Causes and methods of freeing differential pipe stuck

Statement of the problem

Typically, differential sticking is an occurrence that might occur at any level of the drilling process and result in the drill string or casing being stuck. It might happen on a drill pipe, HWDP or drill collar. However, it is most likely to occur on the drill collar, since the collar has a bigger diameter and, accordingly, more contact area than other components of BHA. Consequently, the more contact area between the drill string and the wellbore, the more possibility of getting stuck.
Differential sticking occurs when the hydrostatic pressure exceeds the pore pressure much more than needed and effects outer wall of the pipe, which leads to forcing the pipe into the mud cake.
To acknowledge the reasons of differential pipe sticking, it is necessary to release what is the main factors affecting it. There are a wide range of factors that contribute to the differential pipe sticking and these factors are:

• 
  Overbalance: usually, hydrostatic pressure originating from the mud weight in the well must be more than the formation pressure in order to keep formation fluid under control. However, excessively overbalance of hydrostatic pressure creates more chances of getting stuck.
  
• 
  Pipe movement: if the drill string remains stationary for a certain period of time, the chance of differential sticking increases.
  
• 
  Filter cake: thick mud cake and low lubricity mud cake increase risk of the drill string sticking.
  
• 
  Drill string components: Size as well as shape of used drill collars and drill pipes.
  
• 
  Well geometry: well inclination and dogleg severity.
  
• 
  Porous formations: drilling porous formations such as sandstone, lime, carbonate, and etc.
  

Figure 2: Differential pipe stuck in the permeable zone.



Figure 1: Contact area in the differential pipe stuck.


Although drill string components, well geometry and drilling porous formations impact on pipe sticking force, they are ungovernable variables once they are chosen to achieve well design goals.
As mentioned above, differential sticking force originating from the pressure difference between the hydrostatic and pore pressure at a particular point specially in the permeable zone. Consequently, the drill string or casing is being pushed toward the wellbore by this differential force and which, in turn, the drill string gets in contact with the mud cake. Over time, the contact area becomes larger and as a result, drill string is being captured by the mud cake. The drill string is considered to be differentially stuck if the hydrostatic pressure, Pm, acting on the drill pipe, is much more than the formation-fluid pressure, Pʄʄ, which is typically the case. The differential pressure impact on the portion of the drill string that is embedded in the mud cake can be stated as follows:

The following equation can be used to determine the pull force, Fp, required to release a stuck object:

Where:
Δp - the differential pressure,


f - the coefficient of friction,

A_c - the contact area.

A_c might be calculated using the following equation:

A_c= 2L_ep {(D_h/2-h_mc)² –[ D_h /2-h_mc(D_h-h_mc)/(D_h-D_op)}²}^0.5 (3)
.





D_op ≤ (D_h-h_mc)

Where:

1. 
   L_ep is the length of the permeable interval.
   
2. 
   D_op is the stuck pipe outside diameter.
   
3. 
   D_h is the drilled hole diameter. 
   
4. 
   h_mc is the thickness of the filter cake. 
   
5. 
   The dimensionless coefficient of friction, f, its range from +/- 0.04 for OBM to +/- 0.35 for WBM with no lubricants.
   

The above equations demonstrate the core factors that can lead to an increase in pipe sticking force and the decrease of the likelihood of the pipe freeing procedure.