A Novel Approach to Crosslink Delay of Low-pH Fracturing Fluid

Abstract Carboxylated guar or cellulose fluids are among the most robust fracturing fluids because they crosslink at both high and low pH with transition metal compounds and exhibit tremendous thermal stability. Low-pH fluids are preferred for stimulation treatments using low-quality water or when foamed with CO2 for low-pressure formations. Such systems are often challenged with rapid viscosity development or insufficiently delayed crosslinking, which generates high pipe friction pressure. The ability to effectively control viscosity development in low-pH fracturing fluid will improve the applicability of carboxylated guar- or cellulose-based fracturing fluids. A novel approach is reported in this paper to delay the crosslink of low-pH fracturing fluid using a slow-dissolving, solid organic acid in hydrated polymer containing zirconium crosslinker, to slowly lower the pH of the fluid to the optimum pH where crosslinking occurs. The slow pH reduction results in a slow viscosity build-up (delay) with increasing temperature, without compromising fluid rheology performance. The novel approach will complement existing chemical delay methods. Preliminary results show that crosslink time can be increased by using less organic acid; for instance, decreasing organic loading from 16 to 4 ppt led to crosslink time increase of 97 to 510 s. To further simplify field handling of such organic acid material, the organic acid is slurried in hydrocarbon. Dissolution experiments demonstrate that the slurried acid (1.024-1.710 g/100 mL) dissolves in water more slowly than an approximate amount of solid acid (0.43g/100mL), which offers additional benefits to control delayed crosslinking. This new patent-pending approach to crosslink delay increases the delay options for crosslinked low-pH carboxylated guar and cellulose because the delay can be tailored to suit different well stimulation fluid requirements. The rheology evaluation as a function of varying loading of both solid and slurried organic acid, as well as the rate of pH change are discussed in this paper.