well testing, polito

2009

WELL TESTING COURSE

Francesca VERGA – POLITECNICO DI TORINO

Introduction to well testing

types

objectives

equipment

Recap of flow equations for an infinite acting porous medium

Flow equations for closed reservoirs

Tests description and interpretion for oil and gas wells

Drawdown periods

Buildup periods (Horner plot)

Recap of well productivity and deliverability

Introduction to the pressure derivative method

Pressure derivative analysis: interpretation models for early, middle and late time

Excercises on well tests interpretation (Interpret by Paradigm/Saphir by Kappa)

Tutorials

Real cases

Numerical well testing (Saphir by Kappa)

Basics on unconventional well testing

Decline curves

Tutorials (Topaze by Kappa)

Course Program

S(well + reservoir)

INPUT

Production/Injection flow rates Bottom Hole Pressure

Well Testing

The principle of Well Testing is… to analyze the output signal of a wellon which a known input signal has been applied.

OUTPUT

Rate and Bottom Hole Pressure vs Time

Standard Well Testing

Main Targets of Well Testing

Reservoir description:

define nature of produced fluids;

estimate (initial) static pressure pi (ps )

estimate permeability (horizontal and vertical k);

analyse reservoir heterogeneities (natural fractures, layering, change ofcharacteristics);

evaluate reservoir size (drainage area);

Well description:

define production potential (Productivity Index, well deliverability)

well damage (skin factor S);

design remedial jobs (i.e.: acid treatment, gravel pack, fracturing);

verify completion efficiency;

design surface production facilities;

optimize drilling technique by choosing the best mud in order to minimize formationdamage (for new infilling wells)

Types of Well Test

Mini DST – MDT (Modular Dynamic Tester)

Standard Production Test

Limit Test

Interference Test

areal interference

vertical interference

Mini DST – MDT

Well Test Type

WELL: exploration

COMPLETION: Open/Cased hole DST string

DURATION: very short (few hours)

OBJECTIVES: formation pressure, fluid samples, mobility

MDT – Mini DST

DESCRIPTIONIt is possible to test the fluids in an open hole or cased hole(perforations h=30 cm) by setting packers above and belowthe interval of interest. This way a well interval is isolated (1m) and the formation fluids are allowed to flow into the wellby using a downhole pump. The tools can be run in hole bywireline or drill pipes. The formation pressure and fluidmobility (thus permeability) can be measured and theformation fluids sampled.

Courtesy of Schlumberger

Production test with DST string

Well Test Type

WELL: exploration

COMPLETION: DST string + TCP

DURATION: variable (2-7 days)

OBJECTIVES: formation pressure, fluid samples, permeability, well damage, ……

DESCRIPTIONA drill stem test is a test which uses special tools mounted onthe end of the drill string, comprising a downhole valve,pressure gauges, and fluid samplers.The well can be perforated in underbalance conditions,avoiding mud invasion. The well is opened to flow by a valveat the base of the test tool, and reservoir fluid flows up thedrill string.A common test sequence is to produce, shut in, produceagain and shut in again. DST can be quite short, since thepositive closure of the downhole valve avoids wellborestorage effects.


WELL: production

COMPLETION: final

DURATION: variable (2-7 days)

OBJECTIVES: formation pressure, permeability, well damage, reservoirboundaries, well deliverability

Standard production test

Well Test Type

BuildUpDrawDown

1

Standard P.T.

Well Test Type

BUDD

DRAWDOWN PERIODA well that is static, stable and shut-in is opened to flowand the downhole pressure measured as the pressuredeclines. For the purposes of traditional analysis, theflow rate is supposed to be constant. Many of thetraditional analysis techniques are derived using thedrawdown test as a basis. However, in practice, adrawdown test may be rather difficult to achieve underthe intended conditions. In particular:(a) it is difficult to make the well flow at constant rate,even after it has (more-or-less) stabilized;(b) the well condition may not initially be either static orstable, especially if it was recently drilled or had beenflowed previously.

BUILDUP PERIODA well which is already flowing (ideally at constant rate)is shut in, and the downhole pressure measured as thepressure builds up. Analysis of a buildup test oftenrequires only slight modification of the techniques usedto interpret drawdown test. The practical advantage of abuildup test is that the constant flow rate condition isachieved (since the flow rate is zero).Buildup tests also have disadvantages:(a) It may be difficult to achieve the constant rateproduction prior to the shut in.(b) Production is lost while the well is shut in.

2

1

2

Running gaugein hole

Real Well TestDrawdown Period

Buildup Period

Pulling out gauge

Limit Test

Well Test Type

WELL: exploration/appraisal

COMPLETION: provisional or final

DURATION: variable (days-weeks)

OBJECTIVES: investigate reservoir boundaries, estimate reservoir size

Areal Interference

Well Test Type

FIRST WELL: active (producing) well

SECOND WELL: observation/spy well(s) (shut-in)

COMPLETION: final

DURATION: variable (days-weeks)

OBJECTIVES: recognise reservoir continuity between wells

DESCRIPTIONInterference tests require long-duration production or injection ratechanges in the active well. Theassociated pressure disturbancerecorded in the observation well(s)yields information regarding thedegree of hydraulic communicationwithin the interwell region.

Observation well

Q

tp

tp

t

Areal Interference

Well Test Type

producing well

observation well

observation well

Dt = time lag

Vertical Interference

Well Test Type

FIRST POOL: active (producing) pool

SECOND POOL: observer pool (shut-in)

COMPLETION: final – dual completion

DURATION: variable (hours-days)

OBJECTIVES: recognise comunication between the pools or at the well

DESCRIPTIONThese tests are conducted todetermine crossflow between twolayers separated by a low-permeability layer or to detectleaks behind the casing due topoor cementation or through thepaker.


Well Testing

Field Data

Field Data

SurfaceData

BottomData

ProducedFluids

WellheadPressure

WellheadTemperature

T p

Flow Rates

Field FluidProperties

Separators

DWT

SRO MemoryGauge

Well Testing Equipment

Surface Equipment:

Downhole Gauges:

electronic (Memory , SRO) gauges

bi – three phase separator

choke manifold

Dead Weight Tester (DWT)

heater

piping/burners


Bottom Pressure / Temperature :

Always check the coherence of the instrumental response in terms of both pressureand temperature (option “Validation Gauges“)

Always check gauge pressure against the corresponding wellhead pressures (DeadWeight Tester) taken as a reference.

Flow Rate:

The flow rates selected in the Rate History must be homogeneous with thecorresponding bottomhole pressures ( option “Validation Rates“)

Field Data Quality Control

Well Testing

Downhole Gauge Specifications

Mechanical Gauges (Amerada)…

…are at home to enjoy their pension!


Electronic Gauges (Memory / SRO)

Max W.P.: 20 kpsia

Max W.T.: 185 – 190 °C

Resolution: 0.2 psia

Accuracy: 10 psia

Drift: 3 psi/1° day or 1.5 psi/week

Strain Gauge Quartz Gauge

Resolution: 0.01 psia

Accuracy: 2 psia

Drift: negligible

Memory-The gauge is placed downhole, there is no connection to the surface during the

test. Batteries provide energy to make the tool work. Data are recorded andstored during the test. The gauge is recovered when the test is over. Low cost but

no possibility to monitor the test.

Surface Read Out- Connection to the surface and data transferring is maintained during

the test through an electric wireline cable. High cost (surface wireline unit andpersonnel) but possibility to intervene changing the test sequence if needed.


Microsystems Technology

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