Near-wellbore diversion of fracturing fluid and proppant is a common objective when refracturing horizontal wells for expanding treatment coverage within the lateral. There are five broad categories of near-wellbore diversion methods: (1) particulate diversion for bridging open fractures connected to the wellbore, (2) perforation sealing to limit injectivity into open perforation clusters at the wellbore, (3) filling up the drained fracture system with water for achieving more uniform pressurization (i.e., fill-up), (4) injection rate cycling/hesitation fracturing and (5) mechanical isolation by installing cemented or expandable liners in the lateral followed by plug and perf stimulation. These tactics can be used in isolation or combined. Particulate diverting agents can be additionally categorized by particle type (e.g., granular, fibrous) and solubility characteristics. Perforation sealing agents consist of deformable and rigid/spherical subtypes, both of which can be further categorized by solubility characteristics.
In this study, treatment and production data for 72 company-operated refractured wells in a North America shale play were analyzed to evaluate the effectiveness of the various near-wellbore diversion methods and materials. An index was formulated using information on reservoir depletion to normalize changes in bottom hole fracture pressure over time. This was determined by periodically discontinuing injection to obtain instantaneous shut in pressures (ISIP’s) over the course of the treatment. The calculated indices were plotted for each type of diverting system to compare trends for gaining insight on in-situ stress buildup. Production data grouped by different diversion methods were also analyzed. The near-wellbore diversion methods included mixed-size particulates with and without fibrous materials, deformable and rigid perforation sealers, fill-up tactics in which near-wellbore diverting agents were not utilized and mechanical isolation by cementing a newly installed liner in the lateral followed by plug and perf stimulation.
Frac hit analysis of offset well treatments indicated that refracturing treatments using particulate diverters were heel biased with respect to reservoir re-pressurization. The study showed that the incremental pressure as a result of diverter landing on perforations is a poor indication of diverter efficiency. Non-normalized ISIP trend is misleading as an indicator for post-refracturing well performance. Refractured wells with either particulate diverters or perforation sealers both show initial fluid fill-up into the depleted region before the stress buildup plateaus. Wells that have liners installed and cemented inside the original wellbore and that are then re-stimulated with standard plug-and-perf techniques show superior performance compared to all other diversion methods. Choice of diversion can have a significant impact on results, but not all particulate diverters or perforation sealers behave similarly. Wells refractured using only the fill-up method have long term productivity on par with or better than wells refractured with most types of diverting agents.