05-Introduction to Well Testing

Introduction to Well Testing

Introduction to Well TestingInstructional Objectives1.List 4 objectives of well testing.2.Define testing variables.3.Compute compressibility of reservoir systems.

Introduction To Well TestingOutlineApplications and objectives of well testingDevelopment of the diffusivity equationDefinitions and sources for data used in well testing

What Is A Well Test?A tool for reservoir evaluation and characterizationInvestigates a much larger volume of the reservoir than cores or logsProvides estimate of permeability under in-situ conditionsProvides estimates of near-wellbore conditionProvides estimates of distances to boundaries

How Is A Well Test Conducted?

Well Test ApplicationsExplorationReservoir engineeringProduction engineering

Well Test Objectives Define reservoir limitsEstimate average drainage area pressureCharacterize reservoirDiagnose productivity problemsEvaluate stimulation treatment effectiveness

Types of Well TestsqSingle-WellMulti-Well

Development of the Diffusivity Equation

The Diffusivity EquationDescribes the flow of A slightly compressible fluid Having constant viscosityIn a porous mediumAt constant temperature

The Continuity Equation

Equation of State for a Slightly Compressible Liquid

Flow Equation - Darcys Lawor, in differential form,

The Diffusivity Equation

Data Used in Well Testing

Formation Volume Factor

Viscosity

Fluid Compressibility

Porosity

Permeability

Pore Compressibility

Net Pay Thicknessh = h1 + h2 + h3

Net Pay ThicknessCase 1Case 3Case 4Case 2

Saturations

Wellbore Radius

Total Compressibility

Exercise 1List 4 objectives of well testing

Exercise 2Define variables used in well testing

Exercise 3Calculate total compressibility for an undersaturated oil reservoir

Exercise 4Calculate total compressibility for a saturated oil reservoir

Exercise 5Calculate total compressibility for a low-pressure, high-permeability gas reservoir

Exercise 6Calculate total compressibility for a high-pressure, low-permeability gas reservoir

Introduction to Well TestingIntroduction to Well TestingUpon completion of this section, the student should be able to:1.List 4 major objectives of well testing.2.Define, give the units for, and specify typical sources for each of the following variables: net pay thickness, porosity, saturation, viscosity, formation volume factor, total compressibility, wellbore radius.3.Be able to compute the total compressibility for different reservoir systems.Introduction to Well TestingIntroduction to Well TestingIntroduction to Well TestingA well test is conducted byChanging production rate at a wellMeasuring resulting pressure response at the same well or another well

Introduction to Well TestingExplorationIs this zone economic?How large is this reservoir?Reservoir engineeringWhat is the average reservoir pressure?How do I describe this reservoir in order to estimate reserves?forecast future performance?optimize production?Production engineeringIs this well damaged?How effective was this stimulation treatment?Why is this well not performing as well as expected?Introduction to Well TestingDefine reservoir limitsDistances to boundariesDrainage areaEstimate average drainage area pressureCharacterize reservoir PermeabilitySkin factorDual porosity or layered behaviorDiagnose productivity problemsPermeabilitySkin factorEvaluate stimulation treatment effectivenessSkin factorFracture conductivityFracture half-lengthIntroduction to Well TestingSingle well testsDrawdown test Produce a well at constant rate and measure the pressure response.Buildup test Shut in a well that has been producing and measure the pressure response.Injection test Inject fluid into a well at constant rate and measure the pressure response.Injection-falloff test Shut in an injection well and measure the pressure response.Multi-well testsInterference test Produce one well at constant rate and measure the pressure response at one or more offset wells.Pulse test Alternately produce and shut in one well and measure the pressure response at one or more offset wells.Introduction to Well TestingIntroduction to Well TestingObtained by combiningContinuity equationEquation of state for slightly compressible liquidsFlow equation - Darcys law

Introduction to Well TestingThe continuity equation is a restatement of the conservation of matter. That is, the rate of accumulation of fluid within a volume element is given by the rate at which the fluid flows into the volume minus the rate at which the fluid flows out of the volume.

