Differential sticking is one of the mechanisms of stuck pipe problems and it can be defined as the sticking force generated by the level of the overbalance between the hydrostatic pressure and the formation pressure, this pressure overbalance pushes the static drill string into the filter cake. Many factors can lead to this problem:
- The hydrostatic overbalance exposed on a permeable layer of formation ( sand or fractured limestone)
- Pipe movement stopped for any reason for instance make connection or rig repair which put the string in contact with permeable zone.
- Thick static filter cake
- The large contact area between the drill string and the formation
Analyzing these factors, it can be concluded that differential sticking occurrence is related to the presence of these factors simultaneously, the string has to be static and in contact with a permeable zone and pushed by high force generated by the pressure overbalance.
The filter cake develops when the liquid phase of the drilling fluids can flow in the permeable zone and the solids present in the drilling fluids system deposits on the borehole walls forming the filtered cake. It has to be mentioned here that the filter cake is necessary to manage the fluid loss but the thickness of this filter cake has to be controlled to avoid stuck pipe by this mechanism. There are two types of the filter cake: dynamic and static. Under dynamic conditions, the filter cake thickness decreases due to the mechanical forces of the drill string and the fluids flow which washes the top layer of the filter cake leading to thickness decrease. Under static conditions, the rate of solids depositions increases which leads to thickness increase.
Also, during the dynamic conditions, the hydrostatic pressure is exerted equally around the drill string and the differential sticking force cannot develop but after ceasing drill string motion, the static filter cake starts to develop and string can be pushed toward the borehole walls.
Mathematically, the sticking force is described as follows:
Fs = PDiff x A
Where:
Fs : Sticking force
PDiff: Pressure difference
A : Contact area
There are some indicators of differential pressure stuck pipe:
- Increase in torque and drag while drilling
- In severe situation, getting high level of overpull after making connections
- Inability for reciprocating the drill string
- The circulation is not interrupted
Differential pipe sticking can be prevented by following some precautions:
- Apply the minimum possible safe overbalance while drilling
- Controlling the fluid loss and consequently reducing the filter cake.
- Optimize the drilled solids percentage in the mud system.
- Drill with low friction coefficient mud system (The friction coefficient can vary from 0.04 for oil based drilling fluids to 0.35 for water based muds)
- Reduce the timing while the string is static for example when repairing or making a connection.
The contact area represents the total pipe surface which is in the direct contact with the walls of the open hole. This surface of contact can be affected by many factors:
- Length of the permeable zone
- Hole size
- Pipe size and shape
- Filter cake thickness
It is obvious that the length of the permeable zone cannot be controlled and even the hole size which generally a fixed parameter. Using spiral drill collars and heavy weight drill pipe can contribute in reducing the contact area with the permeable zone and consequently reducing the sticking force.
Controlling the high temperature high pressure filtration rate is an important field practice to minimize the risk of differential sticking. The contact area can increase enormously with important filter cake which increases the risk of such type of stuck pipe.
Fig 2 . Spiral Drill Collar can reduce the contact area by more than 30%
There are many methods used to free the differential stuck pipe. Considering that the differential sticking force increases with time, torque up the drilling string and jarring down can be the effective first action to perform. This action can be applied quickly with the little possibility of creating other problems. Combining the torque and the down- jarring can generate a great load which increases the possibility of freeing the drill string. This action can be applied as follows:
- Circulating at the maximum rate while applying string torque because circulation washes the static filter cake and reduces its thickness.
- The torque has to reach the 80% of the make up torque of the drill pipe, then the string has to be carefully reciprocated to transfer the torque to the deepest point which normally be the stuck point.
- Reduce the pump speed to 1 bbl/min to decrease the annular friction pressures
- Jarring down the torqued drill string
- If the string does not come free , repeat these actions and keep jarring down
- If you decide to jar up for any reason, carefully release the string torque. This is very important because the combined torque and tension reduces the tension capacity of the string.
If the first action described above failed to free the string, then pumping a chemical agent should be pumped down hole and placed at the stuck zone. Pumping this chemical agent is to destroy the filter cake around the stuck point. These products work to loose the contact between the pipe and the filter cake. The chemical products have to be displaced by steps to re-fresh the chemicals around the stuck point and make them more effective. Also, the string has to be worked up and down. This action can be performed with some precautions:
- First, determining the stuck point is important to know where to spot the pill. If the jar can be tripped this means that the stuck point is below the jar. If the drilling jar cannot be tripped this means that the stuck point is at some depth above the drilling jar. Also, the stuck point can be determined by using a free point indicator tool or by performing some pipe torque calculations.
- The chemical agent pill should be 1 to 2 ppg greater than the drilling fluid density to keep the pill on spot. Also the pill volume should be 1.5 times required volume to make sure it is sufficient to cover the stuck area.
- If the well can be pressurized without risking of generating down hole mud losses. The pressure applied at the annular space by closing the BOP has to be held for 10 to 15 minutes in order to pressurize and drive the chemical agent into the filter cake. After waiting the recommended timing, the pressure has to bled off through the choke, then the BOP can be opened.
- After, bleeding the pressure and waiting the recommend timing for the chemical agent, jarring down is the next action to perform. As mentioned above, the string has to be torqued till 80% of the make up torque of drill pipe and torque should be transferred to the stuck point by reciprocating the string up and down, then jarring down with maximum load till getting free.
If the string does not come free after applying these two options, the fishing operation can be the next action after performing a back off.
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