Technology of salt resistance and acid gas pollution resistance drilling fluid for carbonate reservoir in sour gas field

In order to predict the volume of acid gas intruded into drilling fluid for drilling in sour gas field, the acid gas invaded volume and dosage of sulfide scavenger basic zinc carbonate and carbon dioxide scavenger calcium oxide were calculated on-site according to the geological parameters and drilling parameters. To evaluate the impact of acid gas pollution on the drilling fluid performance, an indoor evaluation method "gas perfusion by control cylinder pressure" was designed. And then the required perfusion pressure formulas for infusing different concentration of acid gas were developed according to ideal gas state equation. Using this experiment method we evaluated the influence of acid gas H2S and CO2 on clay-free NaCl brine drilling fluid system and solid-free potassium formate organic salt drilling fluid system that suit for carbonate reservoir of sour gas field. The results show that the two series drilling fluid system have the resistance capacity of 200 mg/L H2S, 1000 mg/L CO2 acid gas pollution, and have good ability of anti-lime stone cuttings, anti-formation brine water and various salt pollution tolerance. In addition, these two systems also have excellent rheological properties, lubricity, anti-corrosion and reservoir protective effects. The two series drilling fluid system can make sure the sour gas field carbonate reservoir safe, fast and efficient drilling.