The Effect of Casing Ovality on Fracture Plug Sealing Element Performance

Abstract

Summary Sealing elements (SEs) of fracture plugs have crucial roles to isolate target zones of a well in hydraulic fracturing. If the zonal isolation by the SE is not adequate, it can result in erosion of the casing. To the best of the authors’ knowledge, the effect of casing deformation on sealing performance is not well researched or understood. To study the effect of casing deformation on sealing performance, finite element analysis (FEA) of SEs in oval casings was conducted in this study. Finite element simulation of a degradable fracture plug with three different casings ovalities (0%, 2%, and 5%) and three different SE designs (O-ring type, short type, and traditional long type) was conducted to evaluate deformation behavior and sealing performance of SEs in deformed casings. Contact pressure (CPRESS) on the casing by the SE after the plug was set in the casing and the risk of leakage were discussed and compared for each design. In the casing with 0% ovality, all the SE designs established contact with the inner surface of the casing when setting force was applied. However, for the O-ring-type design, the area in contact with the casing was small and it may result in leak and erosion in the actual well if there is a small dent or deformation on the casing. When there is ovality in the casing, the minor inside diameter (ID) has a smaller ID and the major ID has a larger ID compared to the nominal ID of the casing. In the casing with 2% and 5% ovality, neither O-ring-type SE (O-SE) nor short-type SE (S-SE) could contact the major ID of the casing and there was a gap between the inner surface of the casing and the SE. This gap can cause erosion of the fracture plug and casing when the fluid passes through the gap. In contrast, the traditional long-type SE (L-SE) contacted both major and minor IDs of the casing, and no gap was observed. This result indicates that there is a potential risk of insufficient isolation of target zones and erosion of casings in actual well conditions if fracture plugs with S-SEs are used. Because there are various types of fracture plugs with different SE designs, this study helps to select proper fracture plugs with good SE design and mitigate the risk of erosion of casings and plugs. As this study is based on FEA simulations, future demonstrations through experiments and field trials are needed.