Stuck wireline tools are costly due the time it takes to retrieve the tool/cable from downhole.
This article focuses on the historical approach to modeling sticking risk by Underhill, Moore & Meeton (1998, ref). This article uses the paper to highlight the critical areas of job design that require attention during the planning and preparation for wireline open-hole operations to mitigate the primary sticking risks in wireline conveyance:
- Differential pressure sticking is common in permeable zones with significant overbalance.
- Mechanical sticking includes key seating where the cable or tools becomes wedged in a groove created by pipe or cable and tool obstruction by borehole ledges or overhangs.
- Formation-related sticking is due to mechanically or chemically unstable formations. Examples include unconsolidated rock, fractured rock or swelling shales.
Differential Pressure Sticking
Differential pressure sticking occurs when there is a significant pressure difference (overbalance) between the borehole and the formation, causing the tool or cable to embed into the mudcake lining the borehole wall. When the tool or cable remains stationary, fluid loss from the mudcake can cause it to become denser and stronger near the formation, increasing friction and leading to sticking. This phenomenon is common in permeable zones with high differential pressures.
Historical work done to understand the mechanisms of drill pipe sticking in relation differential pressure gives us a starting point for understanding the (ref) the primary variables affecting differential pressure sticking:
- Set time
- Differential pressure
- Filter-cake thickness
- Mud composition
- Use of oil and surfactants.
This section delves into each of these variables, elucidating their impact on differential pressure sticking based on laboratory studies and empirical data.
Set Time
Set time, defined as the duration steel is in contact with the mud filter cake, is a critical variable influencing differential pressure sticking. Laboratory experiments reveal that the sticking coefficient, which measures the friction between steel and mud cake, increases with set time (ref). This rise is attributed to the dehydration and compression of the filter cake over time. As the filter cake loses water and compacts, its frictional resistance against the pipe increases, reaching a near-constant value once maximum compression is achieved. Therefore, managing the set time can be an effective strategy to control the severity of differential pressure sticking. However, Underhill et al notes that for wireline stationary measurements, sticking forces increase rapidly right after the tool and cable stop moving, but this growth slows down significantly over time. Doubling the stationary time results in less than a 20% increase in sticking forces. However, reducing the sticking risk by half requires a 95% reduction in stationary time. Field observations confirm that tools often become stuck immediately after stopping, and later increases in sticking forces are minimal. This explains why stationary time appears less critical in sampling applications, as sticking force growth is slower at later stages.
Differential Pressure
Increasing differential pressure between the mud column and the formation is another important factor in increased sticking risk. It creates the force that holds the steel against the borehole wall. Studies indicate that while the absolute pull force required to move the tool increases with differential pressure, the sticking coefficient itself remains relatively constant when corrected for residual adhesive force. This suggests that differential pressure mainly influences the initial sticking force rather than the inherent frictional properties of the mud cake.
Filter-Cake Thickness
The thickness of the mud filter cake formed on the borehole wall significantly impacts differential pressure sticking. Experiments show that as the filter-cake thickness decreases, the maximum sticking coefficient increases linearly. Thinner cakes lead to higher sticking coefficients, likely due to the higher compaction and strength of the thinner layers. Therefore, controlling the filter-cake thickness through mud composition and drilling practices can mitigate the risk of differential pressure sticking.
Mud Composition
Mud composition, particularly the types and amounts of solids and additives used, plays a crucial role in differential pressure sticking. The presence of barite, a common weighting agent in drilling muds, has been found to increase the sticking coefficient significantly. High-yield clays generally result in lower sticking coefficients initially but can reach similar maximum values as low-yield clays over time. Certain chemical additives can temporarily reduce friction but lack long-term effectiveness, while others commonly used for filtration control do not significantly impact friction levels.
Oil and Surfactants
The emulsification of oil into the mud has been shown to reduce the sticking coefficient, with the effectiveness varying among different oil types. Oils create a lubricating film between the steel and mud cake, reducing friction. Surfactants can enhance this effect by making the barite in the mud more oil-wettable, further lowering the sticking coefficient. This approach is particularly useful in reducing the initial frictional forces and delaying the onset of severe sticking.
Spotting Fluids
Spotting clean fluids, such as oil or water, over the stuck interval can mitigate differential pressure sticking, especially when combined with a temporary reduction in differential pressure. This method reduces the build-up of mud cake and decreases the sticking coefficient over extended periods. The temporary reduction in pressure allows the clean fluid to penetrate the mud cake, reducing its adhesion to steel and facilitating movement.
