Introduction to Stuck Pipe

A drillstring is considered stuck if it cannot be pulled out of the hole. There are two main types of sticking mechanisms: differential pipe sticking and mechanical pipe sticking.

I- Differential Pipe Sticking

This type of stuck pipe occurs when the drill string becomes embedded on the mud cake which is formed on the wall of permeable zones during drilling through these formation intervals. As general case, the hydrostatic pressure exerted by the drilling fluid column is greater than the pore pressure of the formation. The required force which has to be applied to free the drill string has to be greater than the force that is related to the differential pressure (the difference between the hydrostatic pressure and the pore pressure - ِClick Here-).

Fig 01- Differential Sticking Mechanism

There are some factors which can be controlled to minimize the applied force on the drill string:

- Reducing the high differential pressure by optimizing the drilling fluid density
- Optimizing the mud cake thickness by reducing the fluid loss
- Reducing the time when the drill sting is not rotated or reciprocated especially when making a connection or rig repair.

Other factors cannot be controllable such as the hole diameter, hole angle and pipe diameter. These parameters, once are selected to get to drilling objectives, they cannot be changed or altered. However, the type of the drill collars can be changed; drill collars with spiral grooves can be used to minimize the contact area between the wellbore walls and the drill string.

The differential pipe sticking can be detected by noticing some indicators when drilling through permeable zones:

- Torque and drag increase
- Difficulties to reciprocate or rotate the drill string
- Full mud return ( the out flow is not interrupted)

The pipe sticking by differential mechanism can be prevented by following some precautions:

- Controlling the thickness of the mud cake 
- Planned for optimum drilling fluid density, to control the differential pressure
- Minimizing the time when the string is static, maintain rotating and reciprocating the drill string as possible

In order to free a stuck string by differential mechanism, some rig practices are followed:

- Reducing the hydrostatic pressure by reducing the mud weight 
- Spotting oil around the zone where string is stuck

II- Mechanical Pipe Sticking

This mechanism can occur due many reasons

- Accumulation of drilled cutting
- Bore hole instabilities
- Key seating

II.1. Accumulation of Drilled Cuttings

Poor hole cleaning can lead to drilled cuttings accumulation into the annulus causing stuck pipe. This problem can be more difficult when drilling deviated wells. The cutting beds can form on the low side of the well bore. It is common as rig site practice to pump sweeps and circulate bottom up before tripping out of the hole to avoid such situations. Increased torque and drag are indications of inappropriate hole cleaning and remedial actions have to be taken to remove the cutting beds by pumping sweeps. The sweeps program depends on the type of the well, high viscosity pill can be pumped when drilling vertical wells, whereas low viscosity and high density pill can be used to clean high deviated wells. The use of pills has to be accompanied by rotating the drill string to create agitation to make cutting beds removal more effective (Click Here).

Fig 02. Packoff

II.2. Borehole Instability

According to the mud weight and mud composition, the shale can slough or plastically move inward and cause mechanical pipe sticking. Another type of formation that can plastically flow is the salt in case when the mud weight is not high enough. Also, using a low drilling fluid density can lead to hole collapsing that can cause stuck pipe. An increase in pressure and torque are indications about borehole instability.

Fig 03. Borehole Instability Mechanism

II.3. Key Seating

Key seating is caused by creating a groove into the wall of wellbore when combining the rotation of the drill sting and applying side force on it. The lateral force tends to push the drill string against the wall causing mechanical erosion and creating what is called a key seat. Long bit runs may cause key seats and as good practice to avoid such problem is making wiper trips or using stiffer bottom hole assemblies in order to minimize high dogleg occurrence. 

Fig 04. Key Seating

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