A Conceptual Study of Finger-Type Slug Catcher for Heavy-Oil Fields J. Márquez, C. Manzanilla, and J. Trujillo, PDVSA Intevep
Summary The gas/liquid-separation processes in heavy- and extra-heavy-oil fields are performed mainly with gravity conventional separators. However, the separation efficiency of this equipment depends on the operating conditions, an appropriate design, and the properties of the fluids. Therefore, the separation efficiency is affected mainly by the high liquid viscosity, low pressures, and the low gas-flow rate that are present in heavy- and extra-heavy-oil fields. Additionally, these conditions increase the probability of slug-flow formation through pipelines, which causes operational problems, mainly in the separation process. This situation raises the need to design a gas/liquid separator able to handle viscous liquid, reduce the effects of slug flow, and perform an efficient separation. There are different technologies that can help improve gas/liquid separation, among them the finger-type slug catcher. This technology is usually used in gas fields or light- and medium-oil fields as a flow conditioner and slug-flow mitigator. However, this paper has changed that focus toward considering a heavy-oil design. This paper presents an improvement in the methodology of Sarica et al. (1990), predicting the dimensions of a finger-type slug catcher for heavy-oil fields. It is derived from the effect from the transition of stratified flow to nonstratified flow when the liquid phase is viscous and only considers slug-flow characteristics under normal flow; on the basis of the improvement, the required diameter and length of the finger are determined. This improvement is used to design a finger-type slug catcher for heavy-oil heavy-o il fields in the Orinoco belt. An economic comparison against conventional separators is presented, demonstrating the finger-type slug catcher to be more economical than the conventional separator. Introduction Multiphase flow through pipelines in heavy-oil fields is a daily occurrence, and the slug-flow pattern is promoted because of the high oil viscosity, the low pressure, and the low gas flow. In fact, the envelope of slug flow in a gas/liquid flow-pattern-transition map is increased with an increase in liquid viscosity or liquid-flow rate. Pipelines transport the production streams to the processing facilities where the separator is the i nitial receiving process device. Separators could be affected by severe operational conditions caused by slug flow. Therefore, it is necessary to use an adequate gas/liquid separator to mitigate the slug-flow effects and perform an efficient separation. Although several gas/liquid-separation technologies have been available for many years, a study was conducted to identify the more-suitable technologies for heavy-oil applications. This study concluded that conventional separators are widely used in heavyoil-field developments and that there are preseparators that can be used as flow conditioners to improve the separation. However, the preseparators also could be used as a primary separator. Among the found technologies, there are conventional separators, slug catchers, T-junctions, ultrasonic equipment, and others, but through a selection process the finger-type slug catcher was chosen to make a conceptual design that considers the viscous-liquid effects.
Copyright © 2010 Society of Petroleum Engineers This paper (SPE 122829) was accepted for presentation at the SPE Latin American and Caribbean Petroleum Engineering Conference, Cartagena, Bolivar, Colombia, 31 May to 03 June 2009, and revised for publication. Original manuscript received 19 February 2009. Revised manuscript received 23 December 09. Paper peer approved 20 January 2010.
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The traditional design of conventional separators is based on typical residence time, depending on the oil density, and does not consider the liquid-viscocity effects on the rise velocity of gas bubbles entrained in the liquid. Even when considering the bubble velocity, traditional design derived simplifications do not reproduce accurately the hydrodynamic behavior of the gas/liquid separation in the equipment when the liquid is viscous. The desired hydrodynamic behavior in a gas/liquid separator is achieved when the gas and liquid phases are stratified, but if slug flow arrives to the separator, the ideal situation is achieved when the effective area of flow is sufficiently increased to reduce the velocity and mitigate the slug affect. Thus, the inviscid Kelvin-Helmholtz (IKH) instability criterion enables identifying if stratified flow can exist under the given conditions, including the pipeline geometry. On the basis of the IKH criterion that was used by Taitel and Dukler (1976), which is widely used in the oil industry to predict the stratified/nonstratified transition, Sarica et al. (1990) proposed a design methodology for sizing a slug catcher. However, Howev er, according to Lin and Hanratty (1986) and Barnea (1990), the IKH criterion does not properly predict the transition when the liquid phase is viscous. On the other hand, Barnea and Taitel (1993) proposed a criterion called viscous Kelvin-Helmholtz (VKH) instability that considers the effects of the shear stress but not the interfacial-tension effects (i.e., the viscous effect is considered). This paper proposes an improvement to the original methodology of Sarica et al. (1990) to design a finger-type slug catcher. The improvement considers the use of the VKH instability criterion proposed by Barnea and Taitel (1993). In this way, the diameter of the finger-type slug catcher is determined to guarantee stratified flow through the device. This methodology was applied to design a slug catcher for heavy oils, and an economic comparison against conventional separators is presented.
