Various methods are used to overcome choking effects in propped fractures to enhance and maintain well productivity (particularly in low-permeability reservoirs). Choking effects can result from permeability damage caused by frac gel residue, proppant crushing from high closure stresses or use of low-quality frac sand, or embedment/intrusion of formation materials into the proppant pack. This paper describes development and field applications of a new well stimulation method for generating stable and highly conductive channels within a propped fracture to maximize transport capability of hydrocarbons from the formation reservoir to the wellbore.
Both extensive laboratory experiments and yard tests were performed to evaluate the formation and stability of proppant aggregates and proppant-free channels. Proppant-laden slurry (prepared by mixing frac sand coated with an agglomerating agent in a gel fluid) and crosslinkable proppant-free spacer fluid were pulsed intermittently to form proppant aggregate masses surrounded by proppant-free gel slugs. Highly conductive channels were formed surrounding the proppant aggregates after crosslinked gel slugs were broken and removed from the propped fracture, leaving behind proppant aggregate masses, supporting the closed fracture.
Field trial testing was performed in unconventional and conventional oil formations. Injecting pressures of proppant-laden slurry and proppant-free spacer using the pulsing approach were found to be significantly lower than those applied with conventional hydraulic fracturing treatments, indicating this new method helps alleviate the risks of screenout, as the proppant-free spacer sweeps and mitigates the proppant buildup in the near wellbore area. Field results showed production in wells treated with the pulsing method increased significantly compared to those of offset wells in which a conventional approach was taken. Additionally, 40% less total proppant was used when the pulsing method was applied.
Potential applications for this new stimulation method include the use of small-sized and low-quality sand. As long as the proppant aggregates remain stable, low-quality sands, such as desert or beach sand can be used. In addition, the operators now drill several horizontal wells from the same pad to target a shale play and perform multistage fracturing treatments in each well. The new method allows the operators to economically reduce the amount of proppant (and water) required as the number of fracturing treatments increases while, at the same time, minimizing the environmental footprint.