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Petroleum Engineering 324 — Reservoir PerformanceDepartment of Petroleum Engineering — Texas A&M U.

Lecture 00 — Orientation for Pressure Transient TestingT.A. Blasingame/Dilhan Ilk (2009)

Thomas A. Blasingame, Ph.D., P.E. Dilhan IlkDepartment of Petroleum Engineering Department of Petroleum EngineeringTexas A&M University Texas A&M UniversityCollege Station, TX 77843-3116 (USA) College Station, TX 77843-3116 (USA)+1.979.845.2292 +1.979.458.1499 [email protected] [email protected]

Petroleum Engineering 324 (2009)Reservoir Performance

Lecture:●Orientation for Pressure Transient Testing:

Objectives.Data (ALL data: reservoir, well, completion data, pressure/rate history).Analysis and Interpretation.

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Notes:___________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

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Provide orientation regarding the purpose, philosophy, and objectives for pressure transient testing.

Objective



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Notes:___________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

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Objectives of Pressure Transient Testing:Evaluate reservoir pressure (initial or average pressure).Evaluate reservoir fluid (fluid samples collected for lab study).Estimate reservoir properties (e.g., k, S, xf, λ, ω, etc.).Estimate reservoir volumetrics (e.g., fluid-in-place, drainage area).

Input Data:BOTTOMHOLE pressure data (accurate to < 1 part in 10,000 or more).SURFACE flowrate data (often poorly measured/recorded).Fluid properties (e.g., FVF, viscosity, compressibility, ... ).Reservoir properties (e.g., h, φ, rw, cf, ... )

Results of PTA Interpretation:Productive capacity of the WELL (damage/stimulation).Productive capacity of the RESERVOIR (transmissibility).Current average reservoir pressure.Reservoir limits (for production to pseudosteady-state).Well interference effects.Well/Reservoir specific parameters (e.g., Cs, xf, λ, ω, Lfault, rcomp, kv/kh, ...).

Orientation: PTA — Purpose, Philosophy, Objectives

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10-4

10-3

10-2

10-1

100

101

p D, p

Dd

and

p Dβ d

10-3 10-2 10-1 100 101 102 103 104

tDxf/CDxf

Type Curve Analysis — SPE 9975 Well 5 (Buildup Case)(Well with Infinite Conductivity Hydraulic Fractured )

Legend: Infinite Conductivity Fracture pD Solution pDd Solution pDβd Solution

Legend: pD DatapDd DatapDβd Data

Reservoir and Fluid Properties:rw = 0.33 ft, h = 30 ft,

ct = 6.37×10-5 psi-1, φ = 0.05 (fraction)μgi = 0.0297 cp, Bgi= 0.5755 RB/Mscf

Production Parameters:qref = 1500 Mscf/D

Match Results and Parameter Estimates:[pD/Δp]match = 0.000021 psi-1, CDf= 0.01 (dim-less)

[(tDxf/CDf)/t]match= 0.15 hours-1, k = 0.0253 md CfD = 1000 (dim-less), xf = 279.96 ft

pDd = 1/2

pDβd = 1/2

10-3

10-2

10-1

100

101

102

p D, p

Dd

and

p Dβ d

10-2 10-1 100 101 102 103 104 105

tD/CD

Type Curve Analysis — SPE 12777 (Buildup Case)(Well in an Infinite-Acting Homogeneous Reservoir)

Legend: Radial Flow Type Curve pD Solution pDd Solution pDβ d Solution

Legend: pD DatapDd DatapDβd Data

Reservoir and Fluid Properties:rw = 0.29 ft, h = 107 ft,

ct = 4.2×10-6 psi-1, φ = 0.25 (fraction)μo = 2.5 cp, Bo= 1.06 RB/STB

Production Parameters:qref = 174 STB/D

Match Results and Parameter Estimates:[pD/Δp]match = 0.018 psi-1, CDe2s= 1010 (dim-less)

[(tD/CD)/t]match= 15 hours-1, k = 10.95 md Cs = 0.0092 bbl/psi, s = 8.13 (dim-less)

pDβd = 1

pDd = 1/2

■ Field Example: (SPE 12777)Data match for a case of radial flow —wellbore storage signature is identified using the pressure β-derivative. (dimen-sionless format — pDβd=1)

■ Field Example: (SPE 9975)Data match for the case of a well with an infinite conductivity vertical fracture — formation linear flow behavior is revealed using the β-derivative. (dimensionless format — pDβd=1/2)

Orientation: PTA Diagnostic ExamplesUnfractured Well Fractured Well

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PVT Properties: (Lab report preferred, correlations acceptable)Black Oil: Bo, Rs, μo, co (correlations require: T, γg,sep, pb, γSTO)Dry Gas: z (or Bg), μg, cg (correlations require: T, γg,sep)Volatile Oil: Black oil equivalent or compositional formulation.Gas Condensate: Dry gas equivalent or compositional formulation.Water: Bw, Rsw, μw, cw (correlations require: T, γg,sep, pbw, salinity)

Reservoir Properties:Porosity (φ) (core and/or well logs)Net pay thickness (h) (core and/or well logs)Wellbore radius (rw) (well completion history (bit diameter))Formation Compressibility (cf) (cf=3x10-6 psia-1 or correlation)

Well Completion History:Drilling records (initial pressures, production tests)Well files (well logs, core, PVT, recompletion, workover records)Annotated production records (records of activities — very useful)

Orientation: Static Data for PTA

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Allocated Rate Data:Common in mature producing environments (e.g., Texas).Common in some offshore operations (manifold rates)."Allocation" depends on records — and consistency checks.

