As a natural organic macromolecular weak acid, humic acid has a molecular structure centered on aromatic nuclei and is connected with aliphatic cyclic structures with multiple groups such as phenolic hydroxyl groups, alcoholic hydroxyl groups, carboxyl groups, enol groups, and methoxy groups. This molecular structure determines the colloidal property, weak acidity, adsorptive property, and complexing property of the modified organic lignite, enabling it to interact with organic and inorganic substances.
In the initial stage of filtration loss of oil-based drilling fluids, fine particles bridge and plug in the wellbore to form a thin mud cake. Subsequently, emulsion droplets deform under the pressure difference to seal the pores between solid particles. Finally, colloids such as filtration loss reducers dissolved in the oil-based drilling fluids fill the areas between solids and emulsion droplets. The combined action of these three aspects achieves the effect of reducing filtration loss. Therefore, as an oil-soluble humic acid filtration loss reducer, it forms a bridge plug through adsorption and cross-linking interaction with clay particles during the instantaneous filtration loss of the drilling fluid. Meanwhile, the lipophilic humic acid colloid will form an oil film on the surface of the mud cake. The oil film forms a liquid resistance effect on the surface of the mud cake, preventing the filtration loss of oil. For modified humic acid particles with the same lipophilic ability, the smaller the particle size, the larger the proportion of the space occupied by the formed oil film, the narrower the actual passage of the filtrate, the greater the resistance, and the lower the permeability of the formed mud cake. Tests have proved that the minimum permeability of the oil-based drilling mud cake can only be obtained when there is an excessive amount of wide and narrow particle sizes in a certain proportion, and it cannot be obtained when the sizes are in a linear relationship. Therefore, the uniform grading of the modified humic acid particle size is particularly important for the filtration loss reducer of oil-based drilling fluids.
Organic lignite also has the effects of assisting emulsification and reducing filtration loss.
The loose sponge-like structure of humic acid plays a good inclusive role in adsorbing small molecules, free groups, and electrically charged particles in the solution. Coupled with the steric hindrance effect formed by large polar groups, the adsorption effect under the inclusive state is further strengthened, promoting the stability of the drilling fluid system. Test results show that the modified humic acid filtration loss reducer has the functions of assisting emulsification and stabilizing the oil-based drilling fluid system.
Through particle size detection, it is proved that the long-chain alkyl chains of the modified humic acid particles can fully extend in white oil to form a steric hindrance layer, greatly reducing the internal cohesion among particles and weakening the agglomeration tendency, thereby improving the dispersibility of the modified humic acid in the oil phase. Scanning electron microscopy further indicates that the multi-stage dispersed modified humic acid particles help improve the compactness of the mud cake. Meanwhile, the demulsification voltage test shows that the modified humic acid has the functions of assisting emulsification and enhancing the stability of the oil-based drilling fluid system. Through the above actions, the modified humic acid filtration loss reducer plays a good role in reducing filtration loss in oil-based drilling fluids.
