What Is The Definition Of Differential Pipe Sticking?
In general, differential sticking is a situation that may happen during any stage of drilling the well and cause the drill string or casing to stuck (no upward or downward movement). It actually happens when the hydrostatic pressure, ph (which acts on the outside wall of the Pipe) is more than the pore pressure, pf (which leads to force the pipe into the mud cake).
This situation can happen on Drill Pipe, Heavy Weight Drill Pipe HWDP, or Drill Collar but it is more likely to take place in the collar as it has a larger diameter than pipes and HWDP.
Causes Of Differential Pipe Sticking
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To understand the causes of differential pipe sticking, you have to understand what is the stuck force and what is the main factors affect it. Differential pipe sticking force is the resultant force from the differential pressure between hydrostatic and pore pressure at a certain point. This Differential force will keep pushing the drill string or casing towards the mud cake. Once you decrease that differential force, your drill string will be free. Below we will explain the force calculation & the factors that affect it.
Differential Sticking Force Calculation
The Differential Pipe Sticking Force can be calculated by multiplying the differential pressure by the contact area.
The differential pressure acting on the part of the Drill Pipe which is stuck in the drilling mud cake can be calculated from the below equation:
The pull force, Fp, which are able to free the stuck is calculated by below equation:
- The differential pressure, Δp.
- The coefficient of friction, f. ( We recommend checking Drill String coefficient of friction article for more infos)
- The contact area, Ac, between the drilling mud cake and the differential sticking pipe surfaces. Ac can be calculated from this equation:
- Lep is the length of the permeable interval.
- Dop is the 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 The Sticking Force
The above Equations show the main factors that will cause the increase in the Sticking force and the decrease of the probability of the pipe freeing process.
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 Pipe Sticking length in the filter cake (time delay in freeing process).
- Hole and pipe diameters.
- Inclination plays a role in the pipe sticking force. they are uncontrolled variables once they got chosen for the well design objectives. However, collar shapes, such as spiral grooves, external-upset tool joints, or squares can minimize the differential pipe sticking force.
Prevention And Mitigation
Simple precautions tips to decrease the possibility of Differential pipe sticking if drilling through high permeable intervals are as following:
- Decrease drilling mud fluid loss. ( Check API Fluid Loss, HPHT Fluid Loss).
- Decrease the amount of drilled solids in the drilling fluid Circulating system, or, if it is OK, remove all drilled solids.
- Use the lowest possible mud weight to decrease hydrostatic pressure so that decrease the stuck forces
- Decrease Friction Coefficient by using a drilling fluid system that will give you a smooth mud filter cake.
- Decrease connection time and keep rotating Drill String all the time.
Differential Sticking Pipe Warning Signs
There will be signs or indicators while drilling permeable or depleted zones. if these indicators happen, you should think that you may get this kind of stuck:
- Torque And Drag increase.
- Unable to reciprocate or in some times rotate the String.
- Uninterrupted mud circulation.
- While drilling highly depleted zones, there may be a high-pressure variation which may cause a stuck.
Differential Pipe Sticking Freeing Procedures & Tips
After you understand the freeing procedures, we recommend that you have a look at the freeing chart.
- Pull the Drill String immediately to the Maximum Safe Overpull for the Bottom Hole Assembly BHA Components.
- Start circulation
- Start jarring up to release the Differential Pipe Sticking. (Check Mechanical jar Operations & Hydraulic Jar Operations)
- If the Overpull and the jarring up didn’t work immediately. Prepare pipe-free pill while that apply right-hand torque to release the stuck Drill String. If using 5 in Drill Pipe, the amount of torque will be around half a turn for every 1000 ft of Drill Pipe above the suspected stuck point ( check also free point calculation for stuck pipe).
- Spot pipe free pill at the drill string Differential Pipe Sticking point as soon as possible.
There are 2 important things that shall be in your mind while freeing the differential pipe sticking problem which are:
- Minimize Hydrostatic Pressure
- Spot pipe releasing agent
Minimize The Hydrostatic Pressure
Decreasing hydrostatic pressure is one of the things to think about for decreasing pipe sticking force and loosening Differential sticking in the string.
But off course take in your consideration the following:
- It should not affect the Well Control issue as it shall not be reduced below the maximum hydrostatic pressure on the well.
- It should not affect or your mud in other meaning don’t decrease your Oil Base Mud weight with water only and this can be done by dilution of your mud with the base fluid or by pumping light weight pills.
- While reducing hydrostatic pressure, take care of any Kick Warning Signs.
Spotting Pipe Release Agents To Free Differential Stuck Pipe
Differentially Pipe Sticking can be solved by spotting a pill called a pipe-free pill as soon as sticking happen. These agents work on attacking and breaking down the mud filter cake so that It reduces the bonding in between the mud cake and the Drill String.
It will be a good practice to pump a volume that covers the suspected Differential Sticking point with an excess of up to 100% and displace it so that it will be around the Differential stuck zone. you can reduce your pumping rate while it passes through the Differential stuck point.
In some cases, you may leave the pill to soak while continuously work the torque and jarring down or jarring up without torque. The soaking time may be up to 12 hours but the problem that you may have other problems from not circulating the well at the same time so you have another choice to circulate at a lower rate and prepare another pill.