Differential sticking – Causes, indications, prevention, and recovery
Nonproductive time has a significant impact on the cost of a well. Based on several studies, the stuck pipe is one of the costliest complications during drilling operations. Deploying suck pipe preventive measures is far more cost-efficient than having to use recovery methods. Recognizing potential drilling problems, understanding their causes, and preparing solutions in advance are key to success. There are primarily three types of string stuck situations faced during drilling. Out of them, Differential sticking is a prevalent issue encountered during well drilling.
Several factors, including the solid content of the drilling fluid, fluid density, filter cake thickness and quality, formation permeability, the length and components of the bottom hole assembly, and the angle of the hole, influence the likelihood of differential sticking. When the hydrostatic pressure exceeds the formation pressure, the drill string lying on the low side of the well can be pushed into a filter cake that has developed against the permeable formation, which leads to the string becoming stuck. Once differential sticking happens, moving the pipe string up or down becomes impossible, although establishing free circulation is still possible.
Causes of Differential Sticking
High Differential Pressure: If the hydrostatic pressure of the working fluid in the well surpasses the formation pressure, the string gets pressed firmly against the wellbore. For example, with a 200 psi difference between the hydrostatic pressure and the formation pressure, each square inch of the Bottom Hole Assembly (BHA) embedded in the filter cake will experience a force of 200 pounds pushing it against the hole's wall. This situation requires a higher pulling force to overcome that pressure.
Mud Properties and Filter Cake: The mud weight should be proactively and precisely managed while drilling through permeable formations. It should be high enough to maintain the overbalance to protect against blowouts but not too high, which could increase the risk of differential sticking.
When drilling fluid encounters a permeable formation, some of the liquid component is pushed into the formation, leaving behind solids that adhere to the wellbore wall and form what is known as a filter cake. The liquid that permeates into the formation is referred to as 'water loss.' The ideal filter cake would be both thin and tough. Among all drilling fluid contaminants, drilled solids are the most detrimental. Therefore, it’s essential to maintain a low percentage of these low-gravity solids. The quality of the wall cake on the borehole wall is influenced by the proportion of low-gravity solids in the mud system and the amount of 'water loss' absorbed by the formation. To minimize water loss, specific chemicals are incorporated into the drilling fluid.
Permeable Formation: The presence of permeable zones allows the formation of a filter cake—a layer of mud solids deposited on the wellbore wall. This cake is vital in enabling differential sticking as it provides the essential surface with which the drill string can contact.
High Contact Area: The sticking force is a product of the differential pressure and the contact area, meaning larger contact areas can significantly exacerbate the issue. The drill string centralization plays a crucial role; effective centralization reduces wall contact. When centralization is inadequate, particularly in deviated sections, the drill string may sag, lying on the low side of the well, further increasing the contact area. Long, plain sections of drill collars heighten the risk of sticking by increasing the contact area, as they place additional weight against the formation. Thick filter cakes also increase the effective contact area by allowing the drill string to penetrate more deeply, intensifying the sticking force.
Wellbore Inclination: In deviated or horizontal wells, gravity causes the drill string to rest against the low side of the wellbore, which increases the contact area with the filter cake, especially when the drill string is stationary.
Static Drill String: If the drill string is idle for too long, it tends to get buried in the filter cake under its own weight or due to high differential pressure. This gives the sticking force more time to intensify and increases the risk of sticking. Share thoughts and additional information
Mechanisms of Differential Sticking
A series of physical interactions result in the differential sticking of a drill string:
Differential pressure-induced force: The column of drilling fluid in the wellbore exerts higher hydrostatic pressure than the surrounding formation pressure. This differential pressure forces the drill string against the filter cake on the wellbore wall. The magnitude of this force is directly proportional to the pressure differential and the area of contact.
Embedding in Filter Cake: With time, the stationary drill string becomes increasingly embedded within the filter cake, expanding the contact area between the string and the filter cake. During this time, the drilling fluid continues to seep into the permeable formation, building the filter cake and further intensifying the sticking force.
