03 - Differential Sticking

8/13/2019 03 - Differential Sticking

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Differential Sticking

Filter Cake

Desired Filter Cake Properties:Impermeable – prevent further filtrate invasionThin – minimum deposit of so lids, effective cake building, min damageTough – stays in place under dynamic conditions

• Filter Cake usually does not form on impermeable formations like shale• Shale is very porou s but th e pore spaces are too small to allow filtrate invasion• Shale adsorbs water into the clay structure by osmotic pressure


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Filter Cake

• Filter Cake usually does not form on impermeable formations like shale• Shale is very porous but the pore spaces are too small to allow filtrate invasion• Shale adsorbs water into the clay structure by osmotic pressure

5 Conditions Required forDifferential Sticking to Occur 1. Permeable formation in the open hole2. Overbalanced to the formation3. Thick filter cake4. Pipe in contact with filter cake

5. Stationary pipe


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Impermeable Shale1. Wellbore pressure > formation pressure (overbalanced).2. Overbalance may cause fluid flow into formation. But since

shale is impermeable, fluid does not flow into the shale andno filter cake forms.

3. Instead, overbalance generates a force which providesmechanical support directly to the formation face.

4. Since no filter cake forms, when the drill string comes incontact with the shale, the hydrostatic pressure very nearlybalances around the drill string because the contact area is

so small.5. As a result, the differential sticking force is small and thestring usually will not get stuck.


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Permeable Sandstone1. Filtrate enters permeable formation because of overbalance

and so filter cake accumulates.2. An effective filter cake becomes impermeable with a thindeposit of solids and stops further filtrate flowing into thesandstone.

3. A poor filter cake remains permeable allowing filtrate to enterthe sandstone and a thick deposit of solids to build up on theformation face.

4. When the drill string is adjacent to the sandstone, it may comein contact with the filter cake deposited on the formation face.

5. While the drill string is moving, sufficient drilling fluid passesbetween the drill string and the filter cake that the force fromhydrostatic pressure still very nearly balances around the drillstring.

6. As a result, the differential sticking force is small and the stringusually will not get stuck.

Permeable Sandstone7. However, since the filter cake is still permeable, the filtrate

continues to flow into the sandstone. The mud that passesbetween the drill string and the filter cake keeps it fromcollapsing by keeping it wet.

8. When the drill string is in contact with the filter cake, the contactarea is much wider since the filter cake deforms around the drillstring.

9. Also, when the drill string becomes stationary while in contactwith the filter cake, there is no longer any fresh drilling fluidpassing between the drill string and the filter cake.


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Permeable Sandstone10.Three events continue to happen:

1. There is no longer any fresh drilling fluid passing between the drillstring and the filter cake.

2. Filtrate continues to drain out of the filter cake between the drill stringand the sandstone.

3. Differential pressure is compressing the drill string against the filtercake.

11.These events cause the filter cake between the drill string andthe sandstone to collapse which further increases the contactarea.

12.As a result, the differential sticking force is large and the stringis more likely to get stuck.

Permeable Sandstone


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Spotting Fluids and Filter Cake

• Differential sticking doesnot typically restrictcirculation in the annulus

• This circulation capabilityis used to free stuck pipe

• Spotting fluids dehydratefilter cake, causing it toget thinner and sometimescrack

• This reduces stickingforce, allowing pipe tocome free more easily

1. Lower Filtration rate means thinner filter cake2. Thinner filter cake means less contact area3. Less contact area means less sticking force

Differential Sticking Warning Signs

• Permeable formations in the open hole.• Mud weight creating high overbalance.• High or uncontrolled mud filtration rates.• Long unstabilized BHA segments• Increasing drag trend coming off the slips after

a connection.

• Long stationary pipe times for connections,surveys, etc.

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03 - Differential Sticking