Analysis and Test of Flow Field of Two-Stage Series Downhole Hydrocyclone

Abstract Hydrocyclone is a critical component of downhole oil-water separation technology. Sometimes, it is necessary to use two hydrocyclones in series in order to improve the effectiveness of downhole oil-water separation and reduce oil concentration in reinjected water. Unlike single-stage hydrocyclones, however, a two-stage series hydrocyclone is characterized of a more complicated flow field, more significant flow field interference and more factors that affect separation effectiveness. The flow field of a two-stage series hydrocyclone is analyzed and, in particular, the distribution of oil phase, pressure field and velocity field near the overflow port of the second stage hydrocyclone is studied by means of a two-stage series hydrocyclone oil-water separation model built based on the hydrodynamic theory. According to our analysis, when the water content of oil-water mixture is higher than 97%, the two-stage series hydrocyclone offers insignificant advantage in separation effectiveness compared to a single-stage counterpart; when the water content is lower than 97%, the effectiveness of separation using a two-stage series hydrocyclone is very significant, where the second stage hydrocyclone provides good repurification of underflow of the first stage hydrocyclone, helping to reduce oil concentration in reinjected water. It is also discovered that the geometry of the connector located between the two hydrocyclones is the major factor affecting the separation effectiveness of the second stage hydrocyclone. Cylindrical connector, which is identified as the best geometry by comparing connectors of three different structures through modeling analysis, can not only improve the separation effectiveness of the series hydrocyclone but also offer decreased pressure loss. The accuracy of analysis and calculation results are further verified through comparison of results of lab tests conducted on the two-stage series hydrocyclone on the basis of analytical calculation.