Case Study: High-Expansion Dissolvable Plug for Casing-Deformation Challenges in Plug-and-Perf Operations

_ 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.