Poor/Incomplete (or Erroneous) Pressure Data:Virtually all production pressure measurements taken at surface.Completion changes often not reflected in surface pressures.Some pressure data are just wrong (poor gauge, poor timing, etc.).

Well Completion Issues:Equipment changes, poor practices, failed equipment, etc.UNREPORTED activities (recompletions, workovers, treatments).

Permanent DOWNHOLE Pressure Measurements:Expense is justified.Provides continuous evaluation of well performance.Data volume/sampling is an issue, but not a major problem.

Orientation: Production Histories

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Example: Production History Plot — East TX Gas WellExcellent rate and pressure histories.Unique case — surface and bottomhole pressure data.MINIMUM data frequency for high resolution analysis ≈ 1 point/day.

PTA Orientation: Tight Gas Production Case

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Example: "Log-Log" Plot (Well Test Analysis) — East TX Gas WellUsed high frequency bottomhole pressure measurements (pws).Consistent match of surface and bottomhole pressure data.Outstanding example of "things done right."

PTA Orientation: Tight Gas Production Case

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Well Deliverability:The first efforts to analyze well performance were an attempt to quantify well potential — not to estimate reservoir properties.The original well deliverability relation was completely empiri-cal (derived from observations), and is given as:

This relationship is rigorous for low pressure gas reservoirs, (n=1 for laminar flow).

nwfppCq )-( 22=

From: Back-Pressure Data on Natural-Gas Wells and Their Application to Production Practices — Rawlins and Schellhardt (USBM Monograph, 1935).

PTA Orientation: Early Well Deliverability

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Darcy's Law:

Separating and Integrating:

Which Reduces to: [(μgz) = constant]

Performing the Pressure Integration:

Discussion: Derivation of Well Deliverability RelationActually an empirical result (see Rawlins and Schellhardt (1935)).Derivation from steady-state flow (above) is useful for illustration.Derivation for pseudosteady-state is similar (variety of results).

PTA Orientation: Well Deliverability Relation

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wegwecgsc

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g ppCqppzpT

zTrr

khq −=→−=μ

π

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Discussion: Well Deliverability (4-point test)Probably oldest "reservoir engineering" technique. Assumption of pseudosteady-state flow is the weakest link in analysis.Does not directly relate time, rate, and pressure performance.

PTA Orientation: Well Deliverability (4-point test)Fr

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a. Typical flow regimes encountered during production (liquid system).

b. Typical "flow-after-flow" or 4-point test, (assumes pseudosteady-state flow (each rate)).

c. "Deliverability" or "Backpressure" plot used to estimate maximum well productivity.

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Overall flow system (after Fonesca). Blasingame axiom: "if there is a problem with the analysis/interpretation of well test and/or production data — the issue most likely stems from the well completion."

Our focus is the reservoir ... but, we also need to consider:

The well completion.The tubulars.The surface facilities.The reservoir fluid(s).

PTA Orientation: Reservoir/Well/Facilities System

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Pressure Transient Analysis: PTAData processing (permanent gauges) (obvious, but...).Numerical modelling (advise caution in applications).Variable-rate analysis (deconvolution).Better data analysis functions (We can always hope...).Continuous Measurement = Continuous Assessment

Production Analysis: PAMore consistent measurement of q and pwf.Pressure conversion (surface → bottomhole).Further implementation of semi-analytical solutions.Diagnostic methods for defining pressure transient behavior in production data (model identification).Continuous Measurement = Productivity Optimization

PTA Orientation: What Advances do We Need?

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PTA Orientation: Questions to ConsiderQ1. Practical applications of Pressure Transient Analysis (PTA)?A1. Estimate/evaluate the following:

●Reservoir properties (e.g., k, S, xf, λ, ω, etc.). (rarely volume)●Productivity efficiency (damage or stimulation). (direct assessment)●Reservoir pressure (initial or average pressure). (pavg → long shut-in)

Q2. What are the major issues or complications in PTA?A2. Major issues/complications in PTA:

●Planning of pressure transient test. (always model prior to testing)●Preparation of well for testing. (execution failures)●Production history is not trivial. (can corrupt interpretation)●Well completion — have records at hand. (leaks, tubulars, placement)●WHY YOU ARE TESTING THE WELL — WHAT IS THE OBJECTIVE?

Q3. Comparison with of PTA with Production Analysis (PA)?A3a. PTA (Pressure Transient Analysis)

●HIGH resolution/HIGH frequency (pressure) data. (quality/quantity)●Gives SNAPSHOT of the well performance at that time. (stress test)

A3b. PA (Production Analysis)●LOW resolution/LOW frequency (pressure) data. (quality/quantity)●LUMPS entire life of well into analysis. (passive monitoring)

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Lecture: End of Presentation●Orientation for Pressure Transient Testing:

ObjectivesData (ALL data: reservoir, well, completion data, pressure/rate history)Analysis and Interpretation

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Notes:___________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

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