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T

he number o proessionals

specializing in pressure-transient

testing is relatively small. Thus,

those who become procient in it will

likely remain in demand throughout

their careers as key technical resources

in operating or service companies.

 What would you be doing should

you decide to look urther into a

career ocused on this specialty?

Traditionally, pressure-transient

testing means testing or drawdown

or buildup in a production well, or

injection or allo in an injection well.

Transient tests are conducted both

in newly drilled exploration and

development wells and in wells that have

been on production or a long time.

The most important tests are

 perormed in exploration and

development wells. Key objectives

or exploration wells are to determine

ormation temperature and pressure,

acquire a bottomhole-fuid sample,

estimate well productivity, and evaluate

seismic eatures that may delineate

reservoir limits. But permeability

 values estimated rom logs are oten

not representative o values apparent

when a well is on production. Likewise,

 permeability measurements rom

cores retrieved during drill ing

are requently unreliable.

However, the logging program

might include advanced ormation tests

designed to acquire a bottomhole-

fuid sample and determine ormation

 pressure, temperature, and permeability,

which may include both horizontal and

 vertical permeability values. Some

 pressure-transient specialists ocus

on these measurements. Formation

tests have become increasingly

important in recent years, in particular

because many regions now restrict

faring during exploration fow tests,

orcing test designs to avoid producing

reservoir fuids to the surace.

Dynamic Interpretation

Drillstem or cased-hole production tests

are required when the test objective

is a more representative permeability

measurement, and especially when

evaluating reservoir limits. These tests

underscore a natural alignment between

testing specialists and geophysicists. The

latter may use seismic data to identiy a

contact or barrier, but only a dynamic

test can conrm the barrier’s actual

impact on fuid fow. This relationship

has become increasingly important

with the rise o 3D seismic, which

reveals both structural and stratigraphic

eatures. When combined with geologic

interpretation that integrates well-log

data with seismic interpretation, a

  pressure-transient-testing specialist’s

dynamic interpretation rom a well test

becomes especially compelling.

The economic importance o 

integrated reservoir characterization

to the E&P business cannot be

overestimated. I tests demonstrate

reservoir continuity over long distances,

the number o wells requi red or

development may be reduced. Or, in

contrast, i structural or stratigraphic

compartmentalization is indicated, the

development plan will need to include

more wells, and pressure maintenance

by waterfooding could be problematic.

 A speciali st who develops a new

model or transient interpretation

may become internationally known

and may be associated with the

model or an entire career. Also, a

ew proessionals in this eld develop

new tools to aid or automate the

interpretation process or to handle and

 process large data sets. These activities

require a h igh level o prociency in

mathematics and/or signal processing

as well as programming skills.

Discover a Career

Christine Ehlig-Economides, a proessor o petroleum

engineering, holds the Albert B. Stevens endowed chair at

Texas A&M University. Previously, she worked 20 years or

Schlumberger in well-test design and interpretation, integrated

reservoir characterization, modern well-construction design,

and well stimulation. She is a member o the US National

 Academy o Engineering. Ehlig-Economides earned a

BA degree in math-science rom Rice University, an MS

degree in chemical engineering rom the University o Kansas, and a PhD degree

in petroleum engineering rom Stanord University. She has worked in more than

30 countries, published more than 60 papers, and received several SPE awards.

Discover a Career inPressure-Transient TestingChristine A. Ehlig-Economides

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1Vol. 6 // No. 3 // 2010

The most successul transient-

test interpreters learn to collaborate

with geophysicists, geologists, and

 petrophysicists, as well as drilling,

 production, and reservoir engineers.

Both in rationalizing and designing

tests and in analyzing them, these

collaborations greatly enrich what

can be learned and usually resolve

ambiguities that can arise when the

transient data are analyzed without

enough external inormation.

