Development and evaluation of high-salinity aqueous fracturing fluids for deep coalbed methane reservoirs

The development of high-salinity aqueous fracturing fluids is crucial for the efficient exploitation of deep coalbed methane (CBM) reservoirs, where conventional fracturing fluids often fail due to high salinity, divalent ion content, and harsh reservoir conditions. This study synthesized a novel zwitterionic copolymer, Poly (AM-co-AMPS-co-HADB-co-DADB) (PAAHD), via solution polymerization, incorporating acrylamide (AM), 2-acrylamido-2-methylpropanesulfonic acid (AMPS), a cationic monomer (HADB), and a hydrophobic monomer (DADB) to enhance salt tolerance through synergistic charge shielding, hydrophobic association, and steric hindrance effects. The optimal synthesis conditions (55°C, 30% monomer concentration, 1.5% initiator, 1% urea, and monomer ratio AM:AMPS:HADB = 4:1:2 with 0.15% DADB) yielded a polymer with a viscosity-average molecular weight of 7.01 × 10⁶ g/mol. PAAHD demonstrated exceptional salt resistance, maintaining stable apparent viscosity (50.6–100.1 mPa·s after 120 min at 100°C and 170 s⁻¹) in simulated formation waters (TDS: 41,640–173,670 mg/L). Rheological tests confirmed elastic-dominated behavior (tan δ < 1) and high drag reduction (>75% at 0.1 wt%). Proppant suspension tests revealed low settling velocities (<1.8 × 10⁻³m/s) across 40–90°C. Gel-breaking with ammonium persulfate (300 ppm) achieved low residual viscosity (<2 mPa·s) and minimal formation damage (<10%), outperforming guar gum-based fluids (20% damage). These results validate PAAHD as a high-performance fracturing fluid for deep CBM reservoirs, combining environmental benefits (produced water reuse) with superior thermal-shear stability and proppant transport.