Study On Displacement Mixing Length Of Cementing In Oil And Gas Wells

A large amount of calcium ions will be produced during the hydration process of the cement.When these calcium ions were mixed with the drilling mud,a viscous mass-like flocculating material will be easily formed,which will result in displacing of the cement slurry not in position,reducing of cement stone compressive strength and cementing strength,these will seriously affect cementing quality.In order to avoid contact between the cement slurry and the drilling mud,a certain amount of spacer fluid needs to be pumped between the cement slurry and the drilling mud during cementing.At present the method for calculating the dosage of spacer fluid mainly determined by experience,and the influence of the mixing length of the two-phase flow in displacement is not considered.In this thesis,based on the theory of Taylor dispersion,the mixing lengths of the spacer fluid and cement slurry were analyzed and calculated.The main researches were as follows:(1)The mechanism of radial flow velocity was analyzed by using micro-mechanics method of the displacement interface.The radial dispersion coefficient was introduced to describe the radial transport capacity of materials;Axial velocity distributions of laminar flow in casing and concentric annular were studied in vertical well by using power law rheology model as the rheological mode of cementing working fluid.Using axial velocity distribution and radial dispersion coefficient,an axial dispersion model which was suitable for casing and concentric annular space was established,and the dispersion of cement slurry in the axial direction was elucidated,and obtained the theoretical formula of the mixing length.The effects of behavior index,average velocity,casing radius and annular space gap,and dimensionless number on dispersion coefficient and mixing length were analyzed.The mixing length of plug flow was obtained by molecular diffusion equation.(2)Using power law rheological model as the rheological model of cementing fluids,the turbulent flow velocity distributions of the vertical wells were studied,and the maximum velocity and the average velocity of concentric annular in vertical well were obtained.A dispersion model was established for centered annulus by using velocity distribution in vertical well,and the theoretical formula of the mixing length were obtained.The effects of behavior index,average velocity,annular space gap,on dispersion coefficient and mixing length were analyzed.(3)By means of numerical simulation,the relationship between radial velocity and density difference was obtained,and the effect of density difference on the mixing length was analyzed by simulation calculation;the relationship between diffusion coefficient of calcium ion and temperature,spacer fluid was studied by experiments.,and the mixing length calculation formula in casing was verified;Combining the calculation formula of mixing length with the actual cementing project,a new method for calculating the dosage of theoretical spacer fluid under different flow state was obtained.and the new method was used to calculate the dosage of spacer fluid in the field.Through the above theoretical and experimental studies,the effects of various parameters of laminar and turbulent displacement on the mixing length were analyzed,and the recommended calculation method for the dosage of spacer fluid after considering the mixing length was proposed,taking into the effects of well deflection,eccentricity,and other factors.It provides a theoretical basis for optimizing the replacement parameters of cementing construction.