Selection of Technology On the basis of a technological survey carried out by Manzanilla (2007), seven gas/liquid-separation technologies are identified that could work properly in a heavy-oil field. In the study, the following technologies were identified: conventional separator as the most used in heavy-oil fields, double-helix separator, auger separator, ultrasonic gas/liquid separator, T-junction, and slug catcher (conventional and finger-type). The general characteristics related to pros and cons of these technologies are given in the Table 1. Derived from this survey, a preselection of two technologies is made on the basis of the available technical information (Table 1). Therefore, four of the technologies found—double-helix, auger, T-junction, and ultrasonic—were rejected for the following reasons: • The design does not consider handling slug-flow conditions. • There is no experience in Venezuelan oil fields with the technology. • The technology is in the experimental development stage. • The technology operation is unstable. Thus, conventional separators and slug catchers were considered as the preselected technologies. The final selection inherent to these technologies was made using the criteria and solutions weighting method proposed by Vilchez (2008). Therefore, seven criteria and two solutions are considered. The criteria are prioritized below according to the level of importance. 1. Capacity to handle viscous liquids (C1) 2. Capacity to handle slug flow (C2) 3. Separation efficiency (C 3) 4. Residence time (C4) December 2010 SPE Projects, Facilities & Construction
TABLE 1—CHARACTERISTICS OF SEPARATION TECHNOLOGIES
Separator Type Conventional horizontal
Separation Principle
Separ ator Char acter istic s
Gravitational force
It is commonly used for low gas/liquid ratios and heavy-oil fields Great sizes and weight Retention time increased to high viscosity
Double helix
Shear and centr if ugal f or ces
Compact Technology in R&D stag e Separation efficiency is moderate
Auger
Centr if ugal f or ce
Compact Susceptible to erosion and plugging Separation efficiency is low Does not work in slug flow
Ultr asonic
Ultr asonic waves
Promotes bubbles agglomeration Generates mechanical vibration between 800 KHz and 20 KHz Could be promoted cavitation phenomenon Reqiures high control levels Does not work in slug flow Does not have experience in Venezuela
Juntion “T”
It is used as a preseparator device Dynamic separation process Compact Requires low control levels Separation efficiency is low
Finger-type slug catcher
Gravitational f or ce
Rows of tubes in par allel It is commonly used as preseparator Handles large amounts of liquid Few operational problems Requires low control levels It is designed to handle slug flow
Conventional slug catcher
Gravitational for ce
It is commonly used as pr esepar ator It can handle large amounts of liquid Few operational problems Requires low control levels It is commonly used in places with limited space It can be used as a gas-liquid-liquid separator It is designed to handle slug flow
5. Operational stability (C5) 6. Complexity of construction (C6) 7. Process control levels (C7) The criteria were evaluated hierarchically. The qualification range was given between 1 and 7. The most important criterion (C1) obtained the greater weighting (seven points), and the less important criterion (C7) received the minimum value. The corresponding weighting is shown in Table 2. The solutions are conventional separator (S1) and slug catcher (S2), and because only two solutions are considered, the numerical qualifying scale will range from 1 to 2. Thus, each criterion (C1 through C7) assesses for each solution, and the highest value (2) is given to the solution that meets this criterion better (Table 3).
Derived from the assessment in Tables 2 and 3, the final value of each probable solution—conventional separator (PS 1) and slug catcher (PS2)—was calculated through the following equations: PS1
= C2S1C + C 2S1C + ... + C 7S1C .
