Study on high-temperature low-cost brine drilling fluid for deep coal measure strata

Abstract:

To address the drilling challenges of insufficient thermal stability and salt resistance as well as high costs of treatment agents in deep coal measure strata， a brine drilling fluid base slurry was prepared using bentonite， NaCl， Na?CO?， and NaOH as raw materials. HCalvis-5， HCalvis-4， S092， cationic starch， and HE-150-were selected to investigate the effects of additive type and concentration on the rheological properties and fluid loss of the drilling fluid. The overall system performance was optimized by compounding antioxidants of different types and dosages， and the optimal formulation was determined. Fourier transform infrared spectroscopy （FT-IR） and Zeta potential analysis were employed to elucidate the synergistic mechanism of the treatment agents. The results indicate that the optimal formulation is "base slurry + 2.0% HCalvis-5 + 1.0% Na?SO?，" with an optimal application temperature of 120 °C. After hot rolling at 120 °C for 16 h， the drilling fluid exhibited an apparent viscosity of 27 mPa·s and an API filtrate loss of only 8.0 mL. The 24 h linear swelling ratio of bentonite was merely 1.67%， and the high-temperature rolling recovery rate reached 58.64%， demonstrating excellent high-temperature resistance， hydration inhibition， and filtration control. Microscopic mechanism analysis reveals that HCalvis-5 adsorbs onto and encapsulates clay particles via hydrogen bonding， forming a dense filter cake， while Na?SO? scavenges high-temperature free radicals to suppress oxidative degradation of polymers. The synergistic action of the two components ensures the high-temperature stability of the system. Featuring low raw material costs and a simple preparation process， this formulation can effectively meet the engineering requirements for drilling in deep coal measure strata， thereby providing experimental evidence and theoretical support for further optimization and upgrading of drilling fluid systems for higher-temperature ultradeep formations.