Vertech Group

_ Casing deformation has become a significant challenge in the development of shale gas reservoirs, particularly in China, the Middle East, and North America. This issue is particularly an issue in unconventional completions, where the combination of geological complexities and operational demands often leads to wellbore-integrity problems. The deformation of casing, often referred to as "casing deformation" or "casing ovality," can severely impact the efficiency of plug-and-perforation operations, leading to increased nonproductive time (NPT) and compromised well productivity. The root causes of casing deformation are multifaceted, involving both geological and engineering factors. Geologically, shale gas reservoirs are characterized by low permeability, high brittleness, and significant natural fracturing. These characteristics make the formations susceptible to deformation under induced stress during hydraulic fracturing operations. The high-pressure injection of fracturing fluids can reactivate natural fractures or faults, leading to shear slippage that causes casing deformation. Additionally, the complex geological structures in deep shale-gas reservoirs, such as those in the Sichuan Basin in China where the case study was executed, further exacerbate the risk of casing deformation due to the higher incidence of faulting and fracturing. From an engineering perspective, wellbore cooling during fracturing operations can significantly reduce the casing's collapse strength, making it more susceptible to deformation. Poor cementing quality can result in inadequate support for the casing, leading to stress concentration and eventual deformation. The design and selection of casing strings also plays a crucial role in mitigating deformation risks. To address these challenges, the industry has been exploring innovative solutions. One such solution is the development of high expansion dissolvable plugs (HEDPs), which are designed to expand significantly while maintaining high pressure ratings. These plugs can navigate through tight spots in deformed casings, providing reliable temporary isolation and reducing the risk of operational failures. Additionally, optimizing pumpdown procedures and using specialized tools like the pumpdown ring can enhance the efficiency of plug deployment and minimize water usage. Design of the HEDP The structural design of the HEDP is engineered to maximize efficiency and effectiveness in challenging downhole environments (Fig. 1). Utilizing high-elongation dissolvable metals, the HEDP features a single slip design, which significantly reduces the overall plug volume. This compact design enhances the plug's ability to navigate tight spots in the wellbore and ensures quicker dissolution post-fracturing, minimizing NPT. The slip teeth are constructed from high-strength, hard ceramic materials, which not only provide anchoring but also facilitate smoother passage during subsequent milling operations compared to traditional alloy teeth, reducing the risk of damage to milling tools. For sealing, the HEDP incorporates either a metal seal or an element seal, depending on the specific application requirements. Once set, the HEDP firmly grips the casing, creating a reliable seal that enhances fracturing efficiency by preventing fluid leakage and ensuring optimal pressure transmission. To address the challenge of low pump efficiency for small-diameter plugs, the HEDP is equipped with a pumpdown ring. This innovative feature assists in the pumpdown operation, saving time and reducing water usage.

_ Over the past few years, dissolvable frac plugs have been effective in addressing unconventional completion challenges by reducing the need for intervention and decreasing overall completion time and cost. However, the majority of dissolvable frac plugs on the market are made up of complex components, which increase their likelihood of failing when run in hole, during stimulation, or not fully dissolving before cleanout. This has resulted in skepticism and challenges with the older generation of dissolvable plugs. To address these issues, Vertechs introduced a new dissolvable collet that can withstand high and low temperatures (100–350°F), has a pressure rating of 10,000 psi, and is less than 2.5 in. in length (Fig. 1). The dissolvable collet combines anchoring and sealing functions into a single component using high-expansion dissolvable metal to create a collet structure with slip segments and a metal seal ring, eliminating the need for elastomers as a sealing material. Compared to traditional frac plug designs, the collet design is simpler and more robust, enabling it to be run and pumped down at higher speed, eliminating issues accompanied with high annular velocities, thereby increasing its running speed in the wellbore. Since it uses less material for manufacturing, it is lighter and easier to dissolve than traditional frac plugs; for instance, the collet only weighs around 1.1 lb, which results in faster cleanout runs, allowing the operators to produce their wells sooner. Many traditional frac plugs weigh around 11–13 lb. Field Test Results An operator in China conducted a field test on a four-well pad in an onshore, southwest shale formation using traditional dissolvable frac plugs in two wells and the new dissolvable collets in the other two. The goal was to speed up completion and minimize coiled tubing operation spending. The results for plug-and-perf and cleanout runs were compared between the two types of plugs, and the new design showed significant improvements. When running conventional frac plugs into the wellbore, the wireline crew adhered to the line speed chart provided by the tool suppliers to minimize the impact of annular velocity on the elastomers. To achieve this, the running speed was maintained within a certain limit. Consequently, the average running time per stage in the vertical section was approximately 50 minutes, while the pumpdown time was about 30 minutes per stage in the horizontal section. In contrast, the dissolvable collets were deployed and pumped down at a higher speed due to their uncomplicated and sturdy structure, leading to an average running time of 32 minutes per stage in the vertical section and 19 minutes per stage in the horizontal section. While it is true that the pumpdown time in the horizontal section decreases as the stages move from the toe to the heel, the average time per stage is used for easy comprehension and visualization. When combined with other aspects of the plug-and-perf process, such as setting the plug, perforating, and pull out of hole, it became apparent that significant time savings were achieved during the running and pumpdown phases in both the vertical and horizontal sections of the well. This resulted in a 15% reduction in time spent per stage (Fig. 2).