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**Liquid Loading Current Status, New Models and Unresolved Questions**

Mohan Kelkar and Shu Luo The University of Tulsa

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**Outline Definition of liquid loading Literature Survey Our Data**

Model Formulation Model Validation Program Demonstration Summary

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**What is liquid loading? Minimum pressure drop in the tubing is reached**

The liquid drops cannot be entrained by the gas phase (Turner et al.) The liquid film cannot be entrained by the gas phase (Zhang et al., Barnea) The answers from different definitions are not the same

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**Traditional Definition**

IPR Stable OPR Unstable Talk about the importance of identifying liquid loading and necessary measurements Transition Point Liquid Loading

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**Traditional Definition**

As gas flow rate increases 𝑑( ∆𝑝 𝑔 ) 𝑑( 𝑉 𝑔 ) and 𝑑( ∆𝑝 𝑓 ) 𝑑( 𝑉 𝑔 ) At low velocities 𝑑( ∆𝑝 𝑔 ) 𝑑( 𝑉 𝑔 ) decreases faster than increase in 𝑑( ∆𝑝 𝑓 ) 𝑑( 𝑉 𝑔 ) When two gradients are equal, minimum occurs

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**Definition Based on Mechanisms**

Two potential mechanisms of transition from annular to slug flow Droplet reversal Film Reversal Models are either based on droplet reversal (Turner) or film reversal (Barnea) 2 mechanisms: droplet and film reversal Droplet model determine liquid loading when droplet falls back Film model determine liquid loading when liquid film falls back

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**Literature Data Air-water data are available**

The data reported is restricted to 2” pipe Very limited data are available in pipes with diameters other than 2” No data are available for other fluids

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**Generalized Conclusions (2” pipe)**

Minimum pressure drop for air-water flow occurs at about 21 m/s The liquid film reversal starts at around 15 m/s The dimensionless gas velocity is in the range of 1.0 to 1.1 at minimum point 𝑢 𝐺 ∗ = 𝑢 𝐺 𝜌 𝐺 1/2 𝑔𝑑 𝜌 𝐿 − 𝜌 𝐺 1/2

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Liquid Film Reversal Westende et al., 2007

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**flow counter current with the gas stream**

Liquid Film Reversal At 15 m/s, liquid starts to flow counter current with the gas stream Westende et al., 2007

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**Liquid Film Reversal Minimum is at 20 m/s (blue line)**

Residual pressure reaches a zero value at lower velocity Zabaras et al., 1986

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**Entrained Liquid Fraction**

Alamu, 2012

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**Inception of Liquid Loading**

For vertical pipe OLGA = 12 m/s Exptl = 14 m/s Belfroid et al., 2013

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Our Data

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Air-Water Flow Skopich and Ajani conducted experiments in 2” and 4” pipes The results observed are different based on film reversal and minimum pressure drop – consistent with literature However, the experimental results are very different for 2” versus 4” pipe

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**Calculation Procedure**

Total pressure drop is measured and gradient is calculated Holdup is measured and gravitational gradient is calculated Subtracting gravitational pressure gradient from total pressure gradient to get frictional pressure gradient By dividing the incremental pressure gradient by incremental gas velocity, changes in gravitational and frictional gradients with respect to gas velocity are calculated.

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**dPG vs. dPF Air-Water, 2 inch, vsl=0.01 m/s**

Minimum

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**Total dp/dz Air-Water, 2 inch, vsl=0.01 m/s**

Film Reversal

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**dP/dz)G vs. dP/dz)F Air-Water, 2 inch, vsl=0.01 m/s**

dp/dz)F is zero

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**dPT - dPG Air-Water, 2 inch, vsl=0.01 m/s**

Transition at 16 m/s

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**Pressure at Bottom Air-Water, 2 inch, vsl=0.01 m/s**

Pressure build up No pressure build up

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**dP/dz)G vs. dP/dz)F Data from Netherlands (2 inch)**

dp/dz)F is zero

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**What should we expect for 3” or 4” pipeline?**

𝑢 𝐺 ∗ = 𝑢 𝐺 𝜌 𝐺 1/2 𝑔𝑑 𝜌 𝐿 − 𝜌 𝐺 1/2 Based on the above equation, the minimum should shift to right as diameter increases If the above equation is correct, the ratio of uG/√d at unstable point should be constant

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**dPG vs. dPF Air-Water, 4 inch, vsl=0.01 m/s**

Minimum

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**Total dp/dz Air-Water, 4 inch, vsl=0.01 m/s**

Film Reversal

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**dP/dz)G vs. dP/dz)F TUFFP (3 inch, vsl=0.1 m/s)**

dp/dz)F is zero

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**dP/dz)G vs. dP/dz)F Air-Water, 4 inch, vsl=0.01 m/s**

dp/dz)F is zero Film reversal

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**Effect of Diameter on Liquid Loading**

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**Why diameter impacts? Film thickness?**

Skopich et al., SPE

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**Liquid Loading Definition**

Liquid loading starts when liquid film reversal occurs We adopt the model of film reversal to predict inception of liquid loading The reason for this adoption, as we will show later, is because we are able to better predict liquid loading for field data using this methodology.

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**Background Turner’s Equation**

The inception of liquid loading is related to the minimum gas velocity to lift the largest liquid droplet in the gas stream. Turner et al.’s Equation: This equation is adjusted upward by approximately 20 percent from his original equation in order to match his data. 𝑣 𝐺,𝑇 = 𝜎 𝜌 𝐿 − 𝜌 𝐺 𝜌 𝐺 Turner proposed two models of removal of liquid. One is liquid film move along the walls of pipe, another is liquid droplets entrained in the gas core. And he found the liquid droplet is the dominating mechanism of liquid removal. Then he developed the equation to calculate terminal velocity. Comparing with field data, he suggested that a upward 20% adjustment of the equation will best fit the data.

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**Background Drawbacks with Turner’s Equation**

Turner’s equation is not applicable to all field data. Coleman et al. proposed equation (without 20% adjustment ) Veeken found out that Turner’s results underestimate critical gas velocity by an average 40% for large well bores. Droplet size assumed in Turner’s equation is unrealistic based on the observations from lab experiments. Turner’s equation is independent of inclination angle which is found to have great impact on liquid loading. 𝑣 𝐺,𝑇 = 𝜎 𝜌 𝐿 − 𝜌 𝐺 𝜌 𝐺 most of Turner’s data had well head flow pressure above 500 psi. Coleman focused on low pressure gas wells, WHFP all below 500psi. He predicted critical rate adequately with Turner’s equation without adjustment. Veeken used TR to examine Turner’s equation. He also proposed a modified Turner expression that best fits this offshore liquid loading field data. He suggested that liquid loading occurs because of liquid film flow reversal rather than droplet flow reversal.

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Approach Film Model Two film models are investigated to predict liquid loading: Zhang et al.’s model(2003) is developed based on slug dynamics. Barnea’s model(1986) predicts the transition from annular to slug flow by analyzing interfacial shear stress change in the liquid film.

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**Approach Barnea’s Model**

Constructing force balance for annular flow and predict the transition from annular to slug flow by analyzing interfacial shear stress changes. The combined momentum equation: Interfacial shear stress with Wallis correlation: 𝜏 𝐼 𝑆 𝐼 1 𝐴 𝐿 𝐴 𝐺 − 𝜏 𝐿 𝑆 𝐿 𝐴 𝐿 − 𝜌 𝐿 − 𝜌 𝐺 𝑔 sin 𝜃 =0 𝜏 𝐼 = 1 2 𝑓 𝐼 𝜌 𝐺 𝑣 𝑆𝐺 2 (1−2𝛿) 4 Schematic of Annular Flow

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**Approach Barnea’s Model**

Solid curves represent Interfacial shear stress from combined momentum equation Broken curves represent Interfacial shear stress from Wallis correlation Intersection of solid and broken curves yields a steady state solution of film thickness and gas velocity at transition boundary Another transition mechanism is liquid blocking of the gas core. Two mechanisms for blockage of core Instability of annular flow that prevents a stable annular configuration Liquid film gets large enough to create spontaneous blockage For low liquid rates, transition occurs due to the first mechanism, whereas, for high liquid rates, transition occurs due to second mechanism Both mechanisms are checked for transition to slug flow Transition

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Model Formulation In inclined wells, the film thickness is expected to vary with radial angle Vertical Well Inclined Well

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**Original Barnea’s Model at Different Inclination Angles**

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**Non-uniform Film Thickness Model**

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**Non-uniform Film Thickness Model**

Let A1=A2, we can find this relationship. If film thickness reaches maximum at 30 degree inclination angle 𝛿 𝑐 = 1 2 [𝛿 0,𝜃 +𝛿 𝜋,𝜃 ]

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**Non-uniform Film Thickness Model**

We will use the following film thickness equation in the new model: 𝑭𝒐𝒓 𝟎≤𝜽≤𝟑𝟎 𝒅𝒆𝒈𝒓𝒆𝒆 𝑭𝒐𝒓 𝜽>𝟑𝟎 𝒅𝒆𝒈𝒓𝒆𝒆 𝜹 𝜱,𝜽 = 𝜽 𝟑𝟎 𝒔𝒊𝒏 𝜱−𝟗𝟎 +𝟏 𝜹 𝒄 𝜹 𝜱,𝜽 = 𝒔𝒊𝒏 𝜱−𝟗𝟎 +𝟏 𝜹 𝒄

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**Non-uniform Film Thickness Model**

Only maximum film thickness will be used in the model because thickest film will be the first to fall back if liquid loading starts. Find critical film thickness δT by differentiating momentum equation. δT equals to maximum film thickness δ(π,30). 𝛿 𝑐 = 1 2 [0+𝛿 𝜋,30 ]= 𝛿 𝑇

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**Non-uniform Film Thickness Model**

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Other Film Shape

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**Interfacial Friction Factor**

Critical gas velocity calculated by Barnea’s model is conservative compared to other methods. Fore et al. showed that Wallis correlation is reasonable for small values of film thickness and is not suitable for larger film thickness liquid film. A new correlation is used in the new model : 𝑓 𝐼 = 𝑅𝑒 𝐺 ℎ 𝐷 −0.0015

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Turner’s Data 106 gas wells are reported in his paper, all of the gas wells are vertical wells. 37 wells are loaded up and 53 wells are unloaded. 16 wells are reported questionable in the paper. Current flow rate and liquid loading status of gas well are reported.

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**Turner’s Model Results Turner’s Data**

Vg < Vg,c Vg > Vg,c This graph shows a plot of boundary velocity calculated by the method vs. actual gas velocity at well head conditions. If boundary velocity is greater than observed velocity, slug flow is present; if less, annular flow is present. The prediction is quite reasonable with 90 % success.

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**Barnea’s Model Results Turner’s Data**

This graph shows a plot of boundary velocity calculated by the method vs. actual gas velocity at well head conditions. If boundary velocity is greater than observed velocity, slug flow is present; if less, annular flow is present. The prediction is quite reasonable with 90 % success.

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**New Model Results Turner’s Data**

This graph shows a plot of boundary velocity calculated by the method vs. actual gas velocity at well head conditions. If boundary velocity is greater than observed velocity, slug flow is present; if less, annular flow is present. The prediction is quite reasonable with 90 % success.

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Coleman’s Data 56 gas wells are reported, all of the wells are also vertical wells. These wells produce at low reservoir pressure and at well head pressures below 500 psi. Coleman reported gas velocity after they observed liquid loading in gas wells.

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**Turner’s Model Results Coleman’s Data**

Coleman examined the data for wells which were only at critical conditions. Which means every one of these wells were loading. As can be seen, the prediction of the new method is quite good in correctly predicting that all the wells are loading.

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**Barnea’s Model Results Coleman’s Data**

Coleman examined the data for wells which were only at critical conditions. Which means every one of these wells were loading. As can be seen, the prediction of the new method is quite good in correctly predicting that all the wells are loading.

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**New Model Results Coleman’s Data**

Coleman examined the data for wells which were only at critical conditions. Which means every one of these wells were loading. As can be seen, the prediction of the new method is quite good in correctly predicting that all the wells are loading.

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**Veeken’s Data Veeken reported offshore wells with larger tubing size.**

67 wells, which include both vertical and inclined wells, are presented. Similar to Coleman’s data, critical gas rate was reported. Liquid rate were not reported in the paper. We assumed a water rate of 5 STB/MMSCF. Liquid rate will not affect

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**Turner’s Model Results Veeken’s Data**

Bad prediction may be pipe diameter and inclination angle

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**Barnea’s Model Results Veeken’s Data**

Well deviation angle : Veekan’s data also only shows wells which are producing at critical conditions. Clearly, the method is not working as well and in many wells, the model is predicting annular flow.

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**New Model Results Veeken’s Data**

Well deviation angle : Veekan’s data also only shows wells which are producing at critical conditions. Clearly, the method is not working as well and in many wells, the model is predicting annular flow.

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**Chevron Data Production data:**

Monthly gas production rate Monthly water and oil production rate 82 wells have enough information to analyze liquid loading Two tubing sizes: and inch Get average gas and liquid production rate when cap string is installed from service history. Assume liquid loading occurred at this point.

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Production Data

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**Turner’s Model Results Chevron Data**

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**New Model Results Chevron Data**

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ConocoPhillips Data Daily production data and casing and tubing pressure data are available Select 62 wells including 7 off-shore wells Two tubing size: and inch Determine liquid loading by casing and tubing pressure divergence.

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**ConocoPhillips Field Data**

liquid loading starts Pc and Pt diverge Liquid Loading starts at 400 MCFD

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**Turner’s Model Results ConocoPhillips Data**

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**New Model Results ConocoPhillips Data**

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**Future Improvements Better interfacial fi correlation**

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**Improvements Liquid Entrainment Collection of 5” data**

Impact on the inception of liquid loading Collection of 5” data Pressure drop inspection for larger diameter pipes Incorporation of foam data in model

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Summary Liquid film reversal is the most appropriate model for defining liquid loading The effect of diameter on liquid loading is significant and is related to square root of diameter The film reversal can be detected either by observation of film or residual pressure drop

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Thank You! Questions…

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