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W ELL TESTI NG COURSE

2 0 0 4

Au t h or s: Enzo Ber e t t aENI E& P

Fr an cesca Ver g a

Pol i t ecn ico d i Tor in o

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Preface

The material gathered in this volume was first presented and discussed during the WellTesting course held in 1998, in S. Donato (MI) and was further reviewed for the subsequenteditions of the Well Testing course.

These slides are aimed at providing with some reference guidelines both those engineersand geologists who are already familiar with well test analysis and those who are just approachingthe subject. It is not meant to be a handbook and, therefore, the authors recommend that the

readers refer to textbooks or to appropriate research papers available from the literature for adeeper understanding of the basic theory and current practice of well testing.

The mathematical theory behind the presented equations and models is not explained intodetails, only the underlying assumptions are summarized. Note that the coefficients appearing inthe formulae are coherent with the S.I. system of units, so readers who use other systems ofunits should adapt the formulae to their particular systems.

SECTI ON 1 provides an overview on well testing targets and standards, equipment andgauge specifications, field data collection and quality control.

SECTI ON 2 deals with flow equations for both oil and gas reservoirs. Reservoirs are firstregarded as infinite acting systems; then bounded reservoirs are taken into account.

The main well testing operating procedures, the flow equations used to interpret the field

data, and the parameters that can be obtained are outlined in the case of both oil and gas wells.Eventually, an introduction to the pressure derivative method, currently used worldwide for

well test interpretation, is presented.

SECTI ON 3 thoroughly describes all the theoretical reservoir models together with therespective pressure derivative responses that have been developed in the literature and areavailable in any software for well test analysis.

The models are classified as early time models to evaluate wellbore and near wellbore effects,middle time models to investigate the reservoir behavior, and late time models to infer possibleboundary effects.

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Th e Pr in cip le o f W e l l Test in g i s ...

to analyse the output signal of a well on which a

known input signal has been applied.

well

INPUTINPUT OUTPUTOUTPUT

Production / Injectionflow rates

Bottom hole pressure

Well Testing

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0

1000

2000

3000

0 2 0 4 0 6 0 8 0 1 20 1 6 0

TI ME , hour s

BH

P,psia

RATE

Standard Well Test

RATE and BHP -vs- time

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define nature of produced fluids and well deliverability

estimate reservoir properties ( i.e : Pf , K , Skin )

evaluate reservoir size ( drenage area )

analyse reservoir heterogeneities ( i.e : sealingboundaries )

design remedial jobs ( i.e : acid treatment / gravel

packs / fracturing )

verify completion efficiency

design the surface production facilities

and for new infilling wells :

optimise drilling technique by choosing the best mud in

order to minimise formation damage

Main Targets of Well Test

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DST

Standard Production Test

Interference Test (areal /vertical)

Limit Test

Types of Well Test

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Well : exploration

Completion : DST string

Duration : very short ( few hours )

Objectives : 1) Fluid samples2) Formation pressure

Well Test Type

DST

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Well : exploration / producing

Completion : final

Duration : variable ( 2 - 7 days )

Objectives : 1) Pf , K , well damage

2) Reservoir boundary

3) Well deliverability

Well Test Type

Standard P.T.

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First Well : active ( producing ) well

Second Well : observer well ( shut - in )

Duration : variable ( days - weeks )

Objective : Recognise reservoircontinuity between wells

Well Test Type

Areal Interference

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First Pool : active ( producing ) pool

Second Pool : observer pool ( shut - in )

Duration : variable ( hours - days )

Objective : Recognise within the wellpools comunication

Well Test Type

Vertical Interference

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Well : exploration / producing

Completion : final

Duration : variable ( days - weeks )

Objectives : 1) Investigate reservoirboundaries

2) Estimate reservoir size

Well Test Type

Limit Test

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Separa to rs

Flow Ra tes

Fie ld Flu id

Proper t ies

ProducedF lu ids

DW T

Wel lheadPressure

Wel lheadT emper a tu r e

Sur faceD a ta

Pr essu r e Tem per a t u r e

SROM e m o r yGauge

B o t t o mD a ta

FieldDa ta

Well Testing: Field Data

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Bo t t o m Pr e ssu r e / Tem p er a t u r e :Bo t t o m Pr e ssu r e / Tem p er a t u r e :

Always check the coherence of the instrumental response in terms ofboth pressure and temperature ( Validation Gauges - INT/2 ) .

Always check bottomhole pressures against the correspondingwellhead pressures (DWT) taken as a reference.

Flow r a tes :Flow r a tes :

The flow rates selected in the Rate History must be homogeneouswith the corresponding bottomhole p ( Validation Rates - INT/2 ).

Well Testing: Field Data Quality Control

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Surface equipment : a) Bi-threephase separators

b) Choke manifoldc) Dead weight tester ( DWT )

Downhole gauges: a) mechanical ( Amerada )b) electronic :

Memory gaugeSRO gauge

Well Testing Equipment

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Downhole gauge specifications

Mechan ica lMechan ica l GaugeGauge ( Am er a da )( Am er a da )

Max W.P.Max W.P. : up to 20 Kpsia

Max W.T.Max W.T. : up to 200 C ResolutionResolution : 0.05% full scale ( 5 psi@ 10Kpsi )

AccuracyAccuracy : 0.4 % full scale ( 40 psia@ 10Kpsi )

DriftDrift : 10 psi/1day - 10 psi/week

NOWADAYS HARDLY EVER USED

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Elect r on ic Gau g esElect r on ic Gau g es ( Mem o r y / SRO )( Mem o r y / SRO )

Max W.P.Max W.P. : 20 Kpsia - Max W.T.Max W.T. : 185 - 190C

Strain Gauge Quartz Gauge

ResolutionResolution : 0.2 psia 0.01 psia

AccuracyAccuracy : 10 psia 2 psia

DriftDrift : 3 psi/1day negligibleor 1.5psi/week

Downhole gauge specifications

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Gauges specifications

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Courtesy of Schlumberger

Microsystems-Technology

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S i m u l a t i o n

Geo m et r i ca l Pet r o p h y si cal PVT

Q.C.

R a t e H i s t o r y

Q.C.

BH P

St a r t i n g A n al y si s : I n p u t Ph a se

Well Testing: Simulation Phase-INT/2