Today, many development wells a re

equipped with a permanent downhole

 pressure gauge, or at least a wellhead

 pressure gauge, providing continuous

 pressure measurements. Since well rates

are usually known directly or by back

allocation rom a gathering system, the

combination o well rate and pressure

may be known over long periods o 

time. Every time the well is shut in

or operational reasons, a pressure-

buildup test is produced. While not

on production, a well equipped with a

 permanent pressure gauge can also

sense pressure transients related to

neighboring wells, thereby acquiring

intererence data that help to characterize

the reservoir between wells.

These data provide well surveillance

that enables detection o well problems

and provides parameters that may be

used in the design o a workover or

stimulation treatment. Also, modern

deconvolution approaches provide a

 virtual drawdown response extending

rom the time well production starts

to the end o the last buildup transient

selected or the deconvolution. The

 virtual drawdown oers a long-term view

o the well’s transient behavior, which

can be sensitive to distant boundaries,

or even provide a characterization

or the entire well-drainage area.

Such data cannot be provided byany single pressure-buildup test.

 

Solving the Puzzle

 Well-test interpretation is oten like

solving a puzzle. The objective is to nd

a suitable conceptual model or observed

 pressure-transient behavior. Conceptual

models can provide considerable insight

about the nature o well damage, well

 productivity, completion geometry,

and reservoir heterogeneity. Such

well-centric insights can be invaluable

to understanding the impact on well

 productivity o a horizontal vs. a

 vertical well, o partial completion,

o hydraulic racture stimulation, or

even o multiple transverse hydraulic

ractures in a horizontal well, which

are typically not discernable through

eldwide reservoir simulation.

 As can be imagined, the complex

understanding required o pressure-

transient-testing proessionals, as well

as the impact that understanding can

have on well productivity and economics,

translates into a demanding career

whose challenges ew may be able to

meet. In addition, well-test gurus in

major multinational companies may be

called on to consult on tests anywhere the

company operates. In my case, working

or the service sector, I part icipated in

test interpretations or many companies

all over the world. When not actively

working on a particular test, I was oten

asked to present the eld’s latest ideas

in seminars, training schools, and SPE

applied technology workshops, orums,

and conerences. Technology transer is

a necessary norm in th is eld. However,each test I worked on nearly always

 presented a new and di erent challenge,

because doing what is known and routine

does not justiy the use o a specialist.

 As the ocus o the industry has

evolved, the needs in pressure-transient-

test design and interpretation have also

evolved. In the 1960s and 1970s, models

were developed to explain pressure-

transient behavior in hydraulically

ractured vertical wells. When horizontal

wells became common, new models

were developed. Today, models can

show pressure-transient behavior

in horizontal wells with longitudinal

or transverse ractures and can also

address the role gas adsorpt ion plays to

adjust the pressure transient in shale gas

wells. Throughout this time, specialists

in both modeling and interpretation

have had a continuous stream o new

challenges to keep them interested.

 And the challenges keep fowing.

For example, using temperature

instead o rate or multilayer

reservoir characterization is a new

development in pressure-transient

analysis that shows a lot o promise.

In the 1980s, tests were developed

or determining properties o individual

layers commingled in a vertical or

slanted well. This work required

combining transient pressure and rate

measurements, with the latter typically

 provided by a spinner fowmeter.

A New Approach

Because it is dicult, i not impossible,

to install multiple fowmeters in a

 production-log string over the depth

span necessary to station them above

each productive zone, multilayer

transient-test designs typically cal l

or as many rate changes as there are

layers to characterize. In recent work

at Texas A&M University, however,

transient pressure and temperature

measurements have been combined or

the same purpose. The new approach

was motivated by the availability o 

distributed temperature sensors that

use ber-optic technology to provide

continuous temperature measurements

along an entire wellbore. While it

has been understood or decadesthat temperature logs can provide an

estimate or the fow prole in a vertical

well with infow rom commingled

layers, modeling temperature transients

has shown that the temperature-log

 prole is really a static snapshot and

that it may be possible to use variations 

in temperature with time to quantiy

 permeability and skin. While this was

already possible using previously

“Because the number of

professionals in this areais relatively small, those

who become proficient in

it remain in demand.”

Continued on page 35 

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