The Effect of Casing Deformation on Frac Plug Sealing Element Performance

Abstract

Sealing element of frac plugs have crucial roles to isolate target zones of the well in hydraulic fracturing. If the zonal isolation by the sealing element is not adequate, it can result in erosion of the casing. The effect of casing deformation on sealing performance of sealing element is not well researched or understood. To study the effect of casing deformation on sealing performance, finite element analysis of sealing element in deformed casing was conducted in this study to assess the effect of casing deformation on sealing performance. In this study, finite element simulation of a full frac plug with three different casings ovalities (0%, 2%, and 5%) and three different sealing element designs (O-ring type, short type, traditional long type) was conducted to evaluate deformation behavior and sealing performance of the sealing elements in the deformed casings. Compression pressure on the casing by sealing element after the plug is set in the casing and the risk of leak were discussed and compared for each design. In the casing with 0% ovality, all the sealing element designs established contact with inner surface of the casing when setting force is applied. However, for the O-ring type design, 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 deformation and ovality in the casing, the minor ID has a smaller ID and a major ID has a larger ID compared to nominal ID of the casing. In the casing with 2% and 5% ovality, neither O-ring type nor short type sealing element could contact the major ID of the casing and there was a gap between inner surface of the casing and the sealing element. This gap can cause erosion of the frac plugs and casing when fluid passes through the gap. In contrast, traditional long type sealing element 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 condition if frac plugs with small sealing element is used. Since there are various types of frac plugs with different sealing element designs, this study helps to select proper frac plugs with good sealing element design and mitigate the risk of erosion of casings and plugs.