Due to the significant difference of water-in-oil emulsion and oil-in-water emulsion in viscosity, conductivity, rheology, thermal resistance, emulsion system has been widely used in the development of oil fields. The reversible phase inversion technique can enhance the performance of emulsified working liquids during the transportation of fluids in porous media. The objective of this study is to study the preparation and the application of the reversible emulsion controlled by inorganic salts.

SDBS, SDS, and SPS were used as emulsifiers in this study. The white oil and deionized water were emulsified into anoil-in-water emulsion by high-speed stirring. Multivalent metal cations were added to the emulsion to control the inversion of the emulsion to water-in-oil type. Then, the multivalent metal cations were removed by anions, leading to the emulsion reversed from water-in-oil to oil-in-water type. The stability of emulsions was evaluated by the stratification time, thermal resistance, conductivity, emulsion-breaking voltage and microstructure.

The results showed that emulsion's dehydration rate was 8.1% at 25°C, which increased to 14.3% as the temperature reached 90°C after 5 hours. The average diameter of droplets was about 6 μm and conductivity was about 619 μs·cm−1. The emulsion processed by CrCl3·6H2O solution inverted to water-in-oil type, and its oil separation rate was 11.9% at 25°C and 18.2% at 90°C after 5 hours. The emulsion-breaking voltage and average diameter of droplets were 153 V and 13 μm, respectively. After Cr3+ was removed by Na2CO3 solution, the type of emulsion became oil-in-water againand the dehydration rate of this emulsion increased from 3.2% to 5.8% when the temperature reached up to 90°C. Under this circumstance, the average diameter of droplets was 5 μm and conductivity was 1907 μs·cm−1. The emulsion was of high stability before and after inversion.

The emulsion controlled by inorganic salts can be reversed between oil-in-water and water-in-oil type efficiently, which can be used as emulsifier within drilling fluid in the low-permeability reservoir. The characteristics of reversible emulsion drilling fluid include inhibition for shale rock, good lubricity, thermal stability, and excellent cementing quality.

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