Promoting wellbore stability in active shale formations by water-based muds: A case study in Pabdeh shale, Southwestern Iran

Minimizing water invasion into shales is a critical property for many water-based muds. By improving drilling fluids rheology in HPHT conditions, it is possible to minimize water invasion and maintain true overbalance pressure conditions at near wellbore scale which endorses wellbore stability. In order to propose an inhibitive mud formulation for drilling troublesome swelling Pabdeh formation frequently encountered in southern to southwestern regions of Iran, a comprehensive experimental approach was adopted. After performing rheology and thermal stability tests for 81 different formulations of drilling fluids, 5 high performance salt/polymer solution muds were chosen. They showed good rheology properties and all of them were thermally stable. In order to find the best formulation which accomplishes all requirements pertaining shale stability and wellbore integrity, some complementary analyses were performed. Using a roller oven simulating down hole temperature conditions, physical integrities of shale samples immersed and rolled in these 5 mud formulations were investigated. Pore pressure transmission tests were also carried out using a membrane efficiency screening equipment to evaluate membrane efficiency of the Pabdeh shale and also inhibitive performances of 5 designed mud formulations. Accordingly, pore pressure evolution in shale samples in contact with designed muds were obtained by direct measurements, which resembles the near wellbore pore pressure alterations. According to the obtained results, Pabdeh shale shows negligible membrane efficiency of about 4%. Although all mud formulations show high surface coating capabilities resulting in almost high shale recovery after exposure, pore pressure transmission tests revealed different inhibitive performances. Among them, a combination of polymers and sodium silicate shows promising mud formulation that hinders filtrate invasion into shale samples by either physical or chemical plugging of the shale pore throats. Regarding both environmental and economical aspects of drilling operations, derived mud formulations can also be advantageous for drilling safe and stable wells in Pabdeh formation.