-
32 Oileld Review
Step Change in Well Testing Operations
In exploration and appraisal environments, one way to gather
data for well
productivity and reservoir characterization is through well or
drillstem testing.
The acquisition of downhole well test data has recently been
enhanced by the
development of an acoustic wireless telemetry system that gives
operators access
to these data in real time.
Amine Ennaifer Palma GiordanoStephane VannuffelenClamart,
France
Bengt Arne Nilssen Houston, Texas, USA
Ifeanyi Nwagbogu Lagos, Nigeria
Andy SooklalCarl WaldenMaersk Oil Angola AS Luanda, Angola
Oileld Review Autumn 2014: 26, no. 3. Copyright 2014
Schlumberger.For help in preparation of this article, thanks to
Michelle Parker Fitzpatrick, Houston; and David Harrison, Luanda,
Angola.CERTIS, CQG, InterACT, IRDV, Muzic, Quartet, RT Certain,
SCAR, Signature and StethoScope are marks of Schlumberger.
By the time Edgar and Mordica Johnston per-formed the rst
commercial drillstem test in 1926, more than two dozen formation
tester pat-ents had been issued. Before the Johnston broth-ers
introduced their innovative methods, if oil did not ow to the
surface, exploration wells were tested through bailinglowering a
hollow tube on a cable to capture a formation uid sampleafter
casing had been set and cemented above the zone of interest. The
brothers success led to the creation of the Johnston Formation
Testing Company, which Schlumberger acquired in 1956.
Today, the most common drillstem tests (DSTs) are temporary well
completions through which operators produce formation uids while
the drilling unit is on location. During DSTs, for-mation uids are
typically produced through drillpipe or tubing to a test separator
or other temporary surface processing facility, where the uids are
metered, sampled and analyzed.
Drillstem tests focus on acquiring various types of data. A
descriptive test may concentrate on acquiring downhole reservoir
uid samples and pressure data from a shut-in well; a produc-tivity
test may focus on identifying maximum ow rates or determining
reservoir extent. In explora-tion and appraisal wells, the primary
well test objectives focus on well deliverability, skin, uid
sampling, reservoir characteristics and identication of reservoir
extent and faults.1 In development wells, the objectives are
typically linked to measurements of the average reservoir pressure
and skin and determination of reservoir characteristics.
Well test operations comprise cycles of well ow and shut-in
while bottomhole pressures (BHPs) are monitored. Reservoir
engineers apply
these data to make early predictions about res-ervoir potential
through a process known as pressure transient analysis, in which
the rate of pressure change versus time during a shut-in and
drawdown cycle is plotted on a logarithmic scale. The resulting
plots indicate reservoir response patterns that can be associated
with specic reservoir models using generalized type curves; the
curves help determine reservoir characteristics such as skin,
permeability and half-length of induced fractures.
The shut-in mechanism must be as close as possible to the point
at which formation uids enter the wellbore to eliminate the inuence
of wellbore storage on the downhole data. Wellbore storage refers
to the volume of uid in the well-bore that may be compressed or
expanded, or to a moving uid/gas interface as a result of a
production rate change. Wellbore storage may exhibit complex
behavior below the point of shut-in, such as phase segregation,
which can hinder true reservoir response by mixing with or masking
reservoir pressure transients.2 A crucial part of the pressure
transient analysis is distin-guishing between the effects of
wellbore storage and the interpretable reservoir response in the
early stages of the test.
At various points during the test, technicians may capture
representative samples of formation uids through the test string;
uid capture may be performed using dedicated inline sample
car-riers equipped with trigger systems or by deploy-ing
through-tubing wireline-conveyed samplers. The samples are then
sent to a laboratory for detailed PVT analysis in a process that
may take several months.
1. Skin is a term used in reservoir engineering theory to
describe the restriction of uid ow from a geologic formation to a
well. Positive skin values quantify ow restriction, whereas
negative skin values quantify ow enhancements, typically created by
articial stimulation operations such as acidizing and hydraulic
fracturing.
2. Al-Nahdi AH, Gill HS, Kumar V, Sid I, Karunakaran P and Azem
W: Innovative Positioning of Downhole Pressure Gauges Close to
Perforations in HPHT Slim Well During a Drillstem Test, paper OTC
25207, presented at the Offshore Technology Conference, Houston,
May 58, 2014.
