Liquid Loading
Current Status, New Models and
Unresolved Questions
Mohan Kelkar and Shu Luo
The University of Tulsa
Foam Flow Meeting, January 23, 2014
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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
Transition
Point
Liquid Loading
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Traditional Definition
• As gas flow rate increases

𝑑(∆𝑝𝑔 )
𝑑(𝑉𝑔 )
and
𝑑(∆𝑝𝑓 )
𝑑(𝑉𝑔 )
• At low velocities
increase in
𝑑(∆𝑝𝑔 )
𝑑(∆𝑝𝑓 )
𝑑(𝑉𝑔 )
decreases faster than
𝑑(𝑉𝑔 )
• 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)
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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
1/2
𝜌𝐺
∗
𝑢𝐺 = 𝑢𝐺
𝑔𝑑 𝜌𝐿 − 𝜌𝐺 1/2
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Liquid Film Reversal
Westende et al., 2007
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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?
𝑢𝐺∗ = 𝑢𝐺
1/2
𝜌𝐺
𝑔𝑑 𝜌𝐿 − 𝜌𝐺 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
25
uG, m/s
20
15
10
5
0
1
1.2
1.4
1.6
1.8
2
2.2
d1/2
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Why diameter impacts?
Film thickness?
0.0006
0.001
0.0005
0.0008
0.0004
0.0003
δ [m]
δ [m]
0.0006
0.0004
0.0002
0.0002
0.0001
0
15.0
20.0
25.0
30.0
0
15.0
20.0
30.0
vSG [m/s]
vSG [m/s]
ID=2in
25.0
ID=4in
(a) vSL=0.01 m/s
ID=2in
ID=4in
(b) vSL=0.05 m/s
Skopich et al., SPE 164477
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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:
𝑣𝐺,𝑇
•
𝜎 𝜌𝐿 − 𝜌𝐺
= 6.558
𝜌𝐺2
0.25
This equation is adjusted upward by
approximately 20 percent from his
original equation in order to match his
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 )
𝑣𝐺,𝑇
•
•
•
𝜎 𝜌𝐿 − 𝜌𝐺
= 5.465
𝜌𝐺2
0.25
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.
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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
•
•
𝜏𝐼 𝑆𝐼
•
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:
1
1
𝑆𝐿
+
− 𝜏𝐿
− 𝜌𝐿 − 𝜌𝐺 𝑔 sin 𝜃 = 0
𝐴𝐿 𝐴𝐺
𝐴𝐿
Interfacial shear stress with Wallis
correlation:
2
1
𝑣𝑆𝐺
𝜏𝐼 = 𝑓𝐼 𝜌𝐺
2
(1 − 2𝛿)4
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Schematic of Annular Flow
34
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.
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Transition
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Model Formulation
•
In inclined wells, the film thickness is expected to vary
with radial angle
Vertical Well
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Inclined Well
36
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.
1
𝛿𝑐 = [𝛿 0, 𝜃 + 𝛿 𝜋, 𝜃 ]
2
If film thickness reaches maximum at 30 degree
inclination angle
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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
𝜹 𝜱, 𝜽 =
𝜽
𝒔𝒊𝒏 𝜱 − 𝟗𝟎 + 𝟏 𝜹𝒄
𝟑𝟎
𝑭𝒐𝒓 𝜽 > 𝟑𝟎 𝒅𝒆𝒈𝒓𝒆𝒆
𝜹 𝜱, 𝜽 = 𝒔𝒊𝒏 𝜱 − 𝟗𝟎 + 𝟏 𝜹𝒄
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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).
1
1
𝛿𝑐 = [0 + 𝛿 𝜋, 30 ] = 𝛿𝑇
2
2
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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 :
𝑓𝐼 = 0.005 1 + 300
Foam Flow Meeting, January 23, 2014
1+
17500 ℎ
− 0.0015
𝑅𝑒𝐺 𝐷
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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
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Vg > Vg,c
46
Barnea’s Model Results
Turner’s Data
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New Model Results
Turner’s Data
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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
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Barnea’s Model Results
Coleman’s Data
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New Model Results
Coleman’s Data
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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.
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Turner’s Model Results
Veeken’s Data
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Barnea’s Model Results
Veeken’s Data
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New Model Results
Veeken’s Data
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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: 1.995 and 2.441 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: 1.995 and 2.441 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
 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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