Mauricio Prado

Two-Phase Flow Performance for Electrical Submersible Pump Stages

Two-phase flow behavior prediction for centrifugal pumps is a difficult task because of the complexity involved in modeling multiphase flow inside turbo machines. An experimental study has been conducted at the U. of Tulsa Artificial Lift Projects (TUALP) with a 513-series, 22-stage, mixed-flow-type pump to gather data for pump performance under two-phase flow conditions. Air and water were used as the working fluids. This study differs from other experimental works because the pressure changes were recorded stage by stage. The results of previous works have been reported as an average of the intake and discharge conditions and depend on the number of stages used. Phenomena such as surging and gas locking were observed during these tests, and their boundaries have been mapped. The pressure increment and total hydraulic horsepower for the average pump, those per stage as a function of the liquid flow rate, and each gas flow rate considered are presented. The pumps average brake horsepower and efficiency also are plotted for the variables mentioned. The results indicate that average pump behavior is significantly different from that observed per stage.

Experimental Investigation of the Viscous Effect on Two-Phase-Flow Patterns and Hydraulic Performance of Electrical Submersible Pumps

Using a visualization prototype built from original electrical-submersible-pump (ESP) components and with minimal geometrical modifications, a pioneer experimental procedure was developed and conducted to address the viscous effect on liquid/gas two-phase flow through these types of pumps. Based on dimensionless groups that govern centrifugal-pump single-phase performance, two-phase experiments were conducted at different shaft speeds (15, 25, and 30 Hz), with nonslip void fractions (up to 5%), and viscosity values of 46 to 161 cp, while liquid rates were kept constant at 60% of the maximum rate at the defined shaft speed. High-speed video footage was taken from the entire impeller flow channel, and stage incremental pressure was measured. The authors identified four liquid/air flow patterns inside the impeller channels: agglomerated bubbles, gas pocket, segregated gas, and intermittent gas. By comparing the images with the differential-pressure data, it was concluded that the agglomerated-bubbles pattern is responsible for the initial head degradation and that the surging event coincides with the gas-pocket structure, indicating that this is an interface-instability problem. Another conclusion made was that the increase in viscosity caused surging to occur at lower void fractions, which could be compensated for by increasing rotational speed. The significance of this work is given by the fact that several authors hae investigated centrifugal-pump performance under two-phase flow; however, previous experiments have been conducted only with water as the liquid, thus neglecting the viscous effect on the two-phase-flow mixture. In most of the petroleum industrys applications, ESPs operate with oil and natural gas. The present work begins the task of addressing this knowledge gap between scientific research and field applications.

A Simple Model To Predict Natural Gas Separation Efficiency in Pumped Wells

A new mechanistic model to predict natural separation efficiency in vertical pumped wells has been developed. The model is based on the combined phase momentum equations and a general slip-closure relationship. New drag-coefficient correlations have been developed that correspond to bubbly (i.e., undistorted particle) and churn-turbulent flow regimes. The model indicates that natural separation efficiency depends strongly on geometry, void fraction, and in-situ gas flow rate.

An improved model for predicting separation efficiency of a rotary gas separator in ESP systems

An improved model capable of predicting the separation efficiency of a rotary gas separator (RGS) in electric submersible pump (ESP) systems is presented. The model incorporates a new, two-phase, flow-inducer model capable of calculating the inducer head. The inducer head, generated by an RGS, has been identified as a key parameter that distinguishes between a separators high- and low-efficiency regions. This information was previously determined empirically but can now be calculated. The new model more accurately predicts the maximum liquid rate at which an RGS should be installed. A comparison of the models predictions with water/air and hydrocarbon/air experimental data indicates that the improved model performs better than earlier ones.

Analysis of Experimental Data of ESP Performance Under Two-Phase Flow Conditions

Electrical Submersible Pumps (ESP’s) are well known to have a good predictable performance for single phase, low viscosity liquids. In the oil industry, oil production is associated with the production of gas. The presence of gas deteriorates the performance of the pump. The performance deterioration varies with the amount of gas and the intake pressure at which the pump is operated. So far no good predictive method is available to predict the performance of the ESP under twophase conditions. Presently the industry is using the homogeneous model, correlations and other models to predict this performance. In the homogeneous model the two-phase flow head is assumed to be the same as single-phase head (provided by the manufacturer) at a total mixture flow rate. The available correlations are specific to the tested type of pump and to the tested number of stages. Finally to predict performance on model, no general model has been developed to predict two-phase performance of the ESP due to complexity of two-phase flow, pump geometry and speed at which it is operated. Researchers, Lea and Bearden (1982), Cirilo (1998), Romero (1999) and Pessoa (2000) have concluded through experimental results that the application of the homogeneous model gives good prediction only at low gas fractions (in the order of 2 to 5 %) at the intake. At higher gas fraction the experimental results shows that performance is well away from the homogeneous model predicted performance. The University of Tulsa Artificial Lift Projects – TUALP is currently conducting experimental and theoretical research on the two-phase behavior of electrical submersible centrifugal pumps, using a 22-stage Mixed flow type, series 513 pump with best efficiency flow rate of 6100 BPD to gather data on stage wise performance under two-phase flow conditions. Air and water were used as working fluids. This is a unique facility that has pressure gauges fitted across each stage. It not only helps to study the stage wise behavior but also the effect of number of stages on cumulative performance of pump under two-phase flow conditions. This paper focuses on analysis of data collected at TUALP facility.

