Study On Filtration Behavior Of Liquid CO₂ Fracturing

Because of no pollution,good compatibility with formation,increasing the reservoir energy and other advantages,Liquid CO₂ fracturing shows a good application prospect in the fracturing of tight gas,shale gas,coalbed gas and low permeability water sensitive reservoir production.However,the low viscosity of CO₂ and its phase change results in the complex rules,which caused some difficulties in the field applications.In this paper,the problem of filtration in the process of liquid CO₂ dry fracturing is studied,and a set of CO₂ dry fracturing fluid system evaluation devices,which include high pressure pipe flow module and pressure filtration device,have been designed to conduct liquid CO₂ fracturing fluid rheology experiments and filtration evaluation experiments.Through the experiments,rheology of CO₂ under different conditions and the influence of thickener concentration have benn studied.The filtration of CO₂ fracturing fluid under different conditions and filtration coefficients were measured.The effects of several factors on the filtration coefficient were analyzed.Filter loss coefficient calculation formula under different conditions were obtained.Based on the law of conservation of mass,Darcy's law and energy conservation law,the model of filtration in the porous media of CO₂ fracturing fluid was established,and the calculation formula of filtration rate,loss of filtration and filter loss coefficient was deduced.The results show that CO₂ is a Newtonian fluid in liquid state and its viscosity increases with the increase of pressure,while the viscosity decreases with temperature.The viscosity of CO₂ increased obviously after adding thickener,and the thickened CO₂ fluid was a power rate fluid.With the increase of CO₂ temperature,the filtration rate is increased and the filtration coefficient increases.With the increase of pressure,the filtration rate of CO₂ decreases and the filtration coefficient decreases.The filtration coefficient of CO₂ decreased greatly after thickening,and the filtration coefficient of CO₂decreased by 2 orders of magnitude under the condition of saturated water and saturated oil.The results show that the filtration coefficient of CO₂ is greatly affected by the filtrate viscosity,formation permeability and formation fluid compressibility.