NomenclatureA =Cross-sectional area open to flow, ft2 =Rate of accumulation of mass within the volume, lbm/secv =Fluid velocity, ft/sec =Density of fluid, lbm/ft3

Introduction to Well TestingThis equation describes the change in density with pressure for a liquid with small and constant compressibility.

Nomenclaturec =Compressibility, psi-1p =Pressure, psi =Density of fluid, lbm/ft3

Introduction to Well TestingNomenclatureA =Cross sectional area open to flow, cm2k =Permeability, darciesL =Length of flow path, cmp =Pressure, atmp =Pressure difference between upstream and downstream sides, atmq =Flow rate, cm3/secux =Flow velocity, cm/secx =Spatial coordinate, cm =Viscosity, cp

Introduction to Well TestingThe diffusivity equation is obtained by combining-The continuity equation-The equation of state for a slightly compressible liquid-Darcys lawOther transient flow equations may be obtained by combining different equations of state and different flow equations-Gas flow equation-Multiphase flow equationIntroduction to Well TestingIntroduction to Well TestingThe formation volume factor is the volume of fluid at reservoir conditions necessary to produce a unit volume of fluid at surface conditions.Symbol Bo, Bg, BwUnits res bbl/STB, res bbl/ MscfSource Lab measurements, correlationsRange and typical valuesOil1 2 res bbl/STB, Black oil2 4 res bbl/STB, Volatile oilWater1 1.1 res bbl/STBGas0.5 res bbl/Mscf, at 9000 psi5 res bbl/Mscf, at 680 psi30 res bbl/Mscf, at 115 psi

Introduction to Well TestingViscosity is a measure of resistance to flow. Specifically, it is the ratio of the shear stress to the resulting rate of strain within a fluid.Symbolso, g, wUnits cpSource Lab measurements, correlationsRange and typical values- 0.25 10,000 cp, Black oil- 0.5 1.0 cp, Water- 0.012 0.035 cp, Gas

Introduction to Well TestingCompressibility is the fractional change in volume due to a unit change in pressure.Symbol co, cg, cwUnits psi-1, microsips (1 microsip = 1x10-6 psi-1)Source Lab measurements, correlationsTypical ValuesOil15x10-6 psi-1, undersaturated oil180x10-6 psi-1, saturated oilWater4x10-6 psi-1Gas1/p, Ideal gas60x10-6 psi-1, at 9000 psi1.5x10-3 psi-1, at 680 psi9x10-3 psi-1, at 115 psi

Introduction to Well TestingPorosity is the ratio of volume of pore space to bulk volume of rock.Symbol - Units Equations - fractionReports - % (or fraction)SourceLogs, coresRange or Typical Value30%, unconsolidated well-sorted sandstone20%, clean, well-sorted consolidated sandstone8%, low permeability reservoir rock0.5%, natural fracture porosityIntroduction to Well TestingPermeability is the measure of capacity of rock to transmit fluid.Symbolk UnitsDarcy or millidarcy (md or mD)SourceWell tests, core analysisRange0.001 md - 10,000 md

Introduction to Well TestingPore volume compressibility is the fractional change in porosity due to unit change in pressure.Symbol cfUnits psi-1, microsipsSource Lab measurement, correlation, guessRange or Typical Value4x10-6 psi-1, well-consolidated sandstone30x10-6 psi-1, unconsolidated sandstone4 to 50 x 10-6 psi-1 consolidated limestones

