# texas am university lecture 07: wellbore 613 (2005a) wellbore phenomena slide —1 t.a....

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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)