Temperature- and alkali-resistant induced domestication of Bacillus pasteurii in drilling fluid and its borehole wall enhancement properties

The microbial induced calcium carbonate precipitation (MICP) technology provides a new approach to solve borehole destabilization in broken formations; however, the high-temperature and alkaline environments inhibit the growth of microorganisms, which in turn affects the performance of their wall enhancement performance. In this study, a pH and temperature-coupled induced domestication method was applied to Bacillus pasteurii, and its wall enhancement performance was evaluated. Post domestication, Bacillus pasteurii exhibited high growth activity at pH 10.3 and temperature 45 °C. In a sodium carboxymethyl cellulose (CMC) drilling fluid environment, bacterial concentration reached 1.373 with urease activity at 1.98 after 24 h, and in a xanthan gum (XG) environment, the figures were 0.931 and 1.76, respectively—significantly higher than those before domestication. The Bacillus pasteurii-CMC system exhibited enhanced performance with the unconfined compressive strength of the specimen up to 1.232 MPa, permeability coefficient as low as 0.024, and calcium carbonate production up to 24.685 g. The crushed specimen portions remained lumpy with even calcium carbonate distribution. In contrast, the Bacillus pasteurii-XG system exhibited the highest unconfined compressive strength of 0.561 MPa, lowest permeability coefficient of 0.081, and the greatest calcium carbonate production of 16.03 g, with an externally cemented shell but internally loose structure and uneven calcium carbonate distribution, resulting in weaker mechanical properties. The Bacillus pasteurii induced predominantly vaterite calcium carbonate crystals in the CMC drilling fluid. In the XG drilling fluid, the crystals were mainly calcite. Both types effectively cemented the broken particles, improving formation strength and reducing permeability. However, under the same conditions, the Bacillus pasteurii-CMC system demonstrated a more pronounced enhancement effect.