Differential Stuck Pipes
Definition Of Differential StuckThis kind of stuck is happened when the hydrosatic pressure, ph , which acts on the outside wall of the Drill Pipe, is more than the pore pressure, pff , which leads to force the Drill Pipe into the mud cake , then the pipe is said to be differentially stuck. This can happen on Drill Pipe, Heavy Weight Drill Pipe HWDP or Drill Collar but the differential stuck is more likely to take place in the Drill Collar as it has larger diameter than Drill Pipe and Heavy Weight Drill Pipe HWDP.
The differential pressure acting on the part of the Drill Pipe which is differential stuck in the drilling mud cake can be calculated from the below equation:
The pull force, Fp, which are able to free the differential stuck pipe is calculated by below equation:
- The differential pressure, Δp.
- The coefficient of friction, f .
- The contact area, Ac, between the drilling mud cake and the differential stuck pipe surfaces.
Ac can be calculated from this equation:
- Lep is the length of the permeable interval.
- Dop is the differential stuck pipe outside diameter.
- Dh is the drilled hole diameter.
- hmc is the thickness of the filter cake.
- The dimensionless coefficient of friction, f, its range from +/- 0.04 for OBM to +/- 0.35 for WBM with no lubricants.
Factors Affecting Differential Stuck Force
Above Equations shows the main factors that will cause the increase in the differential stuck force and the decrease of the probability of Freeing The Differential Stuck Pipe.
These factors are:
- Unnecessarily high differential pressure between hydrostatic pressure and pore pressure.
- Thick filter cake (high fluid loss to the formation)
- Low lubricity filter cake (higher friction coefficient)
- Excessive differential stuck pipe length in the filter cake (time delay in freeing differential stuck)
- Hole and pipe diameters
- Inclination play a role in the differential stuck force. they are uncontrolled variables once they got chosen for the well design objectives. However, Drill Collar shape, such as spiral grooves , external-upset tool joints or square can minimize the differential stuck force.
Differential Stuck Pipe Indicators & Signs
There will be signs or indicators while drilling permeable or depleted zones. if these indicators happen, you should think that you may get a differential stuck :
- Torque And Drag increase.
- Unable to reciprocate or in some times rotate the Drill String.
- Uninterrupted Drilling mud circulation.
Differential Stuck Pipe Prevention And Mitigation
Simple precautions tips to decrease the possibility of Differential pipe stuck if drilling through high permeable intervals are taken:
- Decrease drilling mud fluid loss.
- Decrease amount of drilled solids in the drilling fluid system, or, if it is OK, remove all drilled solids.
- Use the lowest possible mud weight to decrease hydrostatic pressure so that decrease the differential stuck force
- Decrease Friction Coefficient by using drilling fluid system which will give you a smooth mud filter cake.
- Decrease connection time and keep rotating Drill String all the time.