Laboratory Characterization of Gel Filter Cake and Development of Nonoxidizing Gel Breakers for Zirconium-Crosslinked Fracturing Fluids

Summary Borate or Group 4 metal-crosslinked biopolymer fluids constitute the bulk of gelled-fracturing-fluid volumes used in field-fracture stimulation operations to date. Oxidizing chemicals remain the primary gel breakers to break down the filter cake and establish regained formation permeability or fracture conductivity. These breakers presumably oxidize the polymer backbone, causing chain breakdown and facilitating fluid flowback. Safety hazards associated with the use of bulk quantities are one of the many potential problems associated with the use of oxidizing agents as gel breakers. Proper timing of gel breakdown is of utmost importance with respect to oxidizing gel breakers. Conversely, premature polymer oxidation can cause premature proppant settling. On the other hand, a long delay between the completion of the fracturing operation and activation of the gel breaker could cause the deposition of additional filter cake on fracture faces, which could be difficult to break down. A desirable advancement of designing improved fracturing fluids is to develop nonoxidizing gel breakers that focus on decrosslinking the crosslinked gel structure by reacting with the crosslinking agent, rather than only breaking down the polymer chain. In addition, by insolubilizing such breakers, they can be embedded in the filter cake and, when activated, they can degrade the filter cake completely without leaving any residues. In this paper, synthetic disks of different porosities were used to simulate formation-permeability damage caused by filter cakes formed by Zr-crosslinked, carboxylated guar-based gels, and the results were compared with those measured for uncrosslinked fluids. The filter cakes were formed under different applied-pressure regimes and characterized for their polymer content and filter-cake/filtrate-weight ratios. From flow-rate measurements, it was clear that uncrosslinked base polymer solutions did not cause any permeability damage. Uncrosslinked polymer fluids produced from Zr-crosslinked gels by use of decomplexing, nonoxidizer breakers also did not significantly damage disk permeability compared with the fluids produced from conventional, oxidizer-type breakers. Several ortho-disubstituted aromatic compounds containing polar heteroatoms capable of functioning as potentially strong ligands for zirconium were found to be suitable as decomplexing breakers. Biopolymers containing such functional groups, notably lignosulfonate derivatives, show promise as commercially viable alternatives to oxidizing breakers for metal-crosslinked fracturing fluids.