10 / 2021-08-15 11:32:11

Study on multiphase flow law of high CO2 gas invading wellbore

Carbon dioxide; Gas invasion; Equation of state; Volume fraction; Pit Gain

During the drilling and development of high CO₂ gas field, due to the high solubility of CO₂ containing natural gas in drilling fluid, the gas invasion has "concealment" and "sudden" of later expansion, resulting in well control accidents. Therefore, it is of great significance to study the solubility of high CO₂ gas and the law of multiphase flow in annulus after gas invasion. In this paper, the multiphase flow equations considering gas dissolution are established. The solubility of CO₂ + CH₄ in aqueous solution is measured by phase equilibrium experimental device. According to the experimental results, the auxiliary equation suitable for the solubility prediction of CO₂ containing natural gas is optimized. It is found that the fugacity activity method using SRK equation of state is the best. By analyzing the law of multiphase flow through specific examples, it is found that considering the influence of gas solubility, the increment of mud pit changes slowly, the expansion and precipitation of intrusive gas is close to the wellhead, and the expansion burst is stronger. When the CO₂ content of intrusive gas is higher, the time of incremental change of mud pit monitored on the ground is prolonged, and the concealment is further increased, which is more harmful to the safety control of wellbore pressure.