Plug-and-perf multi-stage hydraulic fracturing completions in unconventional reservoirs rely upon complete hydraulic isolation from the previous stage to ensure effective treatment of the active stage. Failure to isolate stages can be a result of partially set plugs, plugs set in wellbore debris or deformed casing, unqualified pressure/temperature rating of plugs, etc. This paper presents a case study with field examples where unexpected casing erosion occurred at the setting depths of the dissolvable frac plugs during hydraulic fracturing and subsequently resulted in loss of inter-stage isolation.
A 12-well, 4-layer, cube pilot was designed with permanent fiber optic cable to collect Distributed Acoustic Sensing (DAS), Distributed Temperature Sensing (DTS) and Distributed Strain Sensing (DSS) data as well as downhole pressure gauges for development insight and future completion optimization. The cable was mapped, and oriented perforation techniques placed entry holes opposite the fiber along the wellbore, and no loss of communication was observed during perforating operations. However, fiber optic signal was lost during hydraulic fracturing operations on one or more stages in all four instrumented horizontal wells. Real-time DAS/DTS analysis indicated the fiber breaks were consistently occurring below the lowermost perforation cluster in the stage, at or very near the frac plug setting depth. Step-down tests were also performed and showed significantly enlarged effective treating area. Based on this observation, post-frac downhole imaging tools were deployed to investigate potential casing and perforation erosion.
Downhole imaging data clearly showed the casing was severely eroded at several locations. Additional interrogation of the damage with respect to plug design components indicated that damage always occurred near the plug sealing element. By integrating the analysis of DAS/DTS, step-down tests and ultrasonic imaging, it was determined that the frac plug bypass is creating a loss of casing integrity at the plug set location. Casing integrity loss resulted in multiple fiber optic cable breaks and lowered the ability to evenly distribute slurry into treatment clusters. Fiber optic data analysis showed that 50% of the larger OD dissolvable frac plugs had bypass compared to 100% bypass for the smaller OD high-expansion dissolvable plugs.
To establish key patterns and identify critical variables that influence stimulation effectiveness, it is important to obtain several different diagnostic datasets and perform an integrated evaluation using all available information. This study also reinforces the need for operators and manufacturers to work together to design and qualify frac plugs against realistic downhole conditions, particularly in areas with potential casing deformation issues. Industry innovation is required to enable fracturing operations to continue through deformed casing. This includes advancing equipment, tools, and techniques for plug-and-perf and other multi-stage completion methods.