Water-in-oil emulsion is a flow pattern that may occur during the oil production and generate flow assurance issues due to its rheological behavior. In addition to the increase of effective viscosity with water content increment, the emulsion has a complex rheological behavior regarding the fluid’s physicochemical properties, and surfactant presence. In this work, the rheology of stable water-in-oil emulsion formed by the shearing of a multistage centrifugal pump is investigated. The emulsion analyzed was composed of a mineral oil, SPAN 80 and tap water for different water cut and temperatures. The shear rate hysteresis analysis on emulsion flow curve were performed to analyze the rheological behavior. From this analysis, the steady-state shear stress condition was investigated regarding water droplet arrangement based on the Peclet number. Furthermore, the droplet arrangement was observed during acquisition of flow curve using a microscopy coupled on rheometer. To investigate the clustering mechanics in emulsion during rheometry, we present a surface boundary to separate the suspending and clustering droplets based on critical shear stress, droplet Reynolds number, and the water cut for different temperatures.
Tag: Effective viscosity
Experimental Investigation Of The Shear Effect On Oil-Water Emulsion Flow In Pipelines
Emulsion flows have been a severe flow assurance issue, mainly in mature oil fields. Its formation occurs due to shear on oil-water flows caused by artificial lift methods, such as Electrical Submersible Pumps (ESP), and/or valves. The shear rate has an important role in emulsion flow behavior related to its relative viscosity and phase inversion. Therefore, this work presented an experimental investigation of the shear effect on three emulsion systems flowing in a pipeline. The shear element used was a combination of an 8-stage ESP and a glob valve. The emulsion systems analyzed were unstable emulsion and stable emulsion with and without a demulsifier. The experimental investigation was carried out for two ESP rotational speeds, 2400 and 3500 rpm, and one total volumetric flow rate, varying the water cut. From this study, it was observed that phase inversion occurred with increasing shear. Moreover, the effective viscosity was the same regardless of the surfactant presence for the three emulsion systems tested.
Understanding ESP Performance Under High Viscous Application and Emulsion Production
Although being widely used as an artificial lift method for heavy oil field developments, Electrical Submersible Pump (ESP) system performance in high viscous applications is not fully understood. A miscomprehension of challenges and equipment performance in such conditions might lead to operation inefficiencies and equipment failures. This paper presents results of single-phase and multiphase tests performed by University of Campinas (UNICAMP). It also presents operation data, lessons learnt, and failure examples gathered over 10 years of ESP operation in Peregrino field which is a heavy oil, high viscous oilfield offshore Brazil operated by Equinor.
Affinity laws commonly used for ESP simulations don’t hold true for high viscosity applications. Hydraulic performance of centrifugal pumps is affected by fluid parameters like viscosity and density; operation parameters such as flow rate and rotational speed; and specific stage design characteristics. To determine degradation in head and efficiency as well as power requirement increase in viscous applications, Equinor performs one-phase high viscosity flow loop test to qualify each stage type prior to deployment in Peregrino field.
For the qualification of ESPs, single phase qualification tests are performed using mineral oil with viscosities specifically chosen to cover the viscosity range of the specific field. Each stage type is qualified using a prototype with reduced number of stages due to flow loop limitations. Qualification tests for the Peregrino field confirmed that affinity laws are not accurate for high viscous applications and provided important insights regarding pump performance that are used in equipment specification and system surveillance.
The UNICAMP research team has designed and performed multiphase flow tests to evaluate emulsion formation inside centrifugal pump stages and effective viscosity behavior. Phase inversion phenomenon investigation was also included in studies. Studies performed using a prototype stage allowed visualization and evaluation of oil drops dynamics inside the impeller in different rotational speeds. Two phase flow loop tests investigated the shear forces influence in effective viscosity inside pump stages and downstream pump discharge. Phase inversion phenomenon was also a point of great interest during the studies. Data gathered during lab tests was used to evaluate accuracy of mathematical models existing in the literature when a centrifugal pump is added to the system. Hysteresis effect associated to catastrophic phase inversion (CPI) was confirmed and replicated during flow loop tests. Such behavior can be related with operation parameters instabilities and equipment failures noticed in actual application in Peregrino field which are also presented in this paper.
A novel criterion based on slip ratio to assess the flow behavior of W/O emulsions within centrifugal pumps
Water-in-oil emulsions usually present complex rheological behavior that depends on the physicochem-ical properties of both phases, the presence of surfactants, and the flow conditions. Thereby, this paperaims to propose a criterion to characterize the rheological behavior of stable and unstable water-in-oil emulsions within the centrifugal pumps. This criterion is based on the slip ratio between the dispersedand continuous phases. For this, the droplet size distribution was measured at the ESP outlet and the slipratio was estimated based on the centrifugal buoyancy-induced flow. A new model was proposed todetermine the Sauter mean diameter for different systems of the water-in-oil emulsion flows withinthe ESP based on operational conditions, which presents good agreement with the experimental data(12.6% of error). Finally, a new dimensionless number parameter named Slip Relevance number was pro-posed to separate the different emulsion flow behaviors within the ESP and its critical value was obtained.
Experimental investigation on the performance of Electrical Submersible Pump (ESP) operating with unstable water/oil emulsions
Electrical Submersible Pump (ESP) is one of the most commonly used artificial lift methods in petroleum production, due to its capacity to operate in several conditions with two or three-phase flows. When the ESP operates with emulsion flow, its performance is degraded, and operational instabilities occur. Therefore, this paper aims to carefully investigate phase inversion and to present, by the first time, the ffective viscosity of unstable mineral oil/water emulsions, both within the ESP. The first part of this work analyzes the phase inversion phenomenon for two oil types in three viscosities, five ESP rotational speeds, and three mixture flow rates. Logistic functions were fitted using the dimensionless head as a water cut function to determine the phase inversion within the ESP. The continuous phase inversion model, developed for emulsion pipe flow, did not present a satisfactory agreement to flow conditions tested. An indirect method to determine the emulsion effective viscosity within the ESP was proposed, which was obtained from the water/oil emulsion performance curves. The viscous performance data were used to determine the geometric coefficients of a dimensionless head empirical model for the tested ESP. Thus, the calculated values were compared with the effective viscosity obtained with oil and water emulsions, as well as the ESP performance, operating with emulsion and oil, which provides similar values for low rotational speeds. The different behavior of the effective viscosity between the pipeline flow and within the ESP was observed for water-in-oil emulsions and may be related to the high centrifugal field in the ESP.