Friction: As the drill string becomes embedded in the filter cake, friction and adhesion with the filter cake occur, requiring higher pulling force for string movement. The sticking force is calculated using the formula Fs=dP×A×Cof, where ‘dP’ is the pressure difference, ‘A’ is the contact area, and ‘Cof’ is the ‘Coefficient of friction’. The sticking force tends to increase over time, making the vertical movement of the pipe nearly impossible.
This mechanism of differential sticking combines pressure dynamics and filter cake interactions with the drill string rather than any physical barrier restricting the string's movement.
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Indicators and early signs
When the drill string is pushed against the mud cake, the friction between the drill pipe and the wellbore increases significantly. Once this friction exceeds the pulling capacity, upward or downward movement becomes impossible, and the pipe is said to be stuck. The following early signs and indicators stem from the increased frictional force.
Higher torque: The dense mud cake decreases the gap between the wellbore wall and the bottom hole assembly. As a result, a more significant part of the assembly comes into contact with the mud cake while rotating, increasing the contact surface area and torque when rotating the drill string. A sudden rise in either suggests that the drill string may start sticking to the wellbore wall, possibly due to pressure differences or filter cake buildup.
Overpull during connection and after surveys: If a thick filter cake builds up on the wellbore wall, the wellbore will narrow across those sections, causing resistance to the drill string movement, which will show up as overpull. The string is temporarily stationary while making connections and surveys. If high differential pressure conditions exist along with the thick filter cake, the drill string could be pushed into the cake, requiring additional pull on the string after the connection or surveys.
Reduced penetration rate: Higher torque consumes a larger part of the mechanical energy supplied to the bit from the surface, leaving less energy for the drill bit to carry out the cutting action.
Inability to Move the Drill String: The drill string cannot be rotated or moved up or down, signaling it is stuck against the wellbore wall.
Free Mud Circulation: Since the sticking is due to pressure and filter cake adhesion, not a physical blockage, the differential sticking allows the drilling fluid to circulate freely through the annulus.
Stationary Pipe in Permeable Zones: Sticking often occurs after the drill string has been stationary, particularly across a permeable zone. It is accompanied by signs of increased torque and drag or mud loss while drilling. Continuous movement, whether rotating or reciprocating the drill string, prevents it from settling into the filter cake.
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Preventive Measures for Differential Sticking
Including measures for avoiding differential sticking in the plan is less costly than freeing a stuck pipe. Effective strategies include:
Monitoring Drilling Parameters: To detect potential sticking risks, look for early warning signs, such as an increasing torque and drag trend or tight spots while tripping.
Minimizing Stationary Time: Keep the drill string moving, especially in permeable zones, to prevent embedding in the filter cake. Avoid prolonged inactivity during drilling operations. Even during pauses in active drilling, periodically moving the string reduces sticking risks.
Optimizing Mud Properties: Adjust drilling mud characteristics to reduce filter cake thickness and permeability. This may involve using additives or selecting mud types (e.g., oil-based mud) that minimize cake build-up. Lowering the drilling fluid's solids content and adjusting its filtration properties would deposit a thinner and less permeable mud cake.
Drill String Design: Employ components like spiral drill collars to reduce the contact area with the wellbore wall, lowering the sticking risk. Unlike standard cylindrical collars with plain outer surfaces, their spiral design minimizes the surface area that contacts the wellbore wall.
Balancing Mud Weight: Maintain a mud weight that minimizes the pressure differential with the formation while ensuring well control, avoiding excessive overpressure that exacerbates sticking.
These proactive measures address the root causes of pressure, permeability, and time, reducing the likelihood of differential sticking. Share thoughts and additional information
Remedial Measures to Release Differentially Stuck Pipe
If the drill string gets differentially stuck despite all precautions, several remedial measures can be used to free the string and minimize the time and cost impact:
Mechanical Efforts: The initial response typically involves mechanical efforts to free the stuck pipe. These efforts aim to break the pipe free using physical force.
Apply maximum allowable torque to the drill string to overcome the static friction between the pipe and the borehole wall. This rotational force can sometimes break the adhesive hold.
