Research and Field Trials of Fracturing-Fluid Formulation Technology Using High-Divalent-Ion Flowback Fluid in Deep Coalbed Methane Reservoirs

ABSTRACT Currently, deep coalbed methane (CBM) development in China has entered a rapid production construction phase. CBM fracturing is characterized by ultra-large fluid volume (2,000-3,000 m³ per stage) and ultra-high proppant volume (300-500 m³ per stage). The extensive use of surface water imposes eco-pressure, making the utilization of flowback fluid an essential alternative. However, flowback fluids exhibit high divalent ion concentrations (typically 5,000-50,000 mg/L). This paper developed a salt-resistant dry powder, established a dry powder dissolution-promoting system, designed a field fluid preparation process, and selected a well for flowback fluid-based fracturing. Pre-fracturing flowback fluid samples were collected to measure salinity. During fracturing, fluid viscosity was monitored in real time, and dry powder dosage was dynamically adjusted based on viscosity and fracturing curve trends. Post-fracturing evaluations included flowback fluid viscosity, gas production, and fracturing effectiveness. The well completed four stages with a total fluid volume of 6,889 m³, proppant volume of 1,200 m³, and displacement rate of 16 m³/min. Excellent gel-breaking performance was achieved, with a measured viscosity of 1.21 mPa·s. The salt-resistant dry powder (70-140 mesh) achieved a viscosity of 45 mPa·s at 0.4% concentration, meeting sand-carrying requirements under 25% proppant concentration and achieving a friction reduction rate >70%. Continuous mixing equipment ensured safe and stable operations at 16 m³/min displacement and 0.4% dry powder concentration. Field flowback fluids exhibited significant variability in divalent ion salinity (5,000-11,000 mg/L), necessitating real-time viscosity monitoring and dosage adjustments. At hardness levels >12,000 mg/L, fluid performance deteriorated sharply, with viscosity dropping and sand suspending capacity lost. Adjusting the fresh water/flowback fluid ratio mitigated risks during abnormal salinity spikes. Post-fracturing gas production reached 26,000 m³/day (cumulative 1.9 million m³), slightly lower than offset wells, suggesting potential formation damage requiring further evaluation. The salt-resistant dry powder + continuous mixing system + fluid preparation process preliminarily met field requirements for flowback fluid fracturing under 10,000 mg/L Ca2+/Mg2+hardness. Future work will focus on developing higher-salinity-tolerant dry powders and multi-stage dissolution-maturation equipment.