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    SPE 103255

    Optimizing the Productivity of Gas/Condensate WellsC. Shi, R.N. Horne, and K. Li, Stanford U.

    Copyright 2006, Society of Petroleum Engineers

    This paper was prepared for presentation at the 2006 SPE Annual Technical Conference andExhibition held in San Antonio, Texas, U.S.A., 2427 September 2006.

    This paper was selected for presentation by an SPE Program Committee following review ofinformation contained in an abstract submitted by the author(s). Contents of the paper, aspresented, have not been reviewed by the Society of Petroleum Engineers and are subject tocorrection by the author(s). The material, as presented, does not necessarily reflect anyposition of the Society of Petroleum Engineers, its officers, or members. Papers presented atSPE meetings are subject to publication review by Editorial Committees of the Society ofPetroleum Engineers. Electronic reproduction, distribution, or storage of any part of this paperfor commercial purposes without the written consent of the Society of Petroleum Engineers isprohibited. Permission to reproduce in print is restricted to an abstract of not more than300 words; illustrations may not be copied. The abstract must contain conspicuous

    acknowledgment of where and by whom the paper was presented. Write Librarian, SPE, P.O.Box 833836, Richardson, TX 75083-3836, U.S.A., fax 01-972-952-9435.

    AbstractGas-condensate reservoirs exhibit complex phase and flow

    behaviors due to the appearance of condensate banking in the

    near-well region. A good understanding of how the condensateaccumulation influences the productivity and the composition

    configuration in the liquid phase is very important to optimize

    the producing strategy, to reduce the impact of condensate

    banking, and to improve the ultimate gas recovery.

    This study addressed several issues related to the behavior of

    the composition variation, condensate saturation build-up and

    condensate recovery during the gas-condensate producingprocess. A key factor that controls the gas-condensate well

    deliverability is the relative permeability, which is influenced

    directly by the condensate accumulation. The accumulatedcondensate bank not only reduces both the gas and liquid

    relative permeability, but also changes the phase composition

    of the reservoir fluid, hence reshapes the phase diagram of

    reservoir fluid and varies the fluid properties. Different

    producing strategies may impact the compositionconfiguration for both flowing and static phases and the

    amount of the liquid trapped in the reservoir, which in turn

    may influence the well productivity and hence the ultimate gas

    and liquid recovery from the reservoir. Changing the manner

    in which the well is brought into flowing condition can affectthe liquid dropout composition and can therefore change the

    degree of productivity loss.

    In this study, compositional simulations of multicomponent

    gas-condensate fluids were conducted at field scale to

    investigate the composition and condensate saturation

    variations. Different producing strategies have been compared,and the optimum producing sequences are suggested for

    maximum gas recovery. A core flooding experiment with two-

    component synthetic gas-condensate was designed andconstructed to model gas-condensate production behavior

    from pressure above the dew-point to below. Experimental

    observations of gas-condensate production confirm the

    dramatic changes in the liquid composition seen in the

    simulations.

    IntroductionLiquid forms in a gas-condensate reservoir when the bottom-

    hole pressure drops below the dew-point pressure. The

    accumulated condensate in the vicinity of the well bore causea blockage effect and reduces the effective permeability

    appreciably, and also causes the loss of heavy components atsurface. These effects depend on a number of reservoir andwell parameters.

    The productivity loss caused by the condensate buildup is

    striking. In some cases, the decline can be as high as a factor

    of two to four, according to the case studies of Afidick et al.and Barnum et al.2. Even in very lean gas-condensate

    reservoirs with a maximum liquid drop out of only 1%, the

    productivity may be reduced by a factor of about two as thepressure drops below the dew-point pressure1. In order to

    predict well deliverability and calculate gas and liquid

    recovery, it is necessary to have a detailed knowledge of liquid

    banking in gas-condensate fields.

    Fevang and Whitson3 addressed the well deliverability

    problem in their gas-condensate modeling, where they

    observed that well deliverability impairment resulting from

    near well-bore condensate blockage depends on PVTabsolute and relative permeabilities, and how the well is being

    produced.

    The relative permeability effect has been reported in field

    observations. Variations of reservoir fluid PVT properties a

    discovered condition have been observed and discussed formany reservoirs around the world (for example reference 4 for

    mid-eastern reservoirs and reference 5 for North Seareservoirs). Lee6 also presented an example to show the

    variation of composition and saturation of the gas-condensate

    system due to the influences of capillary and gravitationaforces.

    Roussennac7illustrated the phase change during the depletionin his numerical simulation. According to Roussennac, during

    the drawdown period, with the liquid building up in the well

    grid cell, the overall mixture in that cell becomes richer in

    heavy components, and the fluid behavior changes from theinitial gas-condensate reservoir to that of a volatile/black oil

    reservoir.

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    2 SPE 103255

    The well producing scheme may impose significant impacts

    on PVT properties. However, the manner by which the

    producing scheme influences the PVT properties has not yetbeen sufficiently addressed. This study aims to investigate the

    producing strategy and its influences on productivity and

    composition.

    Compositional simulations of a multicomponent gas-condensate system and a binary-component gas-condensate

    system were conducted here. To confirm the compositionalvariations resulting from producing strategy, a core flood

    experiment has also been designed and constructed to

    investigate the gas-condensate flow behavior in porous media.

    The simulation models, the experimental apparatus and theprocedures are described here. Following that, the simulation

    results and the experimental results are presented. Finally,

    some conclusions are drawn.

    Simulation models

    Multicomponent simulation model

    The primary objective of the simulation was to understand the

    impact of producing scheme on the condensate banking and

    compositional variations. A hypothetic cylindrical reservoir

    model, with radius of 5200 ft and permeability-thickness of 20

    md50 ft has been chosen, and a simulator E300 (2005a,Eclipse) with fully implicit (FULLIMP) method was used to

    simulate the performance under different producing strategies.

    The multicomponent fluid properties are shown in Table 1.Additional laboratory liquid dropout data were used to

    correlate with equation of state (EOS) phase-behavior

    calculation. The PVT program used in this study was PVTi byGeoquest. The Modified Peng-Robinson EOS was used to

    perform the fluid characterization. Figure 1 shows the liquiddropout calculation from a tuned EOS, which matches well

    with the measured data. Figure 2 shows the phase envelope for

    this multicomponent gas-condensate system. The EOSpredictions were then used as the input to the simulator.

    Table 1: Fluid composition of gas-condensate

    Component

    N2CO2C1C2C3

    iC4nC4iC5nC5C6C7C8C9

    C10+C10+MW

    C10+density (g/cm3)

    Fluid (mol%)

    0.00850.0065

    0.8358

    0.0595

    0.0291

    0.00450.0111

    0.0036

    0.0048

    0.00600.0080

    0.0076

    0.00470.0103

    183

    0.8120

    0.5 1 1.5 2 2.5 3

    x 107

    0

    0.5

    1

    1.5

    2

    2.5

    3

    3.5

    4

    Pressure (MPa)

    Liqu

    idd

    ropout

    liquid dropout (simulated)

    liquid dropout (experimental)

    Figure 1: Liquid dropout from constant volume depletion (CVD)experiment.

    Figure 2: Phase diagram of a multicomponent condensatesystem.

    In the simulations, small radii around the well-bore were

    chosen to allow for accurate pressure drop calculation in the

    near well-bore region.

    In porous media, PVT properties are controlled by the in-situreservoir temperature, pressure and the porous media

    properties. In this study, no temperature change has been

    considered. Hence, the PVT properties are determined by the

    in-situ reservoir pressure and the way the heavy components

    accumulate. In order to investigate how the producing strategyinfluences the condensate blockage and hence, the final gas

    recovery, two sets of simulations were conducted, one with

    fixed bottom-hole pressure (BHP) strategy with different BHPsettings and the other with varying BHP as a function of time,.

    Binary-component simulation model

    To investigate the composition and saturation change resultingfrom the producing scheme, a binary-component gas-

    condensate system was selected to conduct the core flooding

    experiment. Figure 3 shows the phase diagram of the C1/C4(85%/15%) synthetic gas-condensate system. This system haslow critical temperature (Tc = -13.2 C) and critical pressure

    (pc= 1760 psi), which makes the experiment easy to perform

    under room temperature and within relatively low pressure

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    4 SPE 103255

    These four sampling points are shown in Figure 3. All gas

    samples were collected in the sampling bags and sent to the

    gas chromatograph for composition analysis.

    Results and Discussion

    Experiment results

    Figure 5 shows the gas chromatograph results for all the gassamples. Notice that the first batch of gas samples, which were

    collected before the flow test, show slightly differentcomposition for the same component (C1 or C4) at different

    sample ports and these compositions also differ slightly from

    the initial compositions. The samples taken at the sample port

    2 and 6 are the only two samples exactly equal to the initialcompositions. Sample 1, 3 and 4 show higher C4percentage

    and sample 5 shows lower C4 percentages compared to the

    initial 19% C4percentage. This may be due to the fact that the

    core was presaturated with pure methane at pressure 1,800 psi.

    When the upstream pressure drops, more gas condensate drops

    out into the core, and the accumulated condensate liquid,which is richer in heavier component, can not flow until the

    condensate saturation reaches the threshold saturation on the

    relative permeability curve. Hence the flowing phase consists

    of lighter component; this is confirmed by the composition

    decrement in C4component in the second and the third flowtest.

    When the core system pressure drops below the pressure

    corresponding to the maximum liquid drop-out point, thecondensate starts to revaporize. A higher percentage of heavier

    component was expected to be seen at this stage. This is

    confirmed by the composition results from the last batchsamples (batch 4), where the sampling pressure was only 61.5

    psi and the C4composition is as high as 57.5%. At this pointin the experiment, the accumulated heavy component was

    revaporized and recovered.

    0

    10

    20

    30

    40

    50

    60

    70

    1 2 3 4 5 6

    Port

    C4(%)

    Batch 1

    Batch 2

    Batch 3

    Batch 4

    Flow direction

    original

    composition

    0

    10

    20

    30

    40

    50

    60

    70

    1 2 3 4 5 6

    Port

    C4(%)

    Batch 1

    Batch 2

    Batch 3

    Batch 4

    Flow direction

    original

    composition

    Figure 5: Gas sample results for the mole fraction of C 4 in theflowing phase.

    Multicomponent simulation results

    In the field scale simulation results, Figure 6 shows the

    condensate saturation profiles vs. radius rfor different times.

    The region of interest here is the two-phase zone. As expected

    as the production proceeds, the pressure-drop expands to

    regions further away from the producing well. Once thepressure drops below the local dew-point pressure, condensate

    drops into the reservoir and accumulates until the accumulated

    liquid saturation reaches the relative permeability threshold

    From the figure, we can also see that the near well region has

    the greatest liquid accumulation resulting from the early liquiddrop-out.

    Figure 7 shows mole fractions of C7for the liquid phase. The

    trend is similar to that of saturation profiles, noticing that the

    heavy component (C7, in this case) accumulation is more

    prominent in the near well region.

    0

    0.1

    0.2

    0.3

    0.4

    0.5

    0.6

    0.7

    0.1 1 10 100 1000 10000

    Radius r (ft)

    S

    c(fraction)

    t = 0.5 day t = 1.5 days t = 102 days t = 206 days t = 345 days

    t = 575 days t = 670 days t = 755 days t = 840 days t = 900 days

    t = 940 days t = 980 days t = 990 days t = 1000 days

    increasing producing time

    Figure 6: Saturation profiles at different times.

    0

    0.01

    0.02

    0.03

    0.04

    0.05

    0.06

    0.1 1 10 100 1000 10000

    Radius r (ft)

    C7

    inliquidphase(fra

    ction)

    t = 0.5 day t = 1.5 days t = 102 days t = 206 days t = 345 days

    t = 575 days t = 670 days t = 755 days t = 840 days t = 900 days

    t = 940 days t = 980 days t = 990 days t = 1000 days

    increasing producing time

    Figure 7: Mole fraction profiles of C7in liquid phase.

    a) Strategy of fixed BHP

    In a PVT cell, the liquid drop-out from the gas-condensate

    system can be revaporized if we either lower the BHP orincrease the BHP. However, in a porous medium, the

    liquid drop-out is immobile unless the liquid accumulation

    reaches the critical condensate saturation value on the

    relative permeability curve. The accumulated condensate isgenerally made up of heavier components and hence

    changes the local phase composition. Whether the

    condensate build-up can be revaporized is mainlydetermined by the local fluid composition. Figure 8 shows

    the saturation profile for different well BHPs. We can see

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    SPE 103255 5

    that the liquid saturation still accumulates as the BHP

    drops, and no revaporization appears to happen for this

    particular fluid system.

    As the BHP drops, more C7, one of the heavy components,

    drops to the liquid phase (Figure 9). Although the total gas

    production (Figure 10) increases as the BHP decreases, the

    well productivity (Figure 11) drops dramatically as liquidsaturation builds up.

    Figure 12 and Figure 13 show that as the BHP decreases,

    the two-phase region expands, and more heavy-

    component will be left in the reservoir.

    0

    0.1

    0.2

    0.3

    0.4

    0.5

    0.6

    0.7

    0 100 200 300 400 500 600 700 800 900 1000

    Time (days)

    SC

    (fraction)

    BHP = 2000 psi BHP = 500 psi BHP = 1000 psi BHP = 1500 psi

    BHP = 2500 psi BHP = 3000 psi BHP = 3500 psi BHP = 4000 psi

    decreasing BHP

    Figure 8: Condensate saturation profile for different BHP.

    0

    0.01

    0.02

    0.03

    0.04

    0.05

    0.06

    0.07

    0.08

    0.09

    0 100 200 300 400 500 600 700 800 900 1000

    Time (days)

    C7inliquidphase(fracti

    on

    BHP = 2000 psi BHP = 500 psi BHP = 1000 psi BHP = 1500 psi

    BHP = 2500 psi BHP = 3000 psi BHP = 3500 psi BHP = 4000 psi

    decreasing BHP

    Figure 9: Mole fraction profiles of C7in liquid phase.

    0

    5000000

    10000000

    15000000

    20000000

    25000000

    30000000

    35000000

    40000000

    45000000

    50000000

    0 100 200 300 400 500 600 700 800 900

    Time (days)

    W

    GPT(mscf)

    BHP = 2000 psi BHP = 500 psi BHP = 1000 psi BHP = 1500 psi

    BHP = 2500 psi BHP = 3000 psi BHP = 3500 psi BHP = 4000 psi

    decreasing BHP

    Figure 10: Total gas production profile for different BHP.

    0

    20

    40

    60

    80

    100

    120

    140

    160

    180

    200

    0 100 200 300 400 500 600 700 800 900

    Time (days)

    WPIG(mscf/day-p

    si)

    BHP = 2000 psi BHP = 500 psi BHP = 1000 psi BHP = 1500 ps

    BHP = 2500 psi BHP = 3000 psi BHP = 3500 psi BHP = 4000 ps

    decreasing BHP

    single phase

    two phases

    Figure 11: Well productivity index (WPIG) profiles for differenBHP.

    0

    0.1

    0.2

    0.3

    0.4

    0.5

    0.6

    0.7

    0.1 1 10 100 1000 10000

    Radius r (ft)

    Sc

    (fraction)

    BHP = 2000 psi BHP = 500 psi BHP = 1000 psi BHP = 1500 psi

    BHP = 2500 psi BHP = 3000 psi BHP = 3500 psi BHP = 4000 psi

    decreasing BHP

    Figure 12: Comparison of condensate saturation profiles fordifferent BHP at t = 1000 days.

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    6 SPE 103255

    0

    0.01

    0.02

    0.03

    0.04

    0.05

    0.06

    0.07

    0.08

    0.09

    0.1 1 10 100 1000 10000

    Radius r (ft)

    C7

    inliq

    uidphase(fraction

    BHP = 2000 psi BHP = 500 psi BHP = 1000 psi BHP = 1500 psi

    BHP = 2500 psi BHP = 3000 psi BHP = 3500 psi BHP = 4000 psi

    decreasing BHP

    Figure 13: Comparison of mole fraction profiles of C7 in liquidphase at t = 1000 days.

    In this case, we can decrease the BHP to achieve greater

    pressure difference; hence temporarily to produce more gasfrom the reservoir. However, lowering the BHP will cause the

    expansion of the two-phase region, and the accumulation ofmore heavy-component in the reservoir. Hence, lowering the

    BHP may not be a good strategy for maximizing total fluidrecovery.

    b) Strategy of BHP ramping as a function of time.

    Instead of setting BHP at a fixed value, we can also control theBHP such that it ramps as a function of the producing time.

    For all simulation tests in this case, the initial reservoir

    pressure and the final well bottom-hole pressure control werethe same.

    Figure 14 shows the ramping strategies used in this study.

    Increasing the ramping time, the gas production rate increasesat the late producing life, although the initial production rate islow due to the smaller pressure difference (Figure 15). The

    well loses some gas production in total when the ramping time

    increases (Figure 16). However, the well productivity index

    reduction is delayed from the high productivity of single-phase flow to low productivity of two-phase flow (Figure 17).

    The accumulation of condensate saturation (Figure 18) and

    heavy component (Figure 19) are also delayed.

    1500

    2000

    2500

    3000

    3500

    4000

    4500

    5000

    5500

    0 100 200 300 400 500 600 700 800 900 1000

    Time (days)

    WBHP(psi)

    Increase ramping time

    Figure 14: BHP ramps as functions of time.

    0

    10000

    20000

    30000

    40000

    50000

    60000

    70000

    80000

    90000

    0 100 200 300 400 500 600 700 800 900

    Time (days)

    WGPR(mscf/day)

    Increase ramping time

    Figure 15: Gas production rate profiles for different rampingstrategies.

    0

    5000000

    10000000

    15000000

    20000000

    25000000

    30000000

    35000000

    40000000

    45000000

    0 100 200 300 400 500 600 700 800 900

    Time (days)

    WGPT(mscf)

    Increase ramping time

    Figure 16: Total gas production profiles for different rampingstrategies.

    0

    20

    40

    60

    80

    100

    120

    140

    160

    180

    200

    0 100 200 300 400 500 600 700 800 900

    Time (days)

    W

    PIG(mscf/day-psi)

    Increase ramping time

    Figure 17: Well productivity index profiles for different rampingstrategies.

    0

    0.1

    0.2

    0.3

    0.4

    0.5

    0.6

    0.7

    0 100 200 300 400 500 600 700 800 900

    Time (days)

    Sc

    (fraction)

    Increase ramping time

    Figure 18: Condensate saturation profiles for different rampingstrategies.

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    SPE 103255 7

    0

    0.01

    0.02

    0.03

    0.04

    0.05

    0.06

    0 100 200 300 400 500 600 700 800 900 1000

    Time (days)

    C7

    inliquidphase(fraction)

    Increase ramping time

    Figure 19: mole fraction profiles for C7in liquid phase at differentramping strategies.

    From Figures 20 and 21, we can see that by increasing theramping time, the two-phase region can be effectively shrunkby a factor as much as 10, and less heavy-component can beleft in the reservoir. This is very meaningful from the point ofthe long-term field development since it has been reportedfrom many field cases that the heavy components are difficult

    to recover once they have been left in the reservoir.

    0

    0.1

    0.2

    0.3

    0.4

    0.5

    0.6

    0.7

    0.1 1 10 100 1000 10000

    Radius r (ft)

    Sc

    (fraction)

    ramp = 0 ramp = 1000 ramp = 100 ramp = 200 ramp = 300 ramp = 400

    ramp = 500 ramp = 600 ramp = 700 ramp = 800 ramp = 900

    Producer

    Increase ramping time

    Figure 20: Saturation profiles for different ramping strategies att = 1000 days.

    0

    0.01

    0.02

    0.03

    0.04

    0.05

    0.06

    0.1 1 10 100 1000 10000

    Radius r (ft)

    C7inliquidphase(fraction

    ramp = 0 ramp = 1000 ramp = 100 ramp = 200 ramp = 300 ramp = 400

    ramp = 500 ramp = 600 ramp = 700 ramp = 800 ramp = 900

    Producer

    Increase ramping time

    Figure 21: Mole fraction profiles of C7in liquid phase for differentramping strategies at t = 1000 days.

    Two-component simulation results

    Figures 22 to 29 show the simulation results for the binary-component methane/butane system. These are a representationof flow in the experiment described earlier. The generaconclusions for the BHP strategy are the same for bothmulticomponent and binary-component systems. That is, thetotal gas production increases as a result of greater pressure

    difference between the reservoir and the well, but at the sametime, lower BHP also brings more heavy-component into thereservoir and generates a larger two-phase region. For thiparticular binary combination of C1 and C4, the saturationprofile (Figure 22) shows decreases after the accumulatedcondensate saturation reaches a maximum value. Noticing thathis maximum condensate saturation (Scam =0.53) is greaterthan the critical condensate saturation (Sac=0.25) from therelative permeability curve. The mole fraction of C4 in theliquid phase also drops as the well continues producing. Thereason is that some revaporization of the in-place liquid phasetakes place; thus the two-phase zone varies as the well keepsproducing.

    Figure 25 shows that the well gas productivity dropssignificantly from single-phase flow to two-phase flow and thelower the BHP, the greater the drop in gas productivity.

    0

    0.1

    0.2

    0.3

    0.4

    0.5

    0.6

    0.7

    0.8

    0.9

    1

    0 5 10 15 20 25 30

    Distance (cm)

    Sc

    t = 0.005h t = 0.01h t = 0.015h t = 0.02h

    t = 0.025h t = 0.035h t = 0.055h t = 20h

    flow direction

    Figure 22: Saturation vs. distance for binary-componencondensate system at BHP = 75 atm.

    0

    0.1

    0.2

    0.3

    0.4

    0.5

    0.6

    0.7

    0.8

    0.9

    0 5 10 15 20 25 30

    Distance (cm)

    C4inliquidphase

    t = 0.005h t = 0.01h t = 0.015h t = 0.02h

    t = 0.025h t = 0.035h t = 0.055h t = 20h

    flow direction

    Figure 23: Mole fraction of C4 vs. distance at BHP = 75 atm.

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    8 SPE 103255

    0

    10000000

    20000000

    30000000

    40000000

    50000000

    60000000

    0 5 10 15 20 25

    Time (hour)

    WGPT(scc)

    BHP = 75 atm BHP = 25 atm BHP = 35 atm BHP = 45 atm

    BHP = 55 atm BHP = 65 atm BHP = 85 atm BHP = 95 atm

    BHP = 105 atm BHP = 115 atm

    decreasing BHP

    Figure 24: Total gas production profiles for different BHP.

    0

    200000

    400000

    600000

    800000

    1000000

    1200000

    1400000

    1600000

    0.001 0.01 0.1 1 10 100

    Time (hour)

    WPIG(scc/hour-atm)

    BHP = 75 atm BHP = 25 atm BHP = 35 atm BHP = 45 atm

    BHP = 55 atm BHP = 65 atm BHP = 85 atm BHP = 95 atm

    BHP = 105 atm BHP = 115 atm

    increasing BHP

    singlephase

    region

    twophasesregion

    Figure 25: Well gas productivity profiles for different BHP.

    0

    0.1

    0.2

    0.3

    0.4

    0.5

    0.6

    0.001 0.01 0.1 1 10 100

    Time (hour)

    Sc

    (fraction)

    BHP = 75 atm BHP = 25 atm BHP = 35 atm BHP = 45 atm

    BHP = 55 atm BHP = 65 atm BHP = 85 atm BHP = 95 atmBHP = 105 atm BHP = 115 atm

    increasing BHP

    Figure 26: Condensate saturation vs. time for different BHP.

    0

    0.1

    0.2

    0.3

    0.4

    0.5

    0.6

    0.7

    0.8

    0.9

    1

    0.001 0.01 0.1 1 10 100

    Time (hour)

    C4

    inliquidphase

    BHP = 75 atm BHP = 25 atm BHP = 35 atm BHP = 45 atm

    BHP = 55 atm BHP = 65 atm BHP = 85 atm BHP = 95 atm

    BHP = 105 atm BHP = 115 atm

    decreasing BHP

    Figure 27: Mole fraction of C4vs. time for different BHP.

    0

    0.1

    0.2

    0.3

    0.4

    0.5

    0.6

    0.7

    0.8

    0.9

    1

    0 5 10 15 20 25 30

    Distance (cm)

    Sc

    BHP = 75 atm BHP = 25 atm BHP = 45 atm

    BHP = 65 atm BHP = 85 atm BHP = 105 atm

    flow direction

    decreasing BHP

    Figure 28: Saturation vs. distance for different BHP at t = 20h.