Jarring complements torque through a Jar. A jar is a specialized tool to deliver sudden, high-impact forces to the pipe. The direction of jarring depends on the bit’s position. If the bit is at the bottom, jarring upwards may free the pipe. If the bit is off-bottom, jarring downwards may be applied to push the pipe loose.
Circulate mud at the maximum allowable rate to maintain pressure dynamics while working the pipe. This will also help lubricate the stuck section, aiding the mechanical efforts.
If the bit is off the bottom, work the maximum Limit torque down to the stuck depth and hold it in the string. Then, slump the pipe with the right-hand torque held in it.
Stop or reduce pump speed to a minimum. Slack off to the maximum set-down limit. Allow sufficient time for a hydraulic jar to trip. If the string does not come free, hold torques in the string and continue jarring down with the maximum trip load.
If the bit is on the bottom, apply right-hand torque to the stuck point, pull to the maximum, and jar up.
Pressure Reduction: If mechanical efforts fail, reducing the hydrostatic pressure in the annulus becomes a viable strategy. This approach targets the root cause of differential sticking, the pressure differential itself. Lowering the mud weight decreases the differential pressure and reduces the force pushing the pipe against the formation, requiring less pull or push to free it. The hydrostatic head can be reduced by using lighter fluids to replace part of the mud column. It can also be done by injecting nitrogen into the mud, which creates a gasified mixture, lightening the hydrostatic pressure of the mud column and reducing differential pressure. However, this method requires extreme caution due to well-control implications. Lowering mud weight too aggressively could destabilize the well, risking a blowout or other hazardous conditions.
Chemical Interventions: Special chemicals can be pumped downhole to change the interaction between the drill string and the formation. The filter cake, which is formed due to the deposition of mud solids on the borehole wall, is a key factor in differential sticking. Weakening its structure or reducing its friction could help free the string. Chemicals are spotted across the stuck section to facilitate release. Commonly used spotting fluids are ‘Lubricants’ to reduce friction between the pipe and the formation or ‘Diesel Oil,’ which reduces the viscosity and strength of the filter cake, especially in water-based mud systems, making it easier to dislodge the pipe or Acid that can chemically degrade or dissolves the filter cake, weakening its adhesive grip on the pipe. The fluid is pumped into the annulus and left to soak around the stuck section, enhancing its effectiveness over time. Chemical interventions are often combined with mechanical efforts to increase the likelihood of success, particularly in cases where friction or filter cake adhesion is a dominant factor.
Specialized Technique: For severe differential sticking incidents where other methods fail, a more invasive and advanced technique may be necessary to perforate the pipe's stuck section and pump fluid at high pressure. The injected fluid reduces or balances the pressure differential across the stuck section. The high-pressure flow can wash away or erode the filter cake, loosening its hold on the pipe. Perforation-blocking elements (e.g., balls) are added later to control fluid flow, and then the pipe is moved once freed.
Fallback Options: If all previous attempts fail, fallback options focus on recovering the stuck section through more drastic measures. Pipe stretch calculations or running a free point indicator can identify the free point. The free portion of the drill string can be disconnected (backed off) from the stuck section using a controlled explosive or mechanical device. Once the free portion of the drill string is recovered, specialized tools, known as fishing tools, are deployed to latch onto and retrieve the stuck pipe. This process can be slow and costly, require additional equipment, and may require several attempts. If fishing fails or is impractical, sidetracking may be necessary. Sidetracking is the process of drilling a new path around the stuck section may be considered. This is an ultimate fallback but significantly increases time and expense.
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What should you do after the string becomes free?
Circulate at the maximum allowable flow rate to completely clean the hole. The flow rate must be more than the cutting slip velocity to transport cuttings effectively. Reciprocate and work the pipe while cleaning the hole. Ensure you can work the pipe with a complete stand or joint while circulating. Condition mud before drilling ahead because the differential sticking will reoccur if you still drill with poor mud properties. If aggressive mechanical efforts were made to release the string, it is advisable to pull out the string, inspect all components, and replace the damaged components before resuming further drilling. Share thoughts and additional information