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

Well testing part

Books (available @ DITAG library):

Horne R., 2001. Computed aided well test analysis

Bourdet D., 2002. Well test analysis: the use of advanced interpretation models. Elsevier.

Papers (SPE One-Petro library available from DITAG computers)

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 well on 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 of characteristics);

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 formation damage (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 (minutes)

OBJECTIVES: formation pressure, fluid samples, mobility

MDT Mini DST

DESCRIPTION It is possible to test the fluids in an open hole or cased hole (perforations h=30 cm) by setting packers above and below the interval of interest. This way a well interval is isolated and the formation fluids are allowed to flow into the well by using a downhole pump. The tools can be run in hole by wireline or drill pipes. The formation pressure and fluid mobility (thus permeability) can be measured and the formation fluids sampled.

Courtesy of Schlumberger

Production test with DST string

Well Test Type

WELL: exploration

COMPLETION: DST string + TCP

DURATION: variable (hours)

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

DESCRIPTION A drill stem test is a test which uses special tools mounted on the 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 valve at the base of the testing tool, and reservoir fluid flows up the drill string. A common test sequence is to produce, shut in, produce again and shut in again. DST can be quite short, since the positive closure of the downhole valve avoids wellbore storage effects.

Courtesy of Schlumberger

WELL: production

COMPLETION: final

DURATION: variable (2-7 days)

OBJECTIVES: formation pressure, permeability, well damage, reservoir boundaries, well deliverability

Standard production test

Well Test Type

BuildUp DrawDown

1

Standard P.T.

Well Test Type

BU DD

DRAWDOWN PERIOD A well that is static, stable and shut-in is opened to flow and the downhole pressure measured as the pressure declines. For the purposes of traditional analysis, the flow rate is supposed to be constant. Many of the traditional analysis techniques are derived using the drawdown test as a basis. However, in practice, a drawdown test may be rather difficult to achieve under the 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 or stable, especially if it was recently drilled or had been flowed previously. BUILDUP PERIOD A well which is already flowing (ideally at constant rate) is shut in, and the downhole pressure measured as the pressure builds up. Analysis of a buildup test often requires only slight modification of the techniques used to interpret drawdown test. The practical advantage of a buildup test is that the constant flow rate condition is achieved (since the flow rate is zero). Buildup tests also have disadvantages, the main being that production is lost while the well is shut in.

2

1

2

Running gauge in hole

Real Well Test Drawdown 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

DESCRIPTION Interference tests require long-duration production or injection rate changes in the active well. The associated pressure disturbance recorded in the observation well(s) yields information regarding the degree of hydraulic communication within the interwell region.

Observation well

Q

t p

t

p

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

DESCRIPTION These tests are conducted to determine crossflow between two layers separated by a low-permeability layer or to detect leaks behind the casing due to poor cementation or through the paker.

Courtesy of Schlumberger

Well Testing

Field Data

Field Data

Surface Data

Bottom Data

Produced Fluids

Wellhead Pressure

Wellhead Temperature

T p

Flow Rates

Field Fluid Properties

Separators

DWT

SRO Memory Gauge

Well Testing Equipment

Surface Equipment:

Downhole Gauges:

electronic (Memory , SRO) gauges

bi three phase separator

choke manifold

Dead Weight Tester (DWT)

heater

piping/burners

Courtesy of Schlumberger

Bottom Pressure / Temperature :

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

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

Flow Rate:

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

Field Data Quality Control

Well Testing

Downhole Gauge Specifications

Mechanical Gauges (Amerada)

are at home to enjoy their pension!

Downhole Gauge Specifications

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 and stored 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 and personnel) but possibility to intervene changing the test sequence if needed.

Courtesy of Schlumberger

Microsystems Technology