PS2
= C 2S2C + C2S 2C + ... + C 7S 2C . . . . . . . . . . . . . . . . . . . . . (2)
1
1
2
7
2
. . . . . . . . . . . . . . . . . . . . (1)
7
Now, the values in Tables 1 and 2 are substituted into Eqs. 1 and 2 to obtain the algebraic value of each solution: PS 1 = 7 * 2 + 6 *1 + 5 ⋅ 2 + 4 ⋅1 + 3 ⋅1 + 2 ⋅2 +1 ⋅1
= 42 ,
. . . . . . (3)
= 7 *1 + 6 ⋅2 + 5 ⋅ 2 + 4 ⋅2 + 3 ⋅2 + 2 ⋅1 +1 ⋅2 =47 .
. . . . . . (4)
TABLE 2—WEIGHTING OF CRITERIA
Criterion
Weighting
C1
7
C2
6
C3
5
C4
4
C5
3
C6
2
C7
1
December 2010 SPE Projects, Facilities & Construction
PS 2
According to the values of the probable solutions (PS 1 and PS2), the applicable solution is PS2 (i.e., the slug catcher is the best solution to perform the gas/liquid separation in heavy-oil fields). Relative to this solution, a conventional or a finger-type slug catcher could be selected. According to detailed research, a conventional slug catcher is more expensive than a finger-type slug catcher (Vergara and Foucart 2007) designed for the same operational conditions; therefore, the conventional slug catcher is discarded and the finger-type slug catcher is selected. 193
TABLE 3—ASSESSMENT OF SOLUTIONS BY EACH CRITERION
Cr iter ion Solution
(C1)
(C2)
(C3)
(C4)
(C5)
(C6)
(C7)
S1Cn
2
1
2
1
1
2
1
S2Cn
1
2
2
1
1
2
1
Finger-Type Slug Catcher. This slug catcher is defined as a gas/ liquid separator that performs primary separation of gas and liquid and is commonly installed at the end the production lines (i.e., as the first process equipment at flow stations). There are two main types of slug catchers—conventional and finger. Finger-type slug catchers function both as a gas/liquid separator and as a slug-flow mitigator. They are designed to stratify the gas and liquid phases and consist of four parts that are made mainly of pipeline materials (Fig. 1). The different areas are inlet header, separation, the gas- and liquid-outlet header, and they are shown in Fig. 1 and will be explained in the following. The inlet header takes the incoming gas/liquid main stream, reduces its mixture velocity, and splits the main stream into a number of smaller streams according to the size of the separation pipe or finger. The inlet header should allow uniform distribution of flow rate to the separation area. The separation consists of several pipes where gas/liquid separation is accomplished. Separation occurs because the diameter of pipes is made to ensure stratified flow into them. Required design of this part is a function of gas and liquid flow, fluid properties, and other operational conditions to be specified. The gas-outlet header gathers the separated gas to send it to downstream processes. The liquid-outlet header gathers the separated liquid to send it to downstream processes.
Design Methodology The proposed conceptual design for a finger-type slug catcher for heavy oil is based on the methodology proposed by Sarica et al. (1990). The design requires information related to the characteristics of slug flow determined at the slug-catcher inlet-pipeline conditions.
