Causes and methods of freeing differential pipe stuck

Spotting Reduced Density Fluids. Differential pipe sticking could be freed by means of reducing differential pressure, which can be fulfilled in several ways. Spotting low density fluid can also lead to decreasing of the differential pressure and usually depending on mud type oil or water are used to achieve the goal. Well control issues are major concerns in these operations. Therefore, extra care should always be taken to prevent the well blowout.


Applying Drill Stem Test Tools. Using a Drill Stem Test Tool is a safe way to reduce differential pressure, but it is time-consuming because it requires backing off, running a caliper log to select a near-gauge zone for setting the packer, making a conditioning trip, and mobilizing DST equipment. The steps are as follows: Fishing assembly have to be run combined with the DST string filled with a liquid of lower density. Afterwards, the fishing assembly is attached to the fish and the packer is set, then reducing the hydrostatic pressure, which in turn leads to a decrease in the differential pressure. In case the fish is free, the packer must be released, then the pipe needs to be moved up-down in order to prevent differential sticking happening again.
U-Tube Technique. The U-Tube technique refers to the process when differential pressure is decreased by reducing the height of the mud column in the annulus below the bell nipple. First, light
fluid (water, oil) or gas (nitrogen) is pumped down in the drill string, then the pressure with some fluid is bled off the standpipe. Therefore, the heavier mud in the annulus is allowed to ‘U-Tube’ back into the drill string, reducing the hydrostatic pressure in the annulus. The exact volume of the lighter fluid should be calculated carefully. Caution also must be made not to plug the bit, hence this procedure should not be performed with small nozzle bits. Before attempting this technique, formation pressures and possible productive zones must be considered.

Conclusion

As differential sticking became a major concern in the drilling industry, the first objective should be to eliminate it at all. After the mechanism of sticking process was better understood, this objective shifted towards a design principle. This aims that pullout force, which can be exerted by the rig to the pipe without overcoming the tensile strength of the drill pipe, always exceeds sticking force. In order to reduce the likelihood of differential sticking, it is important to determine the drilling parameters, which could be changed in order to avoid sticking. As rock formations are defined by the regional geology, while high overbalance pressures often essential for primary well control and wellbore stability, the most easily changeable parameters are the drilling fluid properties and the drill string.