Programme. In particular it is important that:
HTHP (where relevant) and low pressure fluid loss are be maintained within specification
(These values should be measured 3 times per day at the minimum)
while maintaining consistency with suspension and hole cleaning requirements, the gel strengths are maintained at the minimum value possible.
(Pipe release agents are adversely affected by high gel strengths. The 30 minute gel strength should be measured as well as the 10 second and 10 minute value and should not be significantly higher than the 10 minute value)
drilled solids content are to be minimised.
Lubricants
Since lubricants can reduce the high coefficient of friction for differentially stuck pipe, they should be used in areas where differential sticking is a possibility. There are a wide range of lubricants available for both water based and oil based muds.
Filter Cake
The filter cake thickness can be reduced by the addition of “bridging” material to the mud thus reducing the potential for differential sticking. The Drilling Fluid Contractor can advise on this.
Shale Shaker
Shaker screens must be selected to prevent or minimise a drilled solids build-up, as low gravity solids result in thick and sticky filter cakes.
HTHP Fluid Loss
HTHP fluid loss must be run on the mud when drilling in an area where the potential for differential sticking is high, regardless of the bottom hole temperature.
In such areas, the Drilling Programme shall state the maximum value for the HTHP fluid loss and low pressure fluid loss.
Reciprocation / Rotation
The pipe should be kept moving at all times and should never be left static in high risk areas. Wherever possible the pipe should be reciprocated. If reciprocation is not possible (eg., on connections) rotation should be applied.
Do not programme any unnecessary surveys as they are a high risk operation.
Measurement While Drilling
An MWD survey tool is less likely to become stuck than a single shot because of the reduced amount of stationary drillpipe time. For this reason, in high risk areas, the extra cost of MWD may be justified.
Well Control Activities
Although differential sticking regularly occurs during well control operations due to the increased mud weight, under no circumstances should the fear of becoming stuck dictate the kill weight of the mud to be used. However, sometimes excessive safety margins in kill weight and circulating pressures are used which can increase the chance of stuck pipe.