This also requires calculations of liquid accumulation to obtain the main dimensions of the equipment: diameter and length. The methodology assumes along with Sarica et al. (1990) the following: • When the liquid slug is produced, some of this liquid is spilled to the liquid film of the Taylor bubble because of the velocity difference between the slug and the Taylor bubble. For this reason, the volume of liquid accumulation is less than that calculated. • It is supposed that before producing the liquid slug, the operational holdup is the minimum. In other words, the liquid level into slug catcher is reduced when the Taylor bubble is produced. These two assumptions provide a safety factor to the design of the equipment that slightly increases the slug-catcher dimensions. Another consideration is that the fingers must be in a horizontal position because negative or positive inclinations generate waves of high amplitude, producing early liquid carryover, and under this condition, additionally, the criteria to predict the stratified/no stratified transition do not work properly (Barnea 1990; Barnea and Taitel 1994). The methodology is given for one finger, but can be adapted to more than one finger if the liquid distribution among the fingers is considered. Slug-Flow Characterization. A proper design of a slug catcher requires predicting the characteristics of slug flow at the catcher inlet with the least possible uncertainty. In this sense, a review and selection of the best mathematical models and correlations to predict these characteristics for heavy oil was made. The characteristics include slug holdup, slug length, slug velocity, translational velocity or Taylor-bubble velocity, and slug frequency and basic
Gas-outlet header Separated gas flow
Separated liquid flow
Liquid header
Gas-liquid flow Separation area
Inlet header Fig. 1—Finger-type slug catcher. 194
December 2010 SPE Projects, Facilities & Construction
−0.8606
LU LS
Re SL 0.0365 ⋅ Re SL + Re SG LS = v L ⋅ H LLS −1
LF Film zone
Slug zone
VGTB
VLLS
VGLS
HLTB
Fig. 2—Schematic for a slug unit (Colmenares et al. 2001).
parameters in multiphase flow. Some of these characteristics can be represented through a schema of a slug unit, such as that shown in Fig. 2. Selected mathematical models and correlations are described later. The slug holdup is the fraction of a volume element in the two-phase flow field occupied by the liquid phase in the slug zone (Fig. 2) and is determined according to the liquid viscosity. If the viscosity is less than 500 cp, the holdup is calculated using the Gregory et al. (1978) correlation as H LLS =
1 1.39
v 1 + M 8.66
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (5)
H LLS = 1.0046 ⋅ e
−( 0.0022⋅ ReL )
. . . . . . . . . . . . . . . . . . . . . . . . . . . . (6)
The holdup in the Taylor bubble (the film zone; see Fig. 2) is determined using the equation obtained by Shoham (2000) as H LTB
=
( vTB − v L ) ⋅ H LLS vTB
= 1 − H LLS .
vTB
= cvM + v D,
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .(11)
where c is derived according to the flow type. If the flow is laminar, = 2. If it is turbulent = 1.2. If flow lies between laminar and turbulent, the Xiao et al. (1990) correlation is used: c=
2.0 2
Re L 1+ ReCL
+
1.20 2
ReCL 1+ Re L
. . . . . . . . . . . . . . . . . . . . . . (12)
The drift velocity is the velocity of a phase relative to a surface moving at the mixture velocity and is calculated depending on the pipe inclination, as follows. For pipes slightly inclined, the drift velocity is estimated by the Bendiksen (1984) correlation as
= ( vD )horizontal cos ( ) + ( v D ) vertical sin ( ).
. . . . . . . . . . . . .(13)
The following correlation is used for horizontal pipes:
( v D )horizontal = 0.54
gD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (14)
And for vertical pipes,
( v D )vertical = 0.35
gD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (15)
The slug frequency is the rate of recurrence of the slug through the pipelines and is estimated using the correlation proposed by Colmenares et al. (2001) as f S =
vTB LU
, . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (16)
where slug unit length is given by
. . . . . . . . . . . . . . . . . . . . . . . . . .(7)
The gas-void fraction is the fraction of a volume element in the two-phase-flow field occupied by the gas phase in the slug zone. It is expressed using the following equation proposed by Beggs (1991): S
The next correlations require the estimations of velocities, such as mixture velocity and liquid, and gas superficial velocities. Also, prediction of the t ranslational velocity (Taylor-bubble velocity) and the drift velocity is required. Translational velocity is composed of a superposition of the bubble velocity in stagnant liquid (i.e., the drift velocity and the maximum velocity in the slug body), and it is given by the Nicklin (1962) correlation as
v D
The slug holdup for viscosity greater than 500 cp is determined using a correlation obtained in experiments carried out a 2-in.-testloop facility at PDVSA Intevep. Lubrication oil and air were the testing fluids, and lubrication-oil viscosities from 500 to 1,300 cp were used. Data of slug-flow characteristics are collected through a combination of high-speed video camera in the viewing section and fast signal for pressure drops and wall pressure fluctuations. The holdup is measured with a set of quick-closing valves. The correlation is based on the Shoham (2000) model, and it is given by
. . . . . . . . . . . . . . . . . . (10)
vSL
VTB
VLTB
.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (.8)
The film length of the slug unit (Fig. 2) is predicted by use of a correlation developed from experiments conducted with a 2-in.test-loop facility at PDVSA Intevep using lubrication oil (480 cp) and air as testing fluids. Slug-flow-characteristics data are acquired in the same manner as for Eq. 6. This correlation is based on the Taitel and Barnea (1990) model and is given by −0.8606
Re SL LF = 0.0365 ⋅ Re SL + Re SG
. . . . . . . . . . . . . . . . . . .(9)
The slug-length (Fig. 2) correlation is obtained by inserting LF into the correlation developed by Shoham (2000) to predict the film length. This correlation considers only hydrodynamic slug flow. December 2010 SPE Projects, Facilities & Construction
LU
= LS + LF .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (17)
Other parameters of interest for slug-flow characterization are the liquid and gas instantaneous flow at the inlet of the catcher. They are calculated using the Miyoshi et al. (1988) model. For the liquid, QinsL
= vMins A p H LLS .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (18)
And for the gas, QinsG
= vMins A p (1 − H LLS ).
. . . . . . . . . . . . . . . . . . . . . . . . . . (19)
Prediction of Liquid Accumulation. The liquid accumulation in the slug catcher is estimated by applying a liquid mass balance between the inlet and outlet of the equipment (Sarica et al. 1990), as
Liquid-input Liquid-discharge Liquid-accumulation mass rate − mass rate = mass rate . . . . . . . . . . . . . . . . . . . . . . . . (20) 195
TABLE 4—FIELD DATA
Lfinger
° API
16
Temperature (°F)
72–95
Pressure (psig)
105.80
QL (BPD)
14,343.55
QG (MMSFD)
8,556
BS&W (%)
42.65 0.55
G
Dp (in.)
10
The liquid-input mass rate is determined through the Miyoshi et al. (1988) model, to calculate the liquid instantaneous flow, whereas the liquid-discharge mass rate is related to the outlet liquid flow that depends on the flow-control-valve size. On the basis of the liquid mass balance presented by Sarica et al. (1990), the accumulated liquid volume is given as,
Vaccum
= tsp ⋅ Qacum =
LS max vTB
v M H LLS Ap − Qdis .
. . . . . . . . . . . (21)
Finger-Type-Slug-Catcher Sizing. The most important parameter in the slug-catcher design is the diameter of the slug-catcher fingers, which is calculated to obtain stratified flow. In this sense, models to predict the transition from slug flow to stratified flow are necessary, such as the I KH instability criterion, the VKH instability criterion, and the Taitel and Dukler (1976) model. To predict the transition, Sarica et al. (1990) used the IKH criterion presented by Taitel and Dukler (1976). However, in this work it is proposed to use the VKH criterion presented by Barnea and Taitel (1993) to determine the transition from slug flow to stratified flow because it better predicts transition for a larger range of viscosities (100 to 5,000 cp). The criterion is expressed as 1 / 2
v Gtran
L − G AP ≥ K V ( L RG + G RL ) g L G ∂ A L ∂h L
.
. . . . . . . . (22)
In this expression, K V is a correction factor given as
K V = 1 −
(CV − C IV )
L
− G
2
A g ⋅ cos ⋅ P d A L
.
. . . . . . . . . . . . . . . . . . . (23)
dh L
The VKH criterion provides minimum diameter from which stratification is obtained. When the actual gas velocity is less than the transition gas velocity, stratified flow is expected. Thus, the catcher diameter should be bigger than the minimum diameter to receive the incoming liquid. The catcher diameter is determined by increasing minimum diameter until stratified flow into the equipment is ensured. Also, one must consider the incoming-liquid flow, available space for installation, and the costs. For a given gas superficial velocity, there is a transition liquid holdup and an operation liquid holdup. The first is given by the maximum liquid superficial velocity for stratified flow and is calculate using the VKH criterion. The second is given by the average operation flow rates of liquid and gas at the slug catcher. The difference between these two holdups will provide the available volume to handle the accumulation of liquid in the slug catcher; thus, the catcher length for the designed diameter is given as 196
=
V accum Afinger H L trans − H L oper
.
. . . . . . . . . . . . . . . . . . . . . (24)
Results and Discussion The proposed methodology is used to design a finger-type slug catcher for a heavy-oil field in the Orinoco belt. The designed catcher was compared economically with the design of a conventional horizontal separator that is commonly used in the field. The field data for the design are given in Table 4. The calculated procedure to design the catcher using the methodology presented in the preceding is explained in following. Procedure To Design a Finger-Type Slug Catcher. To obtain the main dimensions of the finger-type slug catcher, a computational procedure is elaborated that is derived from the equations presented previously. 1. As input data are required operational conditions, properties of the fluid and the geometry of the inlet pipeline are required, such as temperature, pressure, API gravity, gas- and liquid-flow rate, and pipeline diameter and roughness (Table 4). 2. The gas-liquid-flow pattern into the inlet pipeline of the slug catcher is determined using the model proposed by Barnea and Taitel (1993). In this sense, a gas/liquid flow-pattern map is generated for the operational conditions given in Table 4. In the map, the only areas represented are intermittent flow, annular flow, and stratified flow. According to Fig. 3, the flow pattern in the inlet line (pipe diameter is 10 in.) of the slug catcher is located in the intermittent-flow area as a yellow point called “operation point” (pipeline). 3. As detailed in the mapping (Fig. 3), the flow pattern is slug flow. Therefore, a slug-catcher design is necessary. Without slug flow, of course, t he slug catcher is not necessary. The procedure to determine the main dimensions of the slug catcher (diameter and length of fingers) is the following. a. Slug-flow characteristics are calculated according to correlations and models discussed in this paper (Eqs. 5 through 17). b. The diameter and quantity of fingers are assumed such that diameter must be large enough that the quantity of fingers must be greater than one. In this scenario, four fingers are considered and an initial diameter of 10 in. c. Gas- and liquid-flow rates through each finger are determined considering an even flow distribution among the fingers. d. The finger diameter is determined using the VKH criterion through an iterative process in which the finger diameter is increased until obtaining a stratified flow. Thus, minimum diameter is obtained when the transition curve between stratified flow and intermittent flow is reached. The point that represents this condition is shown superimposed on the transition curve in the Fig. 4 (green point), and it is called “transition point.” The minimum diameter is increased to the next pipeline commercial diameter to guarantee stratified flow. In this scenario, the operation point (blue point) of the finger-type slug catcher is shown in Fig. 4, which corresponds to a finger-design diameter calculated at 20 in. e. The catcher length is determined by use of the accumulated liquid volume and calculated diameter. In this scenario, the length is 26.2 ft. f. Finally, the catcher weight is determined considering all pipe sections, such as inlet header, separation area, gas-outlet header, and liquid-outlet header. The finger-type slug catcher weight is 7,887 lbm. Economic Comparison. This subsection presents an economic comparison between the designed catcher and a conventional horizontal separator that is commonly used in field applications and is sized under the same conditions as those of the catcher. It is necessary to clarify the cost estimation. It should be used only as a reference point because it is made for a conceptual design. Cost estimation is based on fabrication cost because operation costs are considered comparable because they operate under the same working principle (gravitational sedimentation), use the December 2010 SPE Projects, Facilities & Construction
10
Intermittent
] s / t f [
1 Annular
L s V
Stratified
0.1
VKH
HL/D 0.5
Operation point
=
0.01 1
0.1
10
100
VsG [ft/s] Fig. 3—Flow-pattern map for the inlet conditions of the catcher.
same process-control systems, and will require regular cleaning because of solids accumulation. Therefore, the fabrication cost of the catcher and conventional separator is estimated relating to the vessel weight and steel price, as Powers (1990) proposed: Cost estimated= Weight ( kg ) ⋅ Steel price (USD kg ).
According to Table 5, the fabrication cost of the finger-type slug catcher is 23% less than that of the horizontal conventional separator.
Conclusions On the basis of the methodology of criteria and solutions weighting proposed by Vilchez (2008), the finger-type slug catcher was selected and designed as the separation technology for heavy-oil fields. The slug-flow characteristics must be known to carry out a proper design for a finger-type slug catcher. Thus, various correlations and models are selected in a rigorous manner to predict slug-flow characteristics for heavy oil.
. . . . . . . . (25)
The horizontal conventional separator’s diameter is 72 in., and its length is 20 ft. The separator weight is approximately 10,214 lbm. Steel price is considered approximately 30 USD. Using this referenced price and Eq. 25, the cost of each separator is estimated (Table 5).
10
Intermittent Annular ] s / t f [
1
L s V
Stratified
0.1
VKH
HL/D 0.5
Operation point
Transition point
=
0.01 1
0.1
10
100
VsG [ft/s] Fig. 4—Flow-pattern map for the designed slug catcher. December 2010 SPE Projects, Facilities & Construction
197
TABLE 5—FABRICATION-COST ESTIMATION
Separator Type
Weight (lbm)
Cost (USD)
Finger -type slug catcher
7,887.00
235,818.00
Conventional separator
10,214.00
305,382.00
This work proposed an improvement on the Sarica et al. (1990) methodology on the basis of the use of the VKH criterion to predict the stratified/no-stratified transition in a more rigorous way to determine the dimensions of a finger-type slug catcher to handle viscous liquids. This improvement allows performing a better design of the slug catcher to guarantee the segregation and separation of the phases while slug mitigation is achieved for a heavy-oil field. Economic comparison demonstrates that the slug catcher costs approximately 23% less than the conventional separator. Therefore, it could be used as a separator in heavy-oil fields. However, further studies conducted in laboratory-scale tests and in fluid-dynamics simulations have to be conducted before field applications are feasible.
Nomenclature
= API gravity = cross-sectional area, m2 = wave velocity = slug frequency, slugs/s = gravity acceleration, m/s2 = liquid holdup in the slug zone = liquid holdup in the film zone (Taylor Bubble zone) = coefficient of stability = Length, m = flow rate, m3 /s = Reynolds number = time, s ν = velocity, m/s V = volume, m3
°API A C f s g H LLS H LTB K v L Q Re t
Subscripts accum = accumulation D = drift dis = discharge f = liquid film (Taylor bubble zone) G = gas ins = instantaneous IV = inviscid M = mixture gas-liquid max = maximum oper = operational p = pipe S = slug zone sG = superficial gas sL = superficial liquid TB = translational or Taylor-bubble zone trans = transition U = slug unit V = viscous Greek Letters = specific gravity = inclination angle = density [kg/m3]
Acknowledgment The authors want to express our gratitude to Joe Bradford and Maite Bradford (Gazprom Latin America) and Alexis Gammiero 198
(PDVSA Intevep) for their help and collaboration in the structuring of this work.
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José Márquez is currently a research and development support engineer at PDVSA Intevep, Los Teques, Venezuela. He joined PDVSA Intevep in 2002 and has been working on the development of phase-separation technologies and multiphase-flow modeling. He works for the flow assurance and phase separation research and development project of PDVSA Intevep, focusing on multiphase technologies for heavy-oil applications. Also, he works in the engineering of surfaces facilities of a pilot project of in-situ combustion. He holds an ME degree from
December 2010 SPE Projects, Facilities & Construction
Universidad de Los Andes (ULA), Venezuela, and an MSc degree from Universidad Central de Venezuela. Carlos Manzanilla is a mechanical maintenance supervisor for Nestle of Venezuela. He worked for PDVSA Intevep in the flow assurance and phase separation research and development project from 2007 to 2008. Jorge Trujillo is currently a research and development associate engineer at PDVSA Intevep, Los Teques, Venezuela. He joined PDVSA Intevep in 2002 and since then has been working on the development of high capacity/high efficiency phase-separation technologies and multiphase-flow modeling. He leads the flow assurance and phase separation research and development project of PDVSA Intevep, focusing on multiphase technologies for heavy-oil applications. He holds an ME degree from Universidad Nacional Experimental Politecnica, Venezuela, and an MSc degree from La Universidad del Zulia, Venezuela.
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