Synthesis, characterization, and performance of carboxymethyl chitosan with different molecular weight as additive in water-based drilling fluid

With increasingly intensified energy demand and environmental pressure, it has been of great interest to develop high-performance and environmentally friendly drilling fluid additives, and natural-derived biopolymers are promising candidates. In this work, carboxymethyl chitosan (CM-CS) with different molecular weight , namely high- (CM-HMWCS), medium- (CM-MMWCS1, CM-MMWCS2) and low-molecular-weight (CM-LMWCS) CM-CS, were prepared and the structural factors including molecular weight (MW), degree of deacetylation (DD) and degree of substitution (DS) were determined. The impact of different CM-CS on rheological, filtration and inhibition properties of bentonite-based drilling fluid was evaluated, as well as the bentonite-polymer interactions. The experimental results showed that the lower the MW the higher the DS, while the DD seemed unaffected during reactions. All obtained CM-CS could effectively reduce the yield point of bentonite suspension before and after aging at 135 °C and 150 °C for 16 h, and the lower MW ones had better yield point-reducing effect and showed little influence on plastic viscosity. Besides, different CM-CS possessed the ability to control filtrate loss as well, which showed a high dependence on MW, the CM-HMWCS had the best filtration reduction performance. Meanwhile, all CM-CS had better inhibition properties to prevent clay swelling and shale disintegration than conventional inhibitors, in which the CM-MMWCS2 with medium MW and relatively higher DS exhibited the best inhibition. Therefore, the obtained different CM-CS was versatile and could perform thinning, filtration reduction and shale inhibition properties. These properties were dominated by their amphoteric molecular structures, and also related to the molecular weight.