Controlling Strontium Scaling in the Permian Basin through Manipulation of Base Fluid Chemistry and Additives

Precipitation of secondary mineral species during stimulation is a well-recognized problem in the industry. This problem is exacerbated when “clean” brine derived from the repurposing of produced water is used as a base fluid in hydraulic fracturing fluid (HFF) formulations. Sulfate mineral scaling within the fracture spaces of the stimulated rock volume in particular is a well-known problem in combination with alkaline earth metals such as strontium (Sr), especially in the Permian Basin. To mitigate the impact of Sr mineral scaling, we address two areas of uncertainty: 1) understanding the thermodynamics/kinetics of precipitation under various ionic strengths and with/without HFF additives, and 2) determining if Sr can be removed from clean brines or whether adjustments to HFF additives can provide a mitigation pathway. Precipitation studies were conducted under different solution conditions (0 or 2 M NaCl, with and without various HFF chemicals). Solutions were combined to give different celestite saturation indexes (SI<1.0, SI≅1.0, or SI>1.0) and mixed at 80 °C for 4 days. The concentration of dissolved Sr was measured at different time points by inductively-coupled-plasma mass-spectrometry. Celestite precipitation readily occurred under all scenarios within 12 to 24 h. Results show that celestite (SrSO₄) was the only Sr form that precipitated during these experiments. In an attempt to determine if cost-effective removal of Sr from clean brines for reuse is possible, several water treatment options were explored: Sr removal via the addition of calcium and barium sulfate (to precipitate/sorb Sr from solution), and the addition of illite (to sorb Sr from solution). The reaction of Sr-enriched produced water with Ca-sulfate minerals only showed a small reduction in dissolved Sr. The addition of illite also had no effect on dissolved Sr concentrations due to the high ionic strength of the solution inhibiting sorption. Overall extents of Sr reduction were not found to be cost-effective. An alternative approach to mitigating Sr mineral scaling considers whether additives, primarily persulfate, need to be adjusted/removed from HFF. The presence of the oxidizing breaker (ammonium persulfate) resulted in 2-3 times more Sr precipitation, depending on the solution ionic strength. These experimental results suggest that lowering sulfate rather than alkaline earth metals is the most impactful mitigation strategy for successful reuse of produced water in future hydraulic fracturing jobs.