Persistent CO 2 intrusion during drilling operations in deep carbonate formations frequently leads to excessive thickening of water-based drilling fluids, resulting in significantly increased pump pressure and posing substantial well control risks. Herein, based on field contamination incident tracking and high-temperature high-pressure CO 2 intrusion simulation experiments, the generation and accumulation characteristics of CO 2 -derived contaminants (CO 3 2- and HCO 3 - ) were thoroughly analyzed. The impact of contaminants on the rheological properties of water-based drilling fluids was investigated using a Haake rheometer, a six-speed viscometer, and the glass rod method. The mechanisms of CO 2 -induced thickening in the drilling fluid were elucidated through a combination of zeta potential measurements, particle size distribution analysis, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), environmental scanning electron microscopy (ESEM) and energy-dispersive spectroscopy (EDS). The results indicate that CO 3 2- accumulation is the primary cause of drilling fluid thickening, driven most rapidly through pH adjustments (using sodium hydroxide) back to ∼11 from 10–10.5 following each CO 2 intrusion. The adsorption of CO 3 2- onto bentonite leads to "feathering" and "fine-grainization" transformations within its crystal structure. This adsorption enhances the bonding forces between bentonite particles and water molecules via hydrogen bonds and ion-dipole interactions, which outweigh the repulsive forces between the negatively charged particles. As a result, bentonite particles aggregate and adhere more readily, fundamentally driving the excessive thickening of the drilling fluid, particularly characterized by a significant increase in gel strength. This work provides a theoretical foundation for optimizing drilling fluid formulations against CO 2 contamination through pH management and selective ion control. • CO 3 2- -dominant Mode I causes aberrant thickening of drilling fluids, contrasting with negligible effects in HCO 3 - -rich Mode IV. • CO 3 2- and HCO 3 - generate via NaOH-CO 2 neutralization and accumulate via cyclic pH adjustment. • CO 3 2- /HCO 3 - ratio depend on the pH value of drilling fluids after CO 2 intrusion. • CO 3 2- adsorption triggers bentonite fragmentation and ordered aggregation and therefore causes viscosity surge. • Carbonate-mediated anion adsorption drives particle aggregation despite increased negative surface charges, contradicting classical electrostatic principles
Ren et al. (Sun,) studied this question.