Frac plugs remain the most common zonal isolation tool in multistage stimulation. However, while manufacturers in recent past have focused on improving frac plug drillability and dissolvability, the industry critically overlooked significant plug design flaws. Among these flaws is mechanical damage to horizontal well casing caused by inadequate plug construction, poor isolation integrity, and cycled pressure stress on the pipe throughout the entire fracture treatment. Until recently, casing integrity issues had never even been associated with frac plugs.
Today, significant improvements in ultrasonic wellbore diagnostics, downhole cameras, and fiber optic well monitoring visualizing post-frac casing integrity have revealed widespread casing breaches at plug setting depths. These recently discovered flaws called for a frac plug with non-damaging slips and optimized packing element systems for improved zonal isolation. Three-dimensional finite element analysis was performed to understand casing damage caused by various slip designs currently being utilized in the market. Extensive laboratory tests were conducted on slip and packing element designs to further support and validate the numerical study. Finally, fiber-optic diagnostic data was used to confirm interstage isolation efficiency by a non-damaging frac plug.
This paper will review findings that some slips, under certain scenarios, cause severe casing wall damage. It will also review findings that some packing element designs provide inadequate isolation during the fracture–posing considerable challenges to casing integrity. Based on extensive numerical analyses, laboratory tests, and field trials, a frac plug with non-damaging slips, as well as anti-extrusion packing element systems, limit the frac plug's impact on the casing during high fracture pressure treatments.