Application and mechanism of microbial drilling fluid for wall stabilization and core protection in deep overburden drilling

Abstract:

To address technical challenges such as borehole wall instability and low core recovery in deep overburden drilling， this paper constructs two microbial drilling fluid systems suitable for wire-line coring-sodium carboxymethyl cellulose （CMC） and biopolymer （XC）-based systems—using Bacillus pasteurii as the core microorganism， based on microbially induced carbonate precipitation （MICP） technology and by comprehensively balancing microbial growth characteristics with drilling fluid rheological requirements. Engineering application tests were conducted in deep overburden coring for a major hydropower exploration project in western China， achieving a total footage of 258.75 m. Concurrently， the mechanism of wall stabilization and core protection of microbial drilling fluids was investigated through X-ray diffraction （XRD） and scanning electron microscope （SEM） analyses of core sample surfaces and interiors. The results indicate that a CMC dosage of 0.8% and an XC dosage of 0.3% ensure good microbial growth while meeting the rheological requirements for drilling fluid in the drilling process. The optimized formula is determined as： 0.8% CMC/0.3% XC + Bacillus pasteurii （OD₆₀₀=0.8） + 0.5% NaCl + 2% urea + 2% tryptone. Both microbial drilling fluids exhibit excellent rheological properties and biological activity. Field applications demonstrated effective borehole wall stability maintenance， with core recovery rates exceeding 90%， enabling long-section open-hole drilling in deep overburden and significantly simplifying the borehole structure. The microorganisms in the drilling fluid induced calcium carbonate crystal formation （e.g.， vaterite， calcite） on core surfaces and around borehole walls. These crystals deposited on the surfaces of loose overburden particles and within pores， effectively cementing the loose particles to form a dense protective layer， thereby achieving wall stabilization and core protection. This study provides a novel solution for technical problems such as borehole wall instability and low core recovery in deep overburden core drilling， holding significant engineering application value for improving the quality of deep overburden drilling and exploration.