Pipe flow experiments of unstable oil-water dispersions with three different oil viscosities: Flow pattern, pressure drop and droplet size measurements

The transport of oil-water dispersions in petroleum production pipelines is difficult to predict and requires special attention since it affects the performance of the entire system. For future field developments it is required to generate accurate predictive models to guarantee an optimal field design. The purpose of this work is to present novel experimental data suitable for improving mechanistic flow models in future works. Oil-water pipe flow experiments were conducted in a stainless-steel flow loop with a L/D ratio of 3766, larger than any comparable setups reported in the literature and sufficient to obtain fully developed flow. A novel level of detail measurements included pressure gradients, density profiles and droplet size distributions. Three oils with different viscosities (oil A: 1.3 cP; oil B: 7 cP; oil C: 22 cP) and brine (3.5 wt% NaCl) as the water phase constituted the three fluid systems used. For each fluid system, several flow rates, and a wide range of water fractions were studied. The fluids were not stabilized by any type of chemical additives. The oil viscosity influences the dispersion behavior, especially for oil continuous flow. For higher oil viscosities the dispersion tends to be more homogeneous, and the pressure drop increases due to increasing wall friction. The droplet size decreases as the oil viscosity increases, presumably due to higher shear stress. Water continuous flows, on the other hand, are less affected by the oil viscosity. A strong drag reduction was found for dispersed flow of all three oils and both oil and water continuous flow. A simple model for the dispersion viscosity and drag reduction was developed based on additional bench scale characterization experiments. With this model the pressure drop could be predicted with good agreement. The data reported in this paper will facilitate the development and validation of mechanistic models for predicting oil-water flows. Previous modelling efforts have been hampered by a lack of detailed measurements, in particular droplet size measurements, hence we believe that this data will allow for significant advancements on the modelling side.publishedVersio

Modelling of dispersed oil/water flow in a near-horizontal pipe

A gravity-diffusion model was implemented for predicting water concentration profiles in dispersed oil-continuous oil–water flows. In this model, the measured droplet size distributions were used instead of a droplet size closure law. The turbulent diffusion was modelled assuming single-phase flow while the gravitational drift was based on closure laws from the literature, including hindrance effects. The results showed that including the effect of turbulence on the drag force was important, where the turbulent fluctuations cause an increase in the average drag because of the non-linearity of the drag law. The model yielded a good match with the experimental data reported by Gonzales et al. (Gonzalez et al., 2022), especially at the highest flow rates. We also concluded that the following model simplification could be introduced without changing the results significantly: 1) The droplet size distributions could be replaced by the Sauter mean droplet size. 2) The diffusivity profile model could be replaced by a uniform diffusivity model.publishedVersio

Modelling of the wall film in high-rate low liquid loading flows

In this paper we present a detailed analysis of large scale experimental data from the SINTEF Multiphase Laboratory on high-rate low liquid loading flows. The experimental work [1] was funded by Equinor as part of the Tanzania gas field development project [2] [3] [4], and SINTEF was granted access to use the data for improving the accuracy of the pressure drop predictions in LedaFlow. The experimental results showed that a key element for predicting high-rate low liquid loading flows accurately is to account for the droplets that deposit on the walls in the gas zone, creating a wall film. This wall film can have a profound effect on the hydraulic roughness experienced by the gas, and subsequently the frictional pressure drop. Furthermore, the data showed that this effect was particularly important for high liquid viscosities and in three-phase flows, and simulations showed that LedaFlow had a clear tendency to under-predict the pressure drop in such scenarios. To improve this situation, we used the data to derive a model for predicting this complex phenomenon. This paper summarizes the main parts of the data analysis and the development of the wall film model. We show that by introducing this new model into LedaFlow, we were able to significantly improve the agreement with the measurements.publishedVersio

Modelling of the wall film in high-rate low liquid loading flows

In this paper we present a detailed analysis of large scale experimental data from the SINTEF Multiphase Laboratory on high-rate low liquid loading flows. The experimental work [1] was funded by Equinor as part of the Tanzania gas field development project [2] [3] [4], and SINTEF was granted access to use the data for improving the accuracy of the pressure drop predictions in LedaFlow. The experimental results showed that a key element for predicting high-rate low liquid loading flows accurately is to account for the droplets that deposit on the walls in the gas zone, creating a wall film. This wall film can have a profound effect on the hydraulic roughness experienced by the gas, and subsequently the frictional pressure drop. Furthermore, the data showed that this effect was particularly important for high liquid viscosities and in three-phase flows, and simulations showed that LedaFlow had a clear tendency to under-predict the pressure drop in such scenarios. To improve this situation, we used the data to derive a model for predicting this complex phenomenon. This paper summarizes the main parts of the data analysis and the development of the wall film model. We show that by introducing this new model into LedaFlow, we were able to significantly improve the agreement with the measurements