Toolstring length
The length of the tool and cable significantly impacts the risk of sticking (ref). The force required to free a stuck cylindrical object, such as a tool, is directly proportional to its length. For cables, the situation is more complex. While a cable hanging freely in the borehole center might avoid contact with the mudcake, it often interacts with the mudcake due to tool movement or tension in deviated sections. Even slight deviations in the borehole path can force large cable lengths into the mudcake, leading to sticking when the tool stops. Therefore, the actual borehole geometry, influenced by drilling techniques, plays a crucial role in the likelihood of differential sticking.
Toolstring Diameter
The diameter of the tool significantly affects the differential sticking forces (ref). The force required to free a differentially stuck tool is proportional to the square root of its effective diameter. For small-diameter objects like cables, this effective diameter is simply the cable diameter. However, as the tool’s diameter gets closer to the borehole diameter, the effective diameter increases significantly, causing a rapid rise in sticking forces. Thus, larger tools are more prone to higher sticking forces compared to smaller ones.
Mechanical Sticking
Mechanical sticking occurs when the tool or cable becomes physically trapped within the borehole due to various mechanical interactions. The primary types of mechanical sticking include key seating and borehole obstructions.
Key seating happens when the cable grooves, known as key seats, into the borehole wall. For wireline cables this is exacerbated by the lateral force acting on the side of the borehole wall, which as we shall see is a function of well deviation profile.
Key Seating
The grooves are formed due to sustained lateral pressure from the moving, tensioned logging cable or drill pipe. Larger diameter collars or other bottom hole assembly hardware cannot pass through these grooves, causing the tools to become stuck.
Contributing Factors:
- Directional work, particularly in high dog-leg severity zones.
- Soft formations that are easily cut by the cable.
- High tension in the cable, which increases lateral forces
- Wellbore tortuosity (the degree of deviation or curvature along the path of a drilled wellbore, increasing contact and friction points.
- Frequent traversal of the same section
Borehole Obstructions
Borehole obstructions occur when the tool encounters ledges or overhangs within the borehole, which prevents it from moving freely. These obstructions can trap the tool, especially during tripping out operations, making it difficult to retrieve or continue drilling.
Several factors contribute to borehole obstructions. Irregular borehole geometry with ledges or overhangs is a primary cause, creating physical barriers for the tool. Borehole stability issues, such as collapsing or sloughing formations, can also lead to blockages. Additionally, poor hole cleaning practices can leave debris in the borehole, further increasing the risk of the tool becoming trapped.
Formation-Related Sticking
Formation-related sticking occurs when the toolstring, or wireline becomes stuck due to the unstable nature of the geological formations being drilled through. This type of sticking is primarily caused by mechanically or chemically unstable formations, and can manifest in several specific scenarios:
Unconsolidated Rock
Unconsolidated rocks are loose, non-cohesive sediments such as sand or gravel that have not been cemented into solid rock. When drilling through these formations, the loose sediments can collapse into the wellbore, causing partial or complete obstructions
Fractured Rock
Fractured rock formations contain natural cracks or fractures. These fractures can destabilize the borehole by causing sections of the rock to break off and fall into the wellbore creating an obstruction which interferes with the wireline tool. Additionally, the fractures can allow drilling fluids to escape into the formation, reducing the support provided by the mud column and leading to further collapse or sloughing of the borehole walls.
Swelling Shales
Shale formations that absorb water and expand are known as swelling shales. These shales are chemically reactive and can absorb water from the drilling fluid, causing them to expand and exert pressure on the wellbore walls and reduce the internal diameter of the borehole and creating a restriction for wireline tools to pass.
Technical Advisory
In drilling operations, sticking of tools and cables can lead to significant operational delays and increased costs. A useful way to look at the variables which affect risk is to consider Underhi et al’s (ref) proposed calculation of sticking rick probability. Then considering each of the input variables in turn we can get a clearer picture of the potential contribution (non weighted) to to probability of getting stuck:
Tool Length: For the tool, “The force to free a stuck cylindrical object is proportional to its length”. Longer tools have more surface area in contact with the borehole, increasing the risk of sticking. Mitigation: Use shorter tools or break up contact of the tool with standoffs to reduce contact area.
