Study On Microemulsion Flushing Fluid For Oil-based Drilling Fluid

Oil-based drilling fluid shows the advantages of high temperature resistance,excellent lubricity,strong inhibition and low damage on reservoir,which has been widely used as an important means for the drilling of complex wells such as deep well and horizontal well.With the rapid development of the unconventional oil and gas resources,prominent problem of cementing quality under the condition of oil-based drilling fluid arises,and the problem of oil-based mud removal urgently need to be solved.Aiming at this problem,a novel kind of microemulsion flushing fluid used for removal of Oil-based drilling fluid was developed based on the unique properties of the microemulsion.According to HLB,surface tension,interface tension and solubilization parameter,Gemini quaternary ammonium cationic surfactant CZH,anionic surfactant Sodium Alpha Olefin Sulfonate?AOS?,n-butyl alcohol,n-heptane and KCl was selected as main materials of microemulsion.The molar ratio of CZH/AOS was confirmed and the effects of different alcohols,inorganic salts on the phase behavior of microemulsion formed by a fixed ratio of CZH/AOS were also investigated systematically.Various properties,including interfacial tension,solubilization,contact angle and zeta potential of nanoemulsion obtained by diluting the microemulsion were evaluated.Based on that,combined with flushing efficiency tests,flushing mechanisms of microemulsion was revealed.According to the stability test and rheology test,additive such as weighting agent and polymer were selected,and a novel kind of microemulsion flushing fluid used for removal of Oil-based drilling fluid was developed.The compatibility of microemulsion flushing with cementing slurry and oil-based drilling fluid,the flushing effect were investigated systematically.The results prove that,the best molar ratio of CZH/AOS is 7:3,and the microemulsion phase state changed from Winsor I to Winsor III and then to Winsor II with the increase of mass concentrations of salt and alcohol.The optimal salinity concentration of the middle phase microemulsion is 0.034 g·mL^-1,and the optimal alcohol concentration is 0.065 g·mL^-1.The interfacial tension between nanoemulsion and diesel could reach an ultra-low level,and nanoemulsion possess excellent solubilization,which could pull off oil droplets from the wellbore;nanoemulsion possess excellent wettability reversal capability,which could ensure the water-wetting state of wellbore and provides a favorable environment for the slurry cementation.Taking barite and hydroxyethyl cellulose as main additive,a novel kind of microemulsion flushing fluid with good stability and adjustable rheological property was developed.The microemulsion flushing fluid is compatible,and flushing fluid can remove the OBDF in7 min while remove filter cake in 10 min;the bonding strength of cement-formation interface can be 10.4 time the bonding strength of cement-formation interface after flushed by conventional surfactant flushing fluid.That ensures the normal hydration of the cement slurry and improve the bond strength of the casing-cement-formation interface.The microemulsion flashing fluid would be a new method to improve bond strength of the cementing interface and ensuring cementing quality.