3. Kuchuk FJ, Onur M and Hollaender F: Pressure Transient
Formation and Well Testing: Convolution, Deconvolution and
Nonlinear Estimation. Amsterdam: Elsevier, Developments in
Petroleum Science 57, 2010.
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Autumn 2014 3333
By deploying logging-while-drilling tools such as the
StethoScope formation pressure-while-drilling service, engineers
may ascertain initial information about reservoir properties,
formation uid types and producibility. This information is often
coupled with wireline log analysis and for-mation pressure and
sampling data after the well has been drilled through the section
of interest. In exploration and appraisal wells, these esti-mates
may be associated with some uncertainty, and the reservoir
parameters can be conrmed only by monitoring the reservoir under
dynamic conditions such as is done with DSTs.
Drillstem tests provide complementary data for reservoir and
formation uid characterization and for predicting the reservoirs
ability to pro-duce. Of all the data that operators depend on to
design well completions, these data include the least amount of
uncertainty and the deepest radius of investigation.3 The duration,
producing time and ow rate of a DST provide a deeper investigation
into a reservoir than do other res-ervoir evaluation techniques. As
a consequence, well testing provides the bulk of the information
engineers need to design well completions and production
facilities.
Although more efcient, reliable and robust, the primary
components of DST assemblies today are similar to those deployed by
the Johnston Formation Testing Company in the 1930s. These
components consist primarily of four types of devices: packers to
provide zonal isolation downhole valves to control uid ow pressure
recorders to facilitate analysis devices to capture representative
samples.
Changes to test systems over time have been conned mainly to the
addition of auxiliary components such as circulating valves, jars,
safety joints and other devices aimed at reduc-ing the time
required to recover from a stuck testing string or to provide
options for killing a well. In recent years, service companies have
done much to reduce uncertainty and costs associated with well
testing while increasing safety and efciency. A signicant step in
this progression includes the Quartet downhole res-ervoir testing
system.
The Quartet testing tool allows operators to perform the four
essential functions of a DST assemblyisolate, control, measure and
sam-plein a single run. The system includes the CERTIS
high-integrity reservoir test isolation sys-tem, the IRDV
intelligent remote dual valve, Signature quartz gauges and the SCAR
inline independent reservoir uid sampling tool.
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34 Oileld Review
The CERTIS isolation system provides pro-duction-level isolation
with single-trip retriev-ability. It includes a oating seal
assembly to compensate for tubing movement during well testing and
eliminates the need for slip joints and drill collars (below). The
IRDV dual valve is an intelligent remotely operated tool that
allows
operators independent control of the tester and circulating
valve via commands transmitted by low-pressure annular pulses
(below). Signature gauges that have ceramic electronics boards
provide high-quality pressure and temperature
measurements at the reservoir (next page, top left).4 The SCAR
inline independent reservoir uid sampling tool collects
representative reser-voir uid samples from the ow stream (next
page, top right).
The accuracy of reservoir property analysis and the degree of
reservoir understanding are heavily dependent on the quality of
pressure measurements acquired downhole; obtaining accurate
measurements hinges on metrology and its parameters.
Cornerstone of Pressure Transient Analysis Metrology is the
science of measurements based on physics. Technicians use the
methods of metrology to ascertain that sensors are properly
calibrated to specied or technical parameters (next page, bottom).
In the case of pressure gauge metrology, static parameters include
the following: Accuracy is the algebraic sum of all the errors
that inuence the pressure measurement. Resolution is the minimum
pressure change
that can be detected by the sensor and is equal to the sum of
sensor resolution, digitizer resolu-tion and electronic noise
induced by the ampli-cation chain. Therefore, when determining
gauge resolution, engineers must consider the associated
electronics and specic sampling time. The resolution of the
interpreted range of investigation, or transient drainage radius,
depends on the resolution of the gauge. Gauge metrology could
impact important decisions operators make in evaluating reservoir
size and extent, which is a key objective of well testing
interpretation.5
Stability is the ability of a sensor to retain its performance
characteristics for a relatively long period of time and is the
sensor mean drift in psi/d at a specied pressure and tempera-ture.
