Mud Related Drilling Problems Differential Sticking

•Differential pressure sticking of the drill pipe can be defined as the force that holds the pipe against the wall of the borehole due to the differential pressure between the hydrostatic pressure of the mud column and the formation pressure. •The pressure differential acts in the direction of the lower pressure in the formation. •This pressure pushes the pipe toward the permeable formation. •As the pressure differential gets larger, the force exerted on the pipe gets larger. •Differential stuck pipe occurs most often at a point next to the drill collars. •This is due to the drill collars being larger; hence more surface area is in contact with the side of the wellbore. •The following are major factors in differential pressure sticking: –The pipe becomes stuck opposite a permeable formation. –The sticking occurs after an interruption of pipe movement. –The pipe comes in contact with a soft, mushy or non-resilient type wall cake. •If the pipe is differentially stuck, as opposed to other types of sticking, the following will occur: –Circulation, if interrupted, will be restored and maintained after sticking is noticed. –The pipe cannot be raised or lowered. –No large amounts of cuttings are circulated out The force required to move differentially stuck pipe could exceed the strength of the drill pipe. •Several preventative steps can be taken to minimize the chances of becoming stuck: –The mud density should be maintained as low as practical, taking into consideration wellbore stability and potential well control problems. –Keep the pipe moving or rotating. •Avoid undue shutdowns and/or slow connections. •Use spiral drill collars to reduce the contact area against the well bore. –Maintain a low fluid loss and pay particular attention to the filter cake; i.e.: it should be thin, tough and resilient. •In areas where differential sticking is prevalent, the high temperature / high pressure fluid loss should be maintained below 20 ml. •Adding 2-8% lubricant to the mud system gives preferential oil wetting to the drill string, thereby allowing better lubricity and minimizing the possibility of stuck pipe. •When the drill string become stuck, it is imperative to act quickly as the sticking coefficient increases with time. •To avoid costly and time consuming wash over operations, a couple of methods are generally used to free the pipe.

Mud Related Drilling Problems
Differential Sticking -Spotting Fluid

•Spotting crude oil or diesel oil with a surfactant around the drill collars has gained wide acceptance. –There are many surfactants available are arecommonly called spotting fluids. –If a surfactant is not available on location, a straight diesel oil pill should be spotted across the collars as quick as possible. –If differential sticking is suspected in an area, always keep a supply of a differential sticking surfactant on location in the event it may be required. •Generally enough pill is mixed up to cover the entire length of the drill collars, plus an excess of 1.5 m3(10 bbls) to be left on top of the collars, and another 3.0 m3(20 bbls) to be left inside the drill collars. •Normally 20-25 litresof surfactant is recommended per cubic metreof diesel oil (1-2 gal/bbl). •The pill should be spotted leaving 3 m3(20 bbls) inside the drill string. –The pipe should then be worked by pulling up to a predetermined over pull weight, applying torque and releasing the weight at regular intervals. –The pill across the collars has a tendency to migrate up the hole; therefore approximately 0.1 m3(1/2 -1 bbl) of excess fluid in the pipe should be pumped every half hour. •An average waiting period is generally 10-12 hours. –If the pipe does not come free in a reasonable period of time (maximum of 2 pills), mechanical methods may be required to free the pipe. –If the spotting pill has to be weighted due to an abnormally pressure zone, or to increase the pill density to that of the mud weight to minimize migration, the spotting procedure would be the same although some of the products may be different.