Growth kinetic model of Bacillus pasteurii under different pH and temperature conditions in sodium carboxymethyl cellulose solid-free drilling fluid

Abstract

Increasing the bonding force between broken blocks to form a complete borehole wall is essential to solving borehole wall destabilization in broken strata, and introducing microbial-induced calcite deposition (MICP) technology into the field of drilling fluids establishes a novel research idea for manifesting this technical method. However, during drilling, the pH value and temperature of the drilling fluid constantly change, affecting the activities of microorganisms and the effect of MICP. Therefore, exploring the growth law of microorganisms in drilling fluid environments under different temperatures or pH conditions is necessary. This study used experimental research and theoretical analysis to compare the growth of Bacillus pasteurii in sodium carboxymethyl cellulose (CMC) solid-free and xanthan gum biopolymer solid-free drilling fluid. The growth of B. pasteurii in CMC solid-free drilling fluid in pH = 7.0–9.8 and temperatures from 30 to 50 °C was studied, and the growth kinetic model of B. pasteurii was constructed. The mean square error, decision coefficient (R2), accuracy factor (Af), and deviation factor (Bf) were used to verify the accuracy of the model. CMC solid-free drilling fluid is more suitable for B. pasteurii survival than XG solid-free drilling fluid. The optimum growth temperature of B. pasteurii in CMC solid-free drilling fluid was 32 °C, and the OD600 reached 2.146 at 24 h pH = 8.2 was the optimum pH environment for B. pasteurii in CMC solid-free drilling fluids; the OD600 reached 2.065 at 24 h. As a first-order growth kinetic model, the modified Gompertz model accurately described B. pasteurii growth in CMC solid-free drilling fluid under different temperature strips or pH values. As a secondary growth kinetic model, the square root model accurately described the pattern of pH (7.0–8.2) and temperature effects on the maximum specific growth rate (μmax), with R2 = 0.961, Af = 1.084, Bf = 0.9979, and an error of less than ±0.05. The Arrhenius model accurately described the pattern of pH effects on B. pasteurii (μmax) at pH = 8.6–9.8, with R2 = 0.9996, Af = 1.0036, Bf = 0.999, and an error of less than ±0.02. The effect of temperature was inversely proportional to the growth of B. pasteurii; the higher the temperature, the lesser the B. pasteurii growth. In the pH range of 7.0–8.2, the higher the pH value, the better the B. pasteurii growth; however, at an increased pH range of 8.6–9.8, the higher pH value decreased the B. pasteurii growth. This study is the first to investigate the effect of temperature or pH on B. pasteurii growth in CMC solid-free drilling fluid. Our study provides a scientific basis and theoretical reference for the subsequent research on the induced domestication of B. pasteurii in high-temperature and strong alkaline environments and its engineering application.