The levels of stability include short-term stability for the rst
day of a test, medium-term stability for the following six days and
long-term stability for a minimum of one month.
Sensitivitythe ratio of the transducer output variation induced
by a change of pressure to that change of pressureis the slope of
the trans-ducer output curve plotted versus pressure.
Dynamic parameters include the following: Transient response
during pressure changes
is the sensor response recorded before and after a pressure
variation while the tempera-ture is kept constant.
Transient response during temperature changes is the sensor
response monitored under dynamic temperature conditions while the
applied pressure is kept constant. This param-
Stinger seal
Sealbore
Release ring
Slips
Bypass
Ratchet lock
Hydraulicsetting mechanism
Rupture disc
Stinger release
Stinger
Seal element
Perforating guns
> Isolation system. The CERTIS systems hydraulic setting
mechanism is activated by applying pressure to a rupture disc;
setting does not require string rotation or mechanical movement. To
unset the system, an upward force disengages the ratchet lock and
shears the retaining pins in the release ring, which allows the
slips to relax and release the system. Continued pulling reopens
the bypass, which eliminates swabbing while pulling the packer out
of the hole. The stinger oats inside the sealbore, which
compensates for string movements caused by temperature changes. The
system allows gauges to be positioned below it in the test string.
Tubing-conveyed perforating guns can be suspended below the
body.
>Remote dual valve. The IRDV intelligent remote dual valve
combines a test valve and a circulating valve that may be cycled
independently or in sequence. The test valve, the primary barrier
during the well test buildup period, is activated through wireless
commands or low-pressure pulses. Wireless commands facilitate the
independent operation of both valves without interfering with the
operation of other tools in the test string. In the open position,
the circulating valve allows ow between the tubing and annulus.
Low-pressure pulses are detected by the pressure sensor, and the
electronics conrm the received command by comparing it with those
in a library stored in the tool memory. The IRDV valve may be
congured to provide wireless feedback, conrming command reception.
The activation of both valves is initiated by battery power, which
is augmented by a hydraulic uid circuit that discharges uid from
the atmospheric chamber into the hydrostatic chamber when the valve
is operated.
+-
+-
+-
Circulatingvalve (closed)
Atmosphericchamber
Test valve (open)
Hydrostaticchamber
Pressure sensor
Battery
-
Autumn 2014 35
eter provides the stabilization time required for a reliable
pressure measurement for a given temperature variation.
Dynamic response during pressure and tem-perature changes is the
sensor response recorded before and after a change in both pressure
and temperature.
Pressure data help engineers develop infor-mation about the size
and shape of the reservoir
and its ability to produce uids. Pressure tran-sient analysis is
the process engineers use to convert these data to useful
information. During this process, they analyze pressure changes
over time, particularly those changes that are associ-ated with
small variations in uid volume.
During a typical well test, a limited amount of uid is allowed
to ow from the formation while the pressure measurement at the
sandface is acquired along with downhole and surface ow rate
measurements. After the production period, the well is shut in
while downhole pressure data acquisition continues during the
buildup.
4. For more on Signature gauges: Avant C, Daungkaew S, Behera
BK, Danpanich S, Laprabang W, De Santo I, Heath G, Osman K, Khan
ZA, Russell J, Sims P, Slapal M and Tevis C: Testing the Limits in
Extreme Well Conditions, Oileld Review 24, no. 3 (Autumn 2012):
419.
5. Kuchuk FJ: Radius of Investigation for Reserve Estimation
from Pressure Transient Well Tests, paper SPE 120515, presented at
the SPE Middle East Oil and Gas Show and Conference, Bahrain, March
1518, 2009.
> The Signature quartz gauge. The Signature gauge consists of
a sensor, electronics section and battery. The sensor includes a
multichip ceramic module (not shown).
