Presentation on theme: "Liquid Loading Current Status, New Models and Unresolved Questions"— Presentation transcript:
1 Liquid Loading Current Status, New Models and Unresolved Questions Mohan Kelkar and Shu LuoThe University of Tulsa
2 Outline Definition of liquid loading Literature Survey Our Data Model FormulationModel ValidationProgram DemonstrationSummary
3 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
4 Traditional Definition IPRStableOPRUnstableTalk about the importance of identifying liquid loading and necessary measurementsTransition PointLiquid Loading
5 Traditional Definition As gas flow rate increases𝑑( ∆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
6 Definition Based on Mechanisms Two potential mechanisms of transition from annular to slug flowDroplet reversalFilm ReversalModels are either based on droplet reversal (Turner) or film reversal (Barnea)2 mechanisms: droplet and film reversalDroplet model determine liquid loading when droplet falls backFilm model determine liquid loading when liquid film falls back
7 Literature Data Air-water data are available The data reported is restricted to 2” pipeVery limited data are available in pipes with diameters other than 2”No data are available for other fluids
8 Generalized Conclusions (2” pipe) Minimum pressure drop for air-water flow occurs at about 21 m/sThe liquid film reversal starts at around 15 m/sThe dimensionless gas velocity is in the range of 1.0 to 1.1 at minimum point𝑢 𝐺 ∗ = 𝑢 𝐺 𝜌 𝐺 1/2 𝑔𝑑 𝜌 𝐿 − 𝜌 𝐺 1/2
15 Air-Water FlowSkopich and Ajani conducted experiments in 2” and 4” pipesThe results observed are different based on film reversal and minimum pressure drop – consistent with literatureHowever, the experimental results are very different for 2” versus 4” pipe
16 Calculation Procedure Total pressure drop is measured and gradient is calculatedHoldup is measured and gravitational gradient is calculatedSubtracting gravitational pressure gradient from total pressure gradient to get frictional pressure gradientBy dividing the incremental pressure gradient by incremental gas velocity, changes in gravitational and frictional gradients with respect to gas velocity are calculated.
17 dPG vs. dPF Air-Water, 2 inch, vsl=0.01 m/s Minimum
18 Total dp/dz Air-Water, 2 inch, vsl=0.01 m/s Film Reversal
19 dP/dz)G vs. dP/dz)F Air-Water, 2 inch, vsl=0.01 m/s dp/dz)F is zero
20 dPT - dPG Air-Water, 2 inch, vsl=0.01 m/s Transition at 16 m/s
21 Pressure at Bottom Air-Water, 2 inch, vsl=0.01 m/s Pressure build upNo pressure build up
22 dP/dz)G vs. dP/dz)F Data from Netherlands (2 inch) dp/dz)F is zero
23 What should we expect for 3” or 4” pipeline? 𝑢 𝐺 ∗ = 𝑢 𝐺 𝜌 𝐺 1/2 𝑔𝑑 𝜌 𝐿 − 𝜌 𝐺 1/2Based on the above equation, the minimum should shift to right as diameter increasesIf the above equation is correct, the ratio of uG/√d at unstable point should be constant
24 dPG vs. dPF Air-Water, 4 inch, vsl=0.01 m/s Minimum
25 Total dp/dz Air-Water, 4 inch, vsl=0.01 m/s Film Reversal
26 dP/dz)G vs. dP/dz)F TUFFP (3 inch, vsl=0.1 m/s) dp/dz)F is zero
27 dP/dz)G vs. dP/dz)F Air-Water, 4 inch, vsl=0.01 m/s dp/dz)F is zero Film reversal
29 Why diameter impacts? Film thickness? Skopich et al., SPE
30 Liquid Loading Definition Liquid loading starts when liquid film reversal occursWe adopt the model of film reversal to predict inception of liquid loadingThe 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.
31 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 𝐺Turner 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.
32 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 𝐺most 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.
33 Approach Film ModelTwo 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.
34 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 =0𝜏 𝐼 = 1 2 𝑓 𝐼 𝜌 𝐺 𝑣 𝑆𝐺 2 (1−2𝛿) 4Schematic of Annular Flow
35 Approach Barnea’s Model Solid curves represent Interfacial shear stress from combined momentum equationBroken curves represent Interfacial shear stress from Wallis correlationIntersection of solid and broken curves yields a steady state solution of film thickness and gas velocity at transition boundaryAnother transition mechanism is liquid blocking of the gas core.Two mechanisms for blockage of coreInstability of annular flow that prevents a stable annular configurationLiquid film gets large enough to create spontaneous blockageFor low liquid rates, transition occurs due to the first mechanism, whereas, for high liquid rates, transition occurs due to second mechanismBoth mechanisms are checked for transition to slug flowTransition
36 Model FormulationIn inclined wells, the film thickness is expected to vary with radial angleVertical WellInclined Well
37 Original Barnea’s Model at Different Inclination Angles
39 Non-uniform Film Thickness Model Let A1=A2, we can find this relationship.If film thickness reaches maximum at 30 degree inclination angle𝛿 𝑐 = 1 2 [𝛿 0,𝜃 +𝛿 𝜋,𝜃 ]
40 Non-uniform Film Thickness Model We will use the following film thickness equation in the new model:𝑭𝒐𝒓 𝟎≤𝜽≤𝟑𝟎 𝒅𝒆𝒈𝒓𝒆𝒆𝑭𝒐𝒓 𝜽>𝟑𝟎 𝒅𝒆𝒈𝒓𝒆𝒆𝜹 𝜱,𝜽 = 𝜽 𝟑𝟎 𝒔𝒊𝒏 𝜱−𝟗𝟎 +𝟏 𝜹 𝒄𝜹 𝜱,𝜽 = 𝒔𝒊𝒏 𝜱−𝟗𝟎 +𝟏 𝜹 𝒄
41 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
44 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
45 Turner’s Data106 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.
46 Turner’s Model Results Turner’s Data Vg < Vg,cVg > Vg,cThis 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.
47 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.
48 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.
49 Coleman’s Data56 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.
50 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.
51 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.
52 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.
53 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
54 Turner’s Model Results Veeken’s Data Bad prediction may be pipe diameter and inclination angle
55 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.
56 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.
57 Chevron Data Production data: Monthly gas production rateMonthly water and oil production rate82 wells have enough information to analyze liquid loadingTwo tubing sizes: and inchGet average gas and liquid production rate when cap string is installed from service history. Assume liquid loading occurred at this point.
61 ConocoPhillips DataDaily production data and casing and tubing pressure data are availableSelect 62 wells including 7 off-shore wellsTwo tubing size: and inchDetermine liquid loading by casing and tubing pressure divergence.
62 ConocoPhillips Field Data liquid loading startsPc and Pt divergeLiquid Loading starts at 400 MCFD
65 Future Improvements Better interfacial fi correlation
66 Improvements Liquid Entrainment Collection of 5” data Impact on the inception of liquid loadingCollection of 5” dataPressure drop inspection for larger diameter pipesIncorporation of foam data in model
67 SummaryLiquid film reversal is the most appropriate model for defining liquid loadingThe effect of diameter on liquid loading is significant and is related to square root of diameterThe film reversal can be detected either by observation of film or residual pressure drop