    0

    0.1

    0.2

    0.3

    0.4

    0.5

    0.6

    0.7

    0.8

    0.9

    1

    0 5 10 15 20 25 30

    Distance (cm)

    C4

    inliquidphase(fraction

    BHP = 75 atm BHP = 25 atm BHP = 45 atm

    BHP = 65 atm BHP = 85 atm BHP = 105 atm

    flow direction

    decreasing BHP

    Figure 29: Mole fraction of C4vs. distance for different BHP at t =20h.

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    SPE 103255 9

    In summary from the simulation results, we can conclude that

    there is no standard way to optimize the producing strategy.

    Using low BHP or rapid ramping time for BHP, we canachieve high total gas production temporarily, however, to

    minimize the condensate banking blockage and hence to

    enhance the ultimate gas and liquid recovery, higher BHP or

    slower ramping time for BHP may be a better strategy. The

    optimal approach is likely to be dependent on the originalcomposition.

    Conclusions1. In gas-condensate flow, local composition changes

    due to relative permeability effects.

    2. Composition and condensate saturation changesignificantly as a function of producing sequence.

    The higher the BHP, the less the condensate banking

    and a smaller amount of heavy-component is trapped

    in the reservoir; increasing ramping time of BHP will

    also help to alleviate the condensate banking andheavy-component trapping.

    3. Gas productivity can be maximized with properproducing strategy. The total gas production can be

    achieved by lowering the BHP or dropping the BHP

    quickly instead of ramping slowly to a preset BHP

    value.

    4. Productivity loss can be reduced by optimizing theproducing sequence.

    5. The condensate drop-out will hinder the flow

    capability, due to relative permeability effects.

    NomenclatureN2 nitrogen

    CO2 carbon dioxideC1 methane

    C2 ethaneC3 propane

    iC4 i-butane

    nC4 n-butaneiC5 i-pentane

    nC5 n-pentane

    C6 hexaneC7 heptane

    C8 octane

    C9 nonaneC10+ decene

    MW molecular weight

    CVD constant volume depletion

    BHP bottom-hole pressureScc critical condensate saturationSc condensate saturation

    WPIG gas productivity index of a well (mscf/day-psi or

    scc/hour-atm)

    WGPT well total gas production (mscf or scc)Tc critical temperature (K or C)

    pc critical pressure (psi or atm)

    AcknowledgementsWe would like to express our appreciation to Saudi Aramaco

    and the members of SUPRI-D (Research Consortium onInnovation in Well Testing) for financial support and usefu

    discussions.

    References

    1. Afidick, D., Kaczorowski, N.J., and Bette, S., 1994Production Performance of a Retrograde Gas: A Case Study

    of the Arun Field, paper SPE 28749 presented at the SPEAsia Pacific Oil & Gas Conference held in Melbourne

    Australia.

    2. Barnum, R.S., Brinkman, F.P., Richadson, T.W. andSpillette, A.G., 1995 Gas Condensate Reservoir Behavior

    Productivity and Recovery Reduction Due to Condensation,

    paper SPE 30767 presented at the SPE Annual Technica

    Conference and Exhibition, Dallas, TX.

    3. Fevang, O. and Whitson, C.H., 1996 Modeling Gas-

    Condensate Well Deliverability, paper SPE Res. Eng., P221-230.

    4. Riemens, W.G. and de Jong, L.N.J.: Birba Field PVT

    Variations Along the Hydrocarbon Column and Confirmatory

    Field Tests, paper SPE 13719 presented at the SPE 1985Middle East Oil Technical Conference held in Bahrain, March

    11-14, 1985.

    5. Schulte, A.M.: Compositional Variation within aHydrocarbon Column Due to Gravity, paper SPE 9235

    presented at the 55th Annual Technical Conference and

    Exhibition held in Dallas TX September 21-24, 1980.

    6. Lee, S.T.: Capillary-Gravity Equilibria for HydrocarbonFluids in Porous Media, paper SPE 19650 presented at the

    64thAnnual Technical Conference and Exhibition held in San

    Antonio TX October 8-11, 1989.

    7. Roussennac, B., 2001 Gas Condensate Well Tes

    Analysis, MS report, Stanford University.

    SI metric conversion Factorsatm 1.013250 * E+05 = Paft3 1.589873 E-01 = m3

    F (F-32)/1.8 = C

    in.3 1.638706 E+01 = cm3

    psi 6.894757 E+00 = kPa


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