Battery
Electronics
Sensor > Downhole uid sampler. The SCAR inline independent
reservoir uid sampling tool (left ) captures representative,
contaminant-free, single-phase uid samples directly from the ow
stream close to the reservoir. The tool houses the single-phase
reservoir sampler (right ). Using a rupture disc triggering
mechanism, initiated by applied annular pressure or through
wireless command, the sampler can be activated to open a ow channel
to capture a sample. The single-phase reservoir sampler has an
independent nitrogen charge to ensure each sample remains at or
above reservoir pressure. When the triggering mechanism is
activated, reservoir uid is channeled to ll a sample chamber
bounded by pressure compensation uid. The compensation assembly
comprises the nitrogen precharge, pressure compensation uid and
buffer uid, which ensure that the sample chamber slowly provides
enough volume to capture the reservoir uid without altering its
properties.
Rupture disctrigger
Buffer fluid
Nitrogen
Pressurecompensationfluid
Pressurecompensationfluid
Reservoirfluid
Single-phasereservoirsampler
> Gauge metrology parameters.
Static
Gauge Metrology Parameters
Accuracy
Dynamic
Resolution
Stability
Transient response during pressure changes
Transient response during temperature changes
Dynamic response during simultaneous pressure and temperature
changes
Sensitivity
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36 Oileld Review
The downhole gauges that capture the reser-voir response during
the well test must be highly accurate, but high accuracy is difcult
to achieve because of the complex wellbore environment. During well
tests, uid dynamics and thermal and mechanical string effects
impact tool response.
The technology used to capture pressure data has evolved
considerably over time. In the 1930s, operators deployed mechanical
gauges, which provided resolution of about 35 kPa [5.1 psi].
These gauges operated by recording the displace-ment of a
pressure sensing element on a sensitive surface, which was rotated
by a mechanical clock, thus providing a pressure versus time
mea-surement. The data were digitized manually from the
pressure-time chart.
Following improvements in electronics design and reliability led
by the Hewlett-Packard Company, electronic gauges were introduced
to the oil industry in the 1970s. Development of sta-ble electronic
gauges with higher levels of accu-
racy progressed rapidly, and by the turn of the century, two
main types dominated the industry.
Strain gauges were the rst electronic gauges used widely in the
oil industry. They operated on the principle of a resistance
circuit placed on a pressure sensitive diaphragm. The change in
length of the diaphragm in response to pressure altered the balance
of a Wheatstone bridge cir-cuit. These strain gauges were capable
of 0.7-kPa [0.1-psi] resolution, which may not be sufcient to
resolve reservoir properties.
Vibrating quartz pressure sensors, developed in the 1970s,
signaled a signicant shift in the quality of downhole measurements
in terms of metrology. Because of their superior metrological
characteristics, quartz gauges have become the standard for
downhole pressure and temperature acquisition although their
accuracy may be affected by sudden changes in downhole temper-ature
and pressure. Quartz sensors use the piezo-electric effect to
measure the strain caused by pressure imposed upon the sensing
mechanism. The frequency of vibration in relation to pressure
changes is measured and converted to digital pressure measurements.
The high frequencies of quartz sensors enable measurement of high-
resolution pressure changes and rapid sensor response. Typical
resolution of quartz gauges is 0.07 kPa [0.01 psi]. Today, the
Schlumberger Signature CQG gauge, using a proprietary com-pensated
quartz gaugethe CQG crystalis able to distinguish pressure
measurements as small as 0.021 kPa [0.003 psi] (left).
Signature gauges may be deployed in reser-voir tests at
temperatures up to 210C [410F] and pressures reaching 200 MPa
[29,000 psi]. They may be deployed in real-time or memory mode as
part of the test string and are contained within gauge carrier
mandrels able to hold up to four gauges each. Numerous carriers can
be placed in the test string above and below the CERTIS isolation
system.
The challenge of downhole measurements is not limited to the
harshness of ambient condi-tions; three major sources of
uncertainty affect downhole pressure measurements during well
testing. Uncertainties in gauge resolution and accuracy, which are
typically characterized as functions of the magnitude of pressure
and tem-perature changes downhole, may introduce errors. In
addition, uncertainty in the condition of the environment may
induce error.6 For exam-ple, during the test owing period, a gas
bubble close to the gauge may burst and create high-fre-quency
noise that is of the same order of magni-tude as the gauge accuracy
and several times larger than the gauge resolution. If the
pressure
>The impact of high resolution on data quality. Analysts can
use high-resolution measurements (top ) acