Study On Flow Law Of Water-based Drilling Fluid Considering The Dissolution Of CO₂

During the drilling of gas fields containing acid gases,due to the‘cealment'of acid natural gas invasion into the wellbore and the‘burst'of late expansion,it is easy to cause well control accidents.Therefore,understanding the law of solubility of acid gas in drilling fluid and the law of multiphase flow under drilling gas invasion are of great significance for gas field development.In this paper,the molecular simulation software Materials Studio7.0 was used to simulate the dissolution mechanism of CO₂ in water-based drilling fluid.By analyzing the effects of temperature and pressure on the free volume fraction,solubility coefficient and solubility parameters,the factors affecting the solubility of CO₂ in the drilling fluid are revealed.Then,the self-developed gas-liquid phase equilibrium experimental measuring device was used to measure the solubility of CO₂ in pure water,brine and drilling fluid under the temperature of20-75°C and 5-40 MPa.The effect of pressure,salt content and tackifier content on solubility was analyzed.At the same time,based on the gas-liquid equilibrium,the chemical potential of the gas in the liquid phase and the gas phase is equal.Established a calculation model for the solubility of CO₂ pure gas and CO₂ mixed gas in water and brine.Combined with the corresponding auxiliary equations,the multi-phase flow continuity equation and momentum equation considering gas dissolution under overflow conditions are established.The results show that the free volume fraction,solubility coefficient and solubility parameter of CO₂ gas in drilling fluid show a trend of increasing with the decrease of temperature and pressure,and gradually decrease with the increase of temperature and pressure.The experimental results show that the change law of CO₂in the drilling fluid with temperature and pressure is the same as the change of free volume fraction,solubility coefficient and solubility parameter with temperature and pressure,so the free volume fraction,solubility coefficient and solubility parameter can be used as microscopic factors to determine the solubility..Also found that with increasing salt content and thickener content,CO₂ solubility decreases in the drilling fluid.At the same time,the solubility model of CO₂pure gas and CO₂-containing methane mixed gas established in water and brine is established by the principle of gas-liquid phase equilibrium.It can better predict the solubility of CO₂pure gas and CO₂-containing methane mixture in water and brine.The relative average deviation of the prediction of CO₂ gas dissolution is within 10%.Based on the model of this paper,the calculation results show that if the influence of gas dissolution on the multiphase flow law is considered,it is found that the gas invasion time is later,the‘hiddenness'is higher,and the safety control of the wellbore pressure is more harmful.