Openhole Length: Longer openhole sections raise the risk of sticking due to unstable formations and variations in borehole diameter. Additionally, the frictional co-efficients of open-hole are generally higher, increasing the tension in the cable and resultant forces. Mitigation: Minimize open hole length where practical and maintain wellbore condition.
Tool (effective) Diameter: “The force to free a differentially stuck tool represented by is proportional to the square root of the effective diameter”. Where effective diameter is given by:
For small-diameter cylindrical objects such as the cable, the effective diameter is essentially the cable diameter. However, as the diameter of the object approaches the borehole diameter, the effective diameter becomes large and sticking forces increase rapidly”. Mitigation: Use tools with smaller diameters when feasible.
Cable Diameter: Following the same theory, thicker cables have an increased effective diameter, increasing the potential sticking forces.
Borehole Diameter: Smaller borehole diameters reduce clearance for a fixed tool diameter, therefore increasing the risk of sticking. Mitigation: Larger hole size relative to tool size the better, also maintain optimal borehole diameter whilst drilling.
Mud Density: High mud density can reduce the differential between formation and hydrostatic pressure but can cause issues if too high. Mitigation: Optimise mud density to balance pressure without causing damage.
Differential Pressure: High differential pressure forces the tool or cable into the mudcake, increasing the risk of sticking. Mitigation: Manage mud weight and formation pressure to minimise differential pressure.
Temperature: Higher temperatures can alter mud properties, potentially increasing the risk of sticking. Mitigation: Use temperature-stable mud formulations and monitor downhole temperatures.
Stickance Factor Ratio: A higher ratio indicates a greater tendency for sticking. Mitigation: Use mud additives and lubricants to lower the stickance factor ratio. Consider change in stickance of mud over time and requirement for conditioning/wiper trips over time.
Note: What is stickance and can we really measure it? – In practical terms, stickance quantifies the adhesive properties of the mudcake that forms around the tool or cable, contributing to differential sticking. The stickance factor ratio is used to compare the sticking tendency of a particular mud against a reference mud, helping to assess and mitigate the risk of stuck tools in drilling operations.
The stickance tester is a modified high-temperature, high-pressure mud filtration cell which can be used to measure the stickance properties of muds both in the laboratory and at the wellsite. This device provides a convenient method to evaluate the sticking tendency of the mud.
Set (Stationary Time): Longer stationary time allows for more mudcake buildup, increasing the risk of sticking although this risk, although sticking force growth is slows as time increases. Mitigation: Monitor tensions (head and cable) during any points were the tool is stationary to give any indications of cable/tool sticking at depth. Minimise stationary time by optimizing logging procedures for continuous logs.
Well Inclination: Greater inclination increases the potential for mechanical sticking due to lateral forces. Consider also changes in well direction and consequence lateral forces. Mitigation: Design well path with consideration to lateral forces on wireline.
Dogleg Severity: Significant doglegs will increase lateral forces on the cable raising the risk of sticking. For wireline tools consider minimum rigid length of wireline tools to ensure tools can pass freely. Mitigation: Plan well trajectories to minimize doglegs and use flexible strings to reduce minimum rigid length where necessary.
Number of Logging Runs: More logging runs increase cumulative wear on the borehole, raising the risk of sticking. Mitigation: Combine measurements to reduce the number of runs.
Tool Weight: Heavier tools increase wireline tensions (as does drag), increasing the risk of sticking. Mitigation: Use lighter tool strings where considered necessary, reduce downhole forces due to friction (rollers/centralisers etc).
Unconfined Rock Compressive Strength: Weaker formations are more prone to collapse, increasing the risk of sticking. Mitigation: Stabilize weak formations with appropriate casing and mud programs.
Closing comments
Developing an effective conveyance strategy in wireline operations is crucial for minimising the probability of sticking. While significant progress is still needed to fully quantify this risk, utilising known input variables within a tension model and resolving forces allows us to produce a qualitative risk assessment for individual wellbores. This approach, combined with the application of best practices for tool and job design, offers valuable guidance on designing the conveyance system and executing the job while minimizing the risk of sticking. Our models can be further refined as a well is drilled and logged, enhancing accuracy as confidence in input parameters increases.
The next article in this series will introduce a software solution currently in development that provides a qualitative assessment of sticking risk using known input parameters. This in-house developed solution will enable our customers to better manage and mitigate sticking risks in their future operations.
