Mud filter cake is a layer formed by solid particles in drilling mud against porous zones due to differential pressure between hydrostatic pressure and formation pressure. For the drilling operation, it is preferred to have a filter cake that is impermeable and thin. Practically, the filter cake from API or HTHP fluid loss test should be less than or equal to 1/16 inch. If drilling fluid is not in a good shape, which results in a thick filter cake in the wellbore, it will lead to a stuck pipe situation and high torque/drag.
Mud Filter Cake from HTHP Fluid Loss Test (http://oilfieldpix.com/photo/679/Mud-filter-cake.html)
How will the filter cake impact on a drilling operation?
Differential sticking
If mud filter cake is thick, a contact area between drilling string or any kind of tubular will be increased. When drilling into permeable zones that are severely overbalanced, the drill stem will have high chance to get differentially stuck across these zones. Moreover, not only can the drilling string get stuck, the logging tool could be stuck across the permeable sands as well.
Torque and drag
Under dynamic conditions such as drilling, working pipe, etc., if drilling mud has a thick filter cake across the wall of the wellbore, torque will increase. Furthermore, a thick wall filter cake will result in high drag while tripping out of the hole or logging.
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