Introduction to Well TestingThe net pay thickness is the total thickness of all productive layers in communication with the well.Net pay includes any rock that has sufficient vertical permeability to allow fluid to move to a layer from which it may be produced.Thickness is measured perpendicular to bed boundaries.Symbol h Units ftSource logsRange or Typical ValueMay be as small as 5 ft or even lessMay be as large as 1,000 ft or moreIntroduction to Well TestingIntroduction to Well TestingSaturation is the fraction of pore volume occupied by a particular fluid.Symbol So, Sw, SgUnits fraction or %Source logsRange or Typical Value15 to 25% connate water saturation in well-sorted, coarse sandstones40 to 60% connate water saturation in poorly sorted, fine-grained, shaly, low-permeability reservoir rock

Introduction to Well TestingWellbore radius is the size of wellbore.SymbolrwUnitsfeetSourceBit diameter/2Caliper logRange or Typical Value2 to 8 in.

Introduction to Well TestingThe total compressibility is the sum of pore compressibility and saturation weighted fluid compressibilities.Symbol ctUnitspsi-1, microsipsSourceCalculatedRange or Typical ValueSee exercises

Introduction to Well TestingExercise 1Introduction to Well TestingList 4 Objectives of Well Testing

List 4 objectives of well testing. List as many as possible without referring to the notes.

1.

2.

3.

4.Introduction to Well TestingExercise 2Introduction to Well TestingDefine Variables Used In Well Testing

Define, give the units for, and name a common source for each of the following variables used in well testing. Complete as much of this exercise as possible before referring to the notes.

1.Porosity2.Water saturation3.Total compressibility4.Oil compressibility5.Formation volume factor6.Viscosity7.Wellbore radius8.Net pay thickness9.PermeabilityIntroduction to Well TestingExercise 3Introduction to Well TestingCalculate Compressibility for Undersaturated Oil Reservoir

Calculate total compressibility for the following situation. Assume solution gas/oil ratios do not include stock tank vent gas.Undersaturated oil reservoir (above the bubblepoint)Sw =17%, TDS = 18 wt %, oil gravity = 27API,Rso =530 scf/STB, gas gravity = 0.85, Tf = 185F,p =3500 psi, cf = 3.610-6 psi-1Tsep =75F, psep = 115 psiaFrom fluid properties correlations,pb =2803 psico =1.158 x 10-5 psi-1cw =2.277 x 10-6 psi-1

Introduction to Well TestingExercise 4Introduction to Well TestingCalculate Compressibility for Saturated Oil Reservoir

Calculate total compressibility for the following situation. Assume solution gas/oil ratios do not include stock tank vent gas.Saturated oil reservoir (below the original bubblepoint)Sw = 17%, Sg = 5%, TDS = 18 wt %, oil gravity = 27API,Rso = 530 scf/STB, gas gravity = 0.85, Tf = 185F,p = 2000 psi, cf = 3.610-6 psi-1 Tsep = 75F, psep = 115 psia

From fluid properties correlations,pb=2803 psico=1.429 x 10-4 psi-1cg=5.251 x 10-4 psi-1cw=4.995 x 10-6 psi-1Introduction to Well TestingExercise 5Introduction to Well TestingCalculate Compressibility for Low-Pressure, High-Permeability Gas Reservoir

Calculate total compressibility for the following situation. Assume a dry gas.Low-pressure, high-permeability gas reservoirSw = 20%, gas gravity = 0.74, Tf = 125F, p = 125 psi,cf = 3.610-6 psi-1, cw = 4 x 10-6 psi [Tf is outside range of correlations]

From fluid properties correlations,cg=8.144 x 10-3 psi-1cw=4x10-6 psi-1

Introduction to Well TestingExercise 6Introduction to Well Testing Calculate Compressibility for High-Pressure, Low-Permeability Gas Reservoir

Calculate total compressibility for the following situation. Assume a dry gas.High pressure, low permeability gas reservoirSw = 35%, TDS = 22 wt %, gas gravity = 0.67, Tf = 270F,p = 5,000 psi, cf = 2010-6 psi-1

From fluid properties correlations,cg =1.447 x 10-4 psi-1cw=3.512 x10-6 psi-1