Custom-Designed Coiled Tubing Leads to Success in Extended-Reach Operations
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Advanced horizontal drilling, multistage hydraulic fracturing, and other technologies have helped make the Vaca Muerta shale oil and gas resource economically viable. An option to exploit this resource better is to increase the length of the horizontal section of each well to add hydraulic-fracture stages. Anticipated high angles in deviated well sections, along with the extended horizontal length of the planned wells, formed an expectation that conventional coiled-tubing (CT) designs would not be able to service the new well designs. Using refined, custom-engineered CT-string designs appears to be an economic and efficient solution.
Increased diameter, strength, and reliability have made CT the preferred cost-effective well-intervention solution in the Vaca Muerta unconventional shale development; however, the maximum lateral reach the CT can achieve during operations is a limiting factor for well optimization.
Initially, 2-in. CT strings with a simple wall taper shape were an effective and reliable option to complete wells in the first development stage. However, the need for optimized design and even-larger-diameter CT with enhanced-wall-thickness configurations became apparent when an increase of the well lateral lengths was attempted.
CT-String Design Considerations
The CT-string designs followed a cooperative process between the operator, the CT service companies, tool providers, and the CT-manufacturing engineers. Tubing forces, buckling occurrence, and lockup depths were analyzed in detail to determine the optimal CT size and string design makeup to ensure the likelihood of reaching target depth (TD) in the planned extended-reach wells. Additionally, hydraulic analysis was performed to compare frictional pressure losses (CT, bottomhole-assembly, and annular) using the optimal flow rates for annular velocities adequate to support sufficient well cleaning. The results from the analysis were used to estimate fatigue accumulation in the CT strings, considering anticipated circulating pressures and surface-equipment (reel and gooseneck arch) dimensions.
The primary objective for the new CT-string design is to optimize lateral reach and available weight on bit for post-fracture mill-out operations.
Extended-Reach CT-Design Methodology
Excessive friction between the CT and the casing causes a lockup condition that prevents further penetration in the lateral. Friction lockup indicates that any force applied at surface by the injector and the available CT normal weight is lost because of local drag, especially around the heel, and because of buckling in the vertical and horizontal sections of the wellbore. This friction between the CT and wellbore is produced by excessive wall-contact forces generally because the CT buckled into a helical shape as a result of the axial compressive forces while running in hole. Therefore, preventing helical-buckling occurrence in the CT reduces the wall-contact forces acting against the wellbore, allowing for force transfer to the end of the CT string.
By understanding the theory of buckling, especially helical-buckling load calculations, CT-design engineers can alter the string makeup to reduce CT buckling in extended-reach wells. Helical-buckling load variables that are affected by the CT-string design are CT stiffness (CT geometry), radial clearance, and tubing weight. Therefore, helical buckling can be mitigated by increasing the CT stiffness in the areas of maximum compression (i.e., varying the geometry of the tubing to increase the moment of inertia of that section). Consequently, increased wall thickness is placed strategically to cover the vertical section and the curvature of the well where the tubing tends to buckle more, and then it is tapered down to the minimum wall thickness quickly to reduce weight in the horizontal.
However, the number of wall thicknesses and the lengths of the transitions have significant influence on the performance of the string, particularly on extended reach, weight, and overpull. Technology exists that provides rapid wall-thickness transitions of 300 to 650 ft in length to place specific thicknesses purposefully along the length of the string to enhance force transmission to the end of the tubing, increase strength and stiffness, and reduce fatigue accumulation and weight.
Friction is another variable that can be reduced by using vibration tools (extended-reach tools) and fluid additives such as metal-to-metal lubricants. The advantage of vibration tools or lubricants is so significant that, depending on the complexity of the extended-reach wells, they can enable reaching TDs with smaller CT sizes and lighter string makeups.
To maximize fatigue life, CT-design engineers carefully select the minimum wall thickness that has the proper diameter/wall-thickness ratio for the CT geometry. Increasing wall thickness reduces CT-bending-fatigue accumulation and increases pressure capacity. Therefore, it is recommended to cover the expected working area of the string with increased wall thickness where the fatigue tends to accumulate faster because of short trips (from kickoff point to TD).
2-in. CT Design Optimized for Multistage Hydraulic Fracturing
The design process started by placing specific thicknesses along the tubing length to avoid helical buckling in the well. The effective axial force and the helical-buckling-load limits were analyzed for each wall-thickness change in the designs.
After many design iterations and revisions of the proposed strings with the CT service companies and the operator, the final designs were achieved (Fig. 1). These strings have a unique continuous-taper configuration, which provides rapid wall-thickness transitions to help with the strategic placement of the wall thickness and provide additional tubing strength where it is needed most.
The new string designs feature an hourglass configuration, which reduces some of the heavy wall in the upper section with thinner wall thicknesses. This reduces fluid frictional pressure losses inside the CT because of the restricted inner diameters; reduces weight; reduces tubing costs; and, most importantly, does not affect extended-reach capacity.
Optimized-2-in.-CT-Design Field Performance
The 2-in. customized CT designs have been in operation since 2015. The additional force transfer achieved by the optimal selection of the wall-thickness-transition points allowed for more available set-down weight while milling fracture plugs, which increased the efficiency of the operation. The post-job analysis performed in several wells indicated that the apparent friction factors are very similar to the previously matched friction factors analyzed with the previous CT designs when using lubricants and a vibration tool, which was expected.
2.375-in.-CT Design Optimized for Multistage Hydraulic Fracturing
The proposed design enhances reach by decreasing tubing weight in the horizontal section of the well and increases stiffness in the vertical section to avoid the onset of helical buckling as much as possible. The optimal multitaper configuration has 0.250 in., which is the maximum wall thickness available in the CT market, covering the vertical and the deviated sections of the wells. This is to increase weight in the heel and help push the tubing further in the horizontal.
On the basis of tubing-force simulations, the 2.375-in. CT increased the force transfer of the design, enhancing the downhole axial compressive force at the bottom of the CT. The analysis indicated that this optimized 2.375-in. design will reach the TDs of planned wells with laterals longer than 2500 m and still have sufficient set-down weight to mill isolation plugs. Fig. 2 shows the final configuration of the custom-fit string.
The combination of using a custom-fit CT design, friction-reduction tools and fluid additives, and superior operation techniques had a great effect on developing longer lateral wells in Vaca Muerta economically and efficiently.
Benefits from this exercise include
- Improved reach through optimized CT-string design by tapering wall thicknesses for specific applications
- Understanding the associated friction force eliminated through the use of vibration tools and chemical additives
- Enhanced accuracy of prejob simulations
- Improved understanding of cleanout efficiencies
Custom-Designed Coiled Tubing Leads to Success in Extended-Reach Operations
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