DOI: https://doi.org/10.53766/CEI/2021.43.02.06In the present work, composites based on petroleum coke (shot petcoke) and unmodified lignites as high pressure and high temperature fluid loss control additives (HPHT) in oil-based drilling fluids were synthesized and evaluated. Several petcoke composites were synthesized with coke content among 48 wt% and 85 wt%. Petroleum coke composites with lignites controlled the fluid-loss better than organophilic lignite. Petcoke composite with leonardite (a type of lignite) (FPC-L) was that showed the smaller fluid-loss (6.8 mL) in organophilic lignite (FOL) comparison (5.7 mL), because of colloidal lignite (fouling) helps plug off the permeable parts of filter-cake. Applying the reverse osmosis filtration models (Hermia’s models); the blocking mechanisms that occurred most probably were found. FOL fluid-loss control mechanism is by filter-cake formation, while FPC-L is by filter-cake fouling. Petcoke composites controlling fluid-loss by three mechanisms colloidal fouling of the cake filtration: (i) intermediate blocking, (ii) standard blocking and (iii) complete blocking. Colloidal lignite is a determinant factor in the fouling of pore volume and permeability the filter-cake. Cake filtration permeability was estimated by 1H-NMR. Use lignite-petcoke composites as fluid-loss control additive of lower environment impact for oil-based drilling fluids.  In the present work, composites based on petroleum coke (shot petcoke) and unmodified lignites as high pressure and high temperature fluid loss control additives (HPHT) in oil-based drilling fluids were synthesized and evaluated. Several petcoke composites were synthesized with coke content among 48 wt% and 85 wt%. Petroleum coke composites with lignites controlled the fluid-loss better than organophilic lignite. Petcoke composite with leonardite (a type of lignite) (FPC-L) was that showed the smaller fluid-loss (6.8 mL) in organophilic lignite (FOL) comparison (5.7 mL), because of colloidal lignite (fouling) helps plug off the permeable parts of filter-cake. Applying the reverse osmosis filtration models (Hermia’s models); the blocking mechanisms that occurred most probably were found. FOL fluid-loss control mechanism is by filter-cake formation, while FPC-L is by filter-cake fouling. Petcoke composites controlling fluid-loss by three mechanisms colloidal fouling of the cake filtration: (i) intermediate blocking, (ii) standard blocking and (iii) complete blocking. Colloidal lignite is a determinant factor in the fouling of pore volume and permeability the filter-cake. Cake filtration permeability was estimated by 1H-NMR. Use lignite-petcoke composites as fluid-loss control additive of lower environment impact for oil-based drilling fluids.