Effect of cationic copolyelectrolyte additives on drilling fluids for shales

Wellbore instability remains one of the major challenges in terms of both time and economy during drilling. The issue is more pronounced in reactive shale formations. A set of polytertiary amines (PTA)-ran-polyquaternary amines (PQA) polymers possessing different ratios of tertiary amine to quaternary amine (0:100, 40:60, 60:40, mol:mol) segments were synthesized and their efficacy towards the wellbore stability of shale reservoirs were ascertained. When added in similar weight proportions, the copolymer possessing 60:40 (mol:mol) ratio of tertiary to quaternary amine moieties (PTA-ran-PQA-64) enhanced the apparent (≥58%) and plastic (≥71%) viscosities of silicate drilling fluid to maximum extent. The amount of API filtrate reduced with increasing concentration of the copolymers and filtrate loss in case of the mud containing polyelectrolyte PTA-ran-PQA-64 was lower compared to the other two polyelectrolytes. Hot rolling shale dispersion test showed that the shale recovery improved from 91 to 97% on inclusion of minimum amounts (0.25 wt%) of these polyelectrolytes in the drilling fluid. The Linear Swell Meter (LSM) data demonstrated, the swelling in shale pallet is 11% in absence of polyelectrolytes and gets reduced to 4.3% in presence of PTA-ran-PQA-64. The above studies revealed that, these copolymers are viable candidates as drilling fluid additives to possibly enhance the wellbore stability.