Study On The Mechanism Of High-efficiency Spacer Liquid Selection Under Oil-based Drilling Fluid Conditions

Oil-based drilling fluids (OBMs) have been widely used in shale gas wells and complex drilling deep wells all over the world, but it was not easy to replace and clean. What's more, it also resulted in the decrease of rheological and strength of mixed slurry. So it was the key point to produce a suitable spacer fluid to clean OBMs. For the shortages of the relieved contamination agent and surfactant optimizing method on the efficient spacer fluid used in OBMs, this paper studied the contamination of cement slurries with OBMs and the optimizing method of surfactant, which aimed at producing an efficient spacer fluid applied to OBMs and highlighting the optimizing mechanisms of relieve contamination agent and surfactant.Firstly, the contamination mechanism of cement slurries contact with OBMs was investigated. It was found that the OBMs seriously affected the rheological and the mechanical properties of cement slurries and extended the thickening time. The reasons are clarified by the characterization methods and listed as follow:?the rheological properties of mixture are reduced by the changes of free water induced by the osmotic pressure and dumulsification; ?the oil phase of OBMs occupies some part of cement slurries, which destroys the integrity of cement stone and causes a sharp drop in mechanical property.To solve the contamination, the material with the property of destroying the osmotic pressure and packeting the oil substance was optimized among ten kinds of surfactant. It was concluded that the surfactant name as B4 not only improved the performance of mixed slurry but also not produce new hydrations.Compared to blindness of the existing optimizing method, a suitable and efficient optimizing method was suggested. The character of surfactant were evaluated from four aspects the ion type, hydrophile lipophilebalance value (HLB), surface tension, critical mecellization concentration (CMC) repectively. The surface tension of non-ionic, anionic and non-ionic/anionic surfactant system with HLB value from 12 to 15 was close to the critical surface tension of OBMs. Meantime, the surfactant with the lower CMC displays the better performance. The FPC mixed surfactants and co-cleaner was successfully selected through this method.The efficient spacer fluid made up by the relieved contamination agent B4 and the mixing surfactant FPC exhibited a good suspension stability, compatibility and high cleaning efficiency. Moreover, it also wetted and reversed the bonding interface and improved the bonding strength, which guaranteed the cement quality used in OBMs.