CFD Modeling Inside an Electrical Submersible Pump in Two-Phase Flow Condition

Dynamic multiphase flow behavior inside a mixed flow Electrical Submersible Pump (ESP) has been studied theoretically for the first time. The main goal is to model two-phase flow behavior in an ESP. A three-dimensional CFD model has been developed to describe the operational envelope of the ESP, namely the onset of surging. The theoretical study includes CFD simulations for the prediction of the flow behavior inside the pump. The CFD modeling depends on two important variables, namely the bubble size and the bubble drag coefficient. The bubble size has been measured and a physically based correlation presented in Barrios (2007) is used. A new correlation for the drag coefficient is used (Barrios 2007) as a function of rotational speed and Reynolds number. Single-phase and two-phase flow CFD simulations were carried out to investigate liquid flow field. Results from the CFD simulations are consistent with the experimental data (Barrios 2007).Copyright

Two-Phase Flow Modeling of Inducers

Inducers, which are classified as axial flow pumps with helical path blades, are used within rotary gas separators commonly used in electrical submersible pump installations. A two-phase flow model has been developed to study the inducer performance, focusing on head generation. The proposed model is based on a meridional flow solution technique and utilizes a two-fluid approach. The model indicates that head degradation due to gas presence is a function of flow pattern. The effect of flow pattern diminishes when the void fraction is greater than 15 percent since the centrifugal force dominates the interfacial drag force. In this case, the two-phase flow can be approximated as a homogeneous mixture. The model also suggests that a liquid displacement correction is needed when phase segregation occurs inside the inducer The new model significantly improves the ability to predict separation efficiency of a rotary gas separator over existing models. Hydrocarbon-air and water-air experimental data were gathered to validate the new model.

Modeling Two Phase Flow Inside an Electrical Submersible Pump Stage

Dynamic multiphase flow behavior inside a mixed flow Electrical Submersible Pump (ESP) has been studied experimentally and theoretically for the first time. The overall objectives of this study are to determine the flow patterns and bubble behavior inside the ESP and to predict the operational conditions that cause surging. The theoretical study includes a mechanistic model for the prediction of the flow behavior inside the pump. The model comprises a one-dimensional force balance to predict occurrence of the stagnant bubbles at the channel intake. This model depends on two important variables, namely the stagnant bubble size and the bubble drag coefficient. The bubble size has been measured and a physically based correlation is presented. A new correlation for the drag coefficient is proposed as a function of rotational speed and Reynolds number. The model enables the prediction of the operational envelope of the ESP, namely the transition to surging.Copyright

Experimental Visualization of Two Phase Flow Inside an Electrical Submersible Pump Stage

Dynamic multiphase flow behavior inside a mixed flow Electrical Submersible Pump (ESP) has been studied experimentally and theoretically for the first time. The overall objectives of this study are to determine the flow patterns and bubble behavior inside the ESP and to predict the operational conditions that cause surging. An experimental facility has been designed and constructed to enable flow pattern visualization inside the second stage of a real ESP. Special high speed instrumentation was selected to acquire visual flow dynamics and bubble size measurements inside the impeller channel. Experimental data was acquired utilizing two types of tests (surging test and bubble diameter measurement test) to completely evaluate the pump behavior at different operational conditions. A similarity analysis performed for single-phase flow inside the pump concluded that viscosity effects are negligible compared to the centrifugal field effects for rotational speeds higher than 600 rpm. Therefore, the two-phase flow tests were performed for rotational speeds of 600, 900, 1200, and 1500 rpm. Results showed formation of a large gas pocket at the pump intake during surging conditions.© 2009 ASME

Modelling Downhole Separation Efficiency Using a Stream Function Approach

A new technique for solving simultaneously the mass and momentum balance equations for a gas-liquid mixture is proposed in this paper. The technique is based on the vorticity-stream function approach, under a cylindrical coordinate system. Important variables such as slip velocity in the radial and vertical direction, characteristic of the bottomhole, gas and liquid flow rates, pressures, flow pattern, among others, are considered simultaneously in the solution of the problem. The two-phase model proposed in this work allows obtaining the liquid and gas velocities fields as well as the two-phase flow pressure and gas fraction fields. Experimental data is used to close the model. The model was used to simulate downhole separation efficiency and to determine the influence of liquid flowrate, GLR, casing size and liquid viscosity on the natural separation efficiency. The model agrees very well with experimental data obtained by The University of Tulsa.Copyright