NEWS Demystifying Friction in Oil and Gas Wellbores
The Role of Centralizers in Run-in-Hole Efficiency
The interplay between various components within the wellbore can significantly impact operational efficiency and success. One crucial yet commonly misunderstood aspect is friction, particularly between centralizers, casing or liner, and the wellbore itself. Despite its fundamental importance, there is often a limited understanding of the impact that friction can have on run-in-hole operations and torque management. Addressing this gap in knowledge is essential for optimizing drilling operations and maximizing wellbore integrity.
Understanding Friction in Wellbores
Friction arises when surfaces come into contact and resist relative motion. In oil and gas wellbores, friction occurs between the casing or liner and the wellbore wall, as well as between centralizers and the surrounding formation. This frictional resistance can impede the smooth movement of the casing or liner during installation, leading to challenges such as increased torque, drag, and the risk of differential sticking.
Challenges of Friction in Run-in-Hole Operations
Frictional forces during run-in-hole operations pose several challenges for drilling operations:
- Increased Torque: Friction between the casing or liner and the wellbore wall generates torque, making it more difficult to rotate and advance the casing or liner into the wellbore. High torque levels can strain drilling equipment, increase wear and tear, and necessitate the use of higher-powered drilling rigs.
- Drag: Frictional drag, or the resistance to axial movement, can slow down the advancement of the casing or liner, prolonging the time required for completion and increasing operational costs. Drag forces can also lead to casing deformation or buckling, compromising wellbore integrity.
- Differential Sticking: Excessive friction between the casing or liner and the wellbore wall can cause differential sticking, where the casing becomes lodged in the wellbore due to differential pressure or mechanical obstructions. Differential sticking poses significant operational risks and may require costly remediation efforts to free the casing.
The Role of Centralizers in Friction Management
Centralizers play a crucial role in mitigating friction and optimizing run-in-hole operations. By promoting proper centralization of the casing or liner within the wellbore, centralizers help minimize contact between the casing and the wellbore wall, reducing frictional resistance and associated challenges. The ability of the centralizer remain intact and in the optimal place on the string to center the casing far outweighs any low friction qualities of the product material or coating.
Wear resistance plays a crucial role in the effectiveness and longevity of low friction coatings on casing centralizers. These coatings are applied to reduce friction between the centralizer and the wellbore wall, minimizing drag and torque during casing or liner deployment. However, if the coating lacks sufficient wear resistance, it may degrade prematurely, compromising its low friction properties and diminishing its effectiveness over time.
Centek Group's Low Friction Centralizers: A Game-Changer for Run-in-Hole Efficiency
Centek Group, a leading manufacturer of centralizers for the oil and gas industry, has pioneered a number of innovative solutions to address frictional challenges in wellbore operations. All their bow spring centralizers are designed as low-friction units, featuring specialized coatings and materials that minimize frictional resistance during run-in-hole operations, and flexible bows for smoother casing or liner advancement, reduced torque requirements, and enhanced operational efficiency.
Moreover, many of Centek Group's single piece centralizers are engineered for liner rotation, allowing for improved wellbore integrity and cement coverage. This rotational capability enables operators to overcome frictional challenges associated with casing or liner deployment in deviated or horizontal wellbores, ensuring optimal centralization and cement placement.
Common Steel Coefficient of Friction Values
Steel is a commonly used material for casing, liners, and centralizers in oil and gas drilling operations. The coefficient of friction between steel and various surfaces can vary depending on factors such as surface finish, lubrication, and contact pressure. However, some commonly cited values for the coefficient of friction between steel and different materials include:
- Steel on Steel: Coefficient of friction ranges from 0.15 to 0.8, depending on factors such as surface condition and lubrication.
- Steel on Rock: Coefficient of friction typically ranges from 0.6 to 1.2, depending on the type of rock and surface roughness.
- Steel on Cement: Coefficient of friction ranges from 0.4 to 0.8, depending on factors such as cement composition and surface finish.
Understanding these frictional properties is essential for optimizing wellbore operations and selecting the appropriate centralizers and casing or liner materials to minimize frictional resistance and maximize operational efficiency.
Friction in Action
If the Centek heat treated steel product is placed on a wetted/lubricated steel tubular, according to typical technical textbooks, the coefficient of friction is 0.06 to 0.08. However, it is undoubtedly more complicated to determine the friction coefficient for actual wellbore conditions, due to the drilling mud and formation variabilities. For this reason, common friction coefficients are quoted as 0.3 for OH and 0.2 for cased hole due to variations in the lubrication content.
Centek have measured the friction coefficient between a Centek centralizer and steel casing in water, in order to validate textbook steel on steel values. The test machine was loaded with a 7” casing sub which could be rotated, and upon which a centraliser was placed. Around the outside of this assembly was a 9-5/8” static casing stub, which was side loaded to simulate various lateral loads. The whole mechanism was contained within a tank in which various heat-treated fluids and contaminants were introduced.
Initial testing with a Centek S2 centralizer in water at 60oC and the 9-5/8” stub side loaded to ca 1,000lbf, the recorded torque was calculated back to 0.07 value for the dynamic coefficient of friction. It is believed that the static or ‘Start-up’ friction is of the order 14% higher than ‘dynamic friction; and this equates to a value of ca 0.08 for the Static or Start-up coefficient of friction.
Example:
Torque equivalents for 7” casing in 8-1/2” OH assuming lateral load of a 40ft joint at ca 1,000 lbf:
|
|
Start-up µ 0.08 |
Running µ0.06 |
Commonly used µ 0.25 |
|
Torque (lb.ft) |
24 |
18 |
73 |
As can be seen, the actual figures for steel on steel, wet, give much lower torque values compared to the commonly used figure of 0.25. This is not surprising, given the difference in testing in a controlled clean test plant state and real-life downhole conditions. These differences are not attributable to materials from which Centek centralizers are made but more to do with the debris and mud present.
Conclusion
In conclusion, friction between centralizers, casing or liner, and the oil and gas wellbore significantly influence run-in-hole operations, along with torque, drag, and differential sticking. Centralizers play a vital role in mitigating friction by promoting proper centralization and facilitating smoother casing or liner running. Centek Group's low-friction single piece bow spring centralizers are engineered to address these challenges, offering innovative solutions that optimize run-in-hole efficiency and enhance wellbore integrity. By leveraging advanced materials, coatings, and design features, Centek Group will continue to empower operators to overcome frictional obstacles and achieve optimal performance in oil and gas drilling operations.