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  • PETE 613(2005A)

    Slide 1Wellbore Phenomena

    T.A. Blasingame, Texas A&M U.Department of Petroleum Engineering

    Texas A&M UniversityCollege Station, TX 77843-3116

    +1.979.845.2292 t-blasingame@tamu.edu

    Petroleum Engineering 613Natural Gas Engineering

    Texas A&M University

    Lecture 07:Wellbore Phenomena

  • PETE 613(2005A)

    Slide 2Wellbore Phenomena

    Wellbore PhenomenaCalculation of Bottomhole PressureGeneral relation (energy balance).Static (non-flowing) bottomhole pressure (dry gas).Flowing bottomhole pressure (dry gas).

    Near-Well Reservoir Flow BehaviorSteady-state "skin factor" concept used to represent

    damage or stimulation in the near-well region."Variable" skin effects: non-Darcy flow, well cleanup,

    and gas condensate banking.

  • PETE 613(2005A)

    Slide 3Wellbore Phenomena

    Calculation of Bottomhole PressureProcessGeneral relation (energy balance).Static (non-flowing) bottomhole pressure (dry gas).Flowing bottomhole pressure (dry gas).

    IssuesCalculation approaches:

    Average Pressure and Temperature method. Cullender and Smith method. Numerical methods.

  • PETE 613(2005A)

    Slide 4Wellbore Phenomena

    Calculation of Bottomhole Pressure

    a. Basic energy balance for flow in inclined pipes.

    d. Schematic illustration of wellbore configuration.b. Solution assuming average T and z-values.

    e. Wellbore diagram surface pressure measurement.c. Cullender-Smith solution of the energy balance.

  • PETE 613(2005A)

    Slide 5Wellbore Phenomena

    Calculation of Bottomhole Pressure

  • PETE 613(2005A)

    Slide 6Wellbore Phenomena

    "Skin Factor" ConceptSkin Factor:"Skin" is just a pressure drop due to non-ideal condi-

    tions. The "skin factor" is the "dimensionless" pres-sure drop. Positive Skin = DAMAGE (i.e., the pressure is higher than

    it should be). Negative Skin = STIMULATION (i.e., the pressure is lower

    than it should be).Physical, steady-state "skin factor" concept ("Hawkins"

    formula) used to illustrate near-well damage. Physical limitations, really only valid for damage ... or

    very slight stimulation.Infinitesimal skin concept uses a mathematical "trick"

    to represent additional (positive or negative) pressure.Most practical approach for well performance (Hawkinsformula does not fit in practice).

  • PETE 613(2005A)

    Slide 7Wellbore Phenomena

    Near-Well Behavior Physical Skin Concept

    a. Simple skin concept steady-state flow of a liquidin the "altered" zone.

    c. Schematic pressure behavior in a "steady-state"skin zone.

    b. Governing relations for steady-state flow of a liquidin the "altered" zone (ks=permeability in the "alter-ed" zone).

  • PETE 613(2005A)

    Slide 8Wellbore Phenomena

    a. Concepts of "infinitesimal" skin as well as use of the effective wellbore radius these schematics seek torepresent the concept of "skin" as a physical phenomena, as well as a (simple) mathematical model.

    b. 2-zone, radial "composite" model (condensate bank).

    Skin Concept Generallyspeaking, use of the skin con-cept (or skin "factor") tends to"isolate" pressure behavior thatcannot be directly attributed tothe reservoir. This is an over-simplification, but it is conven-ient, and is widely used.

    Near-Well Behavior Other Skin Concepts

  • PETE 613(2005A)

    Slide 9Wellbore Phenomena

    Comment:This model has been shown to consistently repre-

    sent the kg(r,t) profile.The -parameter is presumed to represent fluid and

    rock-fluid properties.Major concern/issue is that diffusivity (kg/(gcg))

    also varies, and should be considered.

    Using observed performance from numerical simulationof gas condensate reservoirs, we propose the followingmodel for gas mobility (or gas permeability) behavior:

    tr

    kkkk min,gmax,gmin,gg21

    exp1)(

    "Variable" Skin Effects: Gas Condensate Banking

  • PETE 613(2005A)

    Slide 10Wellbore Phenomena

    Variable Skin kg(r,t) Profile

    kg(r,t) Profile:Drawdown performance only (buildup to be addressed later).Exponential model is simple and consistent (valuable for model development).erf(x) model also consistent, but exp(x) model is preferred.

  • PETE 613(2005A)

    Slide 11Wellbore Phenomena

    Variable Skin So(r,t) Profile

    So(r,t) Profile: (from Roussennac Thesis Fig. 2.6)Region 1: Near well region affected by condensate "bank" (liquid dropout).Region 2: "Transition" of condensate "bank" to "dry gas" region.Region 3: "Dry gas" region.

  • PETE 613(2005A)

    Slide 12Wellbore Phenomena

    Variable Skin pD(D) "Type Curve"

    pD(D) Type Curve:Drawdown performance only.D-parameter is a "scaling parameter" note behavior for large (rD2/DtD).Near-well behavior is controlled by kmin/kmax ratio (as would be expected).

  • PETE 613(2005A)

    Slide 13Wellbore Phenomena

    Variable Skin pDd(D) "Type Curve"

    pDd(D) Type Curve:Drawdown performance only.D-parameter is a "scaling parameter" only influences large (rD2/DtD).kmin/kmax controls |DdpD/dD)| in "near-well" region (note horizontal trends).

  • PETE 613(2005A)

    Slide 14Wellbore Phenomena

    Variable Skin pDd(tD) "Type Curve"

    pDd(tD) Type Curve:Drawdown performance only.D-parameter only influences early time behavior (i.e., small values of (DtD)/rD2).|tDdpD/dtD)| reflects kmin/kmax influence as mobility profile moves.

  • PETE 613(2005A)

    Slide 15Wellbore Phenomena

    Validation: Literature Data Case 1

    "Roussennac Fig. 2.7" (log(pp(r,t)) versus log(r2/t) (s=0))Good agreement in data and model trends (with the noted exception of the non-

    line source" portion of the pp data recall that the new model is based on theline source assumption (rw0), the simulated data have no such constraint).

  • PETE 613(2005A)

    Slide 16Wellbore Phenomena

    Validation: Literature Data Case 2

    "Roussennac Fig. 2.9" (log(pp(r,t)) versus log(r2/t) (s0 case))Excellent agreement in data and model trends for r2>1x100 (note that the FINITE

    skin zone is 0.53 ft

  • PETE 613(2005A)

    Slide 17Wellbore Phenomena

    "Vo Fig. 3.1" (log(pp(r,t)) versus log(r2/t) (s=0 case)) Very good agreement in data and model trends (entire range of data). Excellent match validates model for this particular case.

    Validation: Literature Data Case 3

  • PETE 613(2005A)

    Slide 18Wellbore Phenomena

    Validation: Other Reservoir Models

    Combination Plot: Radial Composite, Sealing Faults, New ModelSealing fault responses (blue) appear similar to new model for various cases.2-zone radial composite model (diffusivity ratio (1ct,1)/(2ct,2) of unity) is a

    limiting case of our new model (i.e., the interface is fixed at a radius for all time).

  • PETE 613(2005A)

    Slide 19Wellbore Phenomena

    T.A. Blasingame, Texas A&M U.Department of Petroleum Engineering

    Texas A&M UniversityCollege Station, TX 77843-3116

    +1.979.845.2292 t-blasingame@tamu.edu

    Petroleum Engineering 613Natural Gas Engineering

    Texas A&M University

    Lecture 07:Wellbore Phenomena

    (End of Lecture)