Pseudo slug flow in viscous oil systems – experiments and modelling with LedaFlow

An experimental two-phase flow campaign with a viscous oil (100 cP) was conducted in the Large Scale 8" loop at the SINTEF Multiphase Flow Laboratory. The loop consisted of three main test sections, with pipe inclinations 0, 0.5 and 90 degrees, and with approximate lengths of 380 m, 380 m and 50 m, respectively. The experiments were performed at system pressures 20, 45 and 85 bara. The primary focus of the campaign was on liquid dominated flows in horizontal/near-horizontal pipes, with particular emphasis on the laminar-to-turbulent transition in the liquid. The main instruments were DP-cells for pressure drop measurements, and narrow beam gamma densitometers to measure the liquid height. In addition, traversing gamma densitometers were mounted on each of the two near-horizontal sections to measure the time-averaged liquid distribution and volume fractions. In this campaign, it was found that at high pressure, the slug flow region was very narrow. In particular, at low gas-liquid ratios, the prevailing flow regime was determined to be a kind of "pseudo slug flow", which was characterized by large waves that did not quite extend to the top of the line. Simulations with the commercial multiphase simulator LedaFlow [1] did not reproduce this at the time, and the penalty for this discrepancy was that the predicted pressure drop was too high. Consequently, it was concluded that the physical models in LedaFlow needed improvement to predict these conditions better. Through detailed analysis of the experimental data, it was found that at these conditions, slugs were often not able to form because the waves did not have sufficient inertia to sustain a slug front. This limitation was not accounted for in the flow regime criteria in LedaFlow, so to improve the situation, the existing slug flow model in LedaFlow was generalized to cover pseudo slug flow in addition to regular slug flow. By introducing this new pseudo slug flow regime, the pressure drop predictions in viscous oil systems became significantly more accurate than before.publishedVersio

Onset of water accumulation in oil/water pipe flow – experiments and modelling with LedaFlow

In oil/water flows with very low water rates, the steady-state water fraction jumps discontinuously from a low value to a high value when the oil rate falls below the critical value. This jump is understood to be connected to the existence of multiple solutions, and generally takes place when the oil rate becomes too low to sustain a low water fraction. We refer to this critical oil flow rate as the onset of water accumulation. Water accumulation in oil transport lines is undesirable because it can lead to corrosion problems that can threaten the integrity of the installation, potentially leading to oil leaking undetected into the environment, jeopardizing nearby wildlife and ecosystems. It is therefore critical to maintain a flow rate that is high enough to prevent water from accumulating in oil lines, and the ability to predict the minimum allowable flow rate accurately is thus of great importance. To address this challenge, a new and unique set of experiments were conducted at the SINTEF Multiphase Laboratory. The experiments were specially designed to measure the critical conditions for water accumulation in oil/water flows and were performed with a pipe diameter of 8 inches (194 mm) and a pipe inclination of 2.5 degrees. The fluid system consisted of Exxsol D60 as the oil phase and regular tap water as the aqueous phase. In these experiments, the measured critical superficial water velocities were in the range 0.1-2.6 mm/s, while the critical superficial oil velocities were in the range 0.3-0.5 m/s. We found that the customary approach of modelling the oil/water interfacial shear stress as a smooth wall was inadequate for predicting these experiments, and that interfacial waves must be considered. The data analysis showed that the onset of interfacial waves is well predicted by Viscous Kelvin-Helmholtz theory, and that a model for the interfacial shear stress can be constructed with this theory as a starting point. A new model for oil/water interfacial shear stress was developed based on this data and the associated data analysis. The new model was able to match the experimental data well and a slightly modified version of it was ultimately implemented in the commercial multiphase flow simulator LedaFlow

Pseudo slug flow in viscous oil systems – experiments and modelling with LedaFlow

An experimental two-phase flow campaign with a viscous oil (100 cP) was conducted in the Large Scale 8" loop at the SINTEF Multiphase Flow Laboratory. The loop consisted of three main test sections, with pipe inclinations 0, 0.5 and 90 degrees, and with approximate lengths of 380 m, 380 m and 50 m, respectively. The experiments were performed at system pressures 20, 45 and 85 bara. The primary focus of the campaign was on liquid dominated flows in horizontal/near-horizontal pipes, with particular emphasis on the laminar-to-turbulent transition in the liquid. The main instruments were DP-cells for pressure drop measurements, and narrow beam gamma densitometers to measure the liquid height. In addition, traversing gamma densitometers were mounted on each of the two near-horizontal sections to measure the time-averaged liquid distribution and volume fractions. In this campaign, it was found that at high pressure, the slug flow region was very narrow. In particular, at low gas-liquid ratios, the prevailing flow regime was determined to be a kind of "pseudo slug flow", which was characterized by large waves that did not quite extend to the top of the line. Simulations with the commercial multiphase simulator LedaFlow [1] did not reproduce this at the time, and the penalty for this discrepancy was that the predicted pressure drop was too high. Consequently, it was concluded that the physical models in LedaFlow needed improvement to predict these conditions better. Through detailed analysis of the experimental data, it was found that at these conditions, slugs were often not able to form because the waves did not have sufficient inertia to sustain a slug front. This limitation was not accounted for in the flow regime criteria in LedaFlow, so to improve the situation, the existing slug flow model in LedaFlow was generalized to cover pseudo slug flow in addition to regular slug flow. By introducing this new pseudo slug flow regime, the pressure drop predictions in viscous oil systems became significantly more accurate than before

Pipe flow experiments of unstable oil-water dispersions with three different oil viscosities: Flow pattern, pressure drop and droplet size measurements

The transport of oil-water dispersions in petroleum production pipelines is difficult to predict and requires special attention since it affects the performance of the entire system. For future field developments it is required to generate accurate predictive models to guarantee an optimal field design. The purpose of this work is to present novel experimental data suitable for improving mechanistic flow models in future works. Oil-water pipe flow experiments were conducted in a stainless-steel flow loop with a L/D ratio of 3766, larger than any comparable setups reported in the literature and sufficient to obtain fully developed flow. A novel level of detail measurements included pressure gradients, density profiles and droplet size distributions. Three oils with different viscosities (oil A: 1.3 cP; oil B: 7 cP; oil C: 22 cP) and brine (3.5 wt% NaCl) as the water phase constituted the three fluid systems used. For each fluid system, several flow rates, and a wide range of water fractions were studied. The fluids were not stabilized by any type of chemical additives. The oil viscosity influences the dispersion behavior, especially for oil continuous flow. For higher oil viscosities the dispersion tends to be more homogeneous, and the pressure drop increases due to increasing wall friction. The droplet size decreases as the oil viscosity increases, presumably due to higher shear stress. Water continuous flows, on the other hand, are less affected by the oil viscosity. A strong drag reduction was found for dispersed flow of all three oils and both oil and water continuous flow. A simple model for the dispersion viscosity and drag reduction was developed based on additional bench scale characterization experiments. With this model the pressure drop could be predicted with good agreement. The data reported in this paper will facilitate the development and validation of mechanistic models for predicting oil-water flows. Previous modelling efforts have been hampered by a lack of detailed measurements, in particular droplet size measurements, hence we believe that this data will allow for significant advancements on the modelling side

Modelling of dispersed oil/water flow in a near-horizontal pipe

A gravity-diffusion model was implemented for predicting water concentration profiles in dispersed oil-continuous oil–water flows. In this model, the measured droplet size distributions were used instead of a droplet size closure law. The turbulent diffusion was modelled assuming single-phase flow while the gravitational drift was based on closure laws from the literature, including hindrance effects. The results showed that including the effect of turbulence on the drag force was important, where the turbulent fluctuations cause an increase in the average drag because of the non-linearity of the drag law. The model yielded a good match with the experimental data reported by Gonzales et al. (Gonzalez et al., 2022), especially at the highest flow rates. We also concluded that the following model simplification could be introduced without changing the results significantly: 1) The droplet size distributions could be replaced by the Sauter mean droplet size. 2) The diffusivity profile model could be replaced by a uniform diffusivity model

Experiments and modelling of three-phase vertical pipe flow

A set of two- and three-phase experiments were conducted in a 50 m long 400 vertical pipe using nitrogen, Exxsol D60 and water at 45 bara pressure. The results show that the liquid content and pressure drop are highly sensitive to the injected water cut. The proposed explanation for this surprising result is that the presence of liquid droplets constrains the gas bubble capacity of the liquid so that the concentration of small bubbles inside the liquid becomes smaller in three-phase flows than in two-phase flows. To test this hypothesis, a simple flow model was implemented using closure laws from the public literature, combined with the assumption that the concentration of gas bubbles is reduced by the presence of liquid droplets. The model shows that the observed three-phase effects can be reproduced very accurately using this assumption.publishedVersio