Muhammad R. Ridlah (Master of Science in Petroleum Engineering)
Study of Oil/Water Emulsion in Electric Submersible Pump — Emulsion Rheology and Its Effect on Pump Boosting Pressure
Conclusions and Recommendations
Electric Submersible Pump (ESP) operation may run under up-thrust or down-thrust conditions in case of inaccurate model design thus shortening the ESP run life. The formation of tight oil/water emulsion throughout the production lifetime is inevitable and consequently induces pressure loss in the pump due to increased friction loss deteriorating the pump performance. Currently, the emulsion rheology in the ESP is still not well understood. The effective viscosity of the emulsion is commonly estimated with empirical correlations, which are only valid for a production pipeline without considering high shear in the system. This study will present the experimental investigation of emulsion rheology in the ESP along with mechanistic model development for emulsion rheology and pump hydraulic performance.
Water/oil emulsions at different water fractions and different oil viscosities are formed and circulated through a General Electric (GE) TE-2700 14-stage radial type ESP, which has 1600 specific speed (NS), in a 3-inch closed flow loop. A pipe-in-pipe heat exchanger is installed to control the loop temperature during the experiments. Mass flow rate and fluid density are measured with the mass flowmeter. The pressure transmitters are placed over each pump stage to measure the pressure increment.
The temperature sensors are located at the pump inlet, pump outlet, and downstream of the pipe viscometer (PV). The emulsion viscosity is derived from the in-line PV downstream of the ESP discharge. The emulsion rheology and pump hydraulic performance are characterized. The comparisons of experiment results and model predictions are presented for the emulsion effective viscosity as a ftlnction of water fraction and the ESP boosting pressure at different flow rates.
Single-phase oil tests are performed at different oil viscosities and pump rotational speeds (3000 RPM, 3500 RPM) to validate the feasibility of the flow loop and pipe viscometer measurements. The PV measured viscosity shows discrepancy compared to oil viscosity from rotational viscometer measurement. Experimental results show a significant increase of emulsion viscosity at water fraction close to the inversion point due to the interactions of a great number of water droplets. The oil/water emulsion experiments are performed with two different oil viscosities, 45 cp, and 70 cp, at 3000 RPM pump speed. Higher oil viscosity reaches the inversion point at a lower water fraction. The experiment results indicate the inversion point at around 35% 40% and 30% — 35% water-fractions, respectively.
The dimensional analysis performed by the Buckingham Pi theorem reveals that the Reynolds number, Weber number, and Strouhal number represent the combination of correction factors to the mechanistic model of emulsion viscosity. The new emulsion rheology model predictions show a good agreement with the experimental data with discrepancy. The ESP boosting pressure model begins with the Euler Equation for centrifugal pump. The mechanistic model introduces a conceptual best match flow rate (QBM) at which the outlet flow direction of the impeller matches the designed flow direction. The ESP hydraulic performance model further incorporates recirculation loss, friction loss, turn loss, and leakage loss. The increasing of water fraction up to the inversion point deteriorates pump boosting pressure since the high friction loss occurs due to the increase of emulsion viscosity. Nevertheless, as the water fraction passes the inversion point, the boosting pressure starts to rebound with the emulsion viscosity decrease since the water changes from the dispersed phase to the continuous phase. The experiment results are compared with the mechanistic model predictions of pump hydraulic performance for oil/water emulsions with a standard deviation error less than ±20%. Additional experiment data are needed to expand the model validation for different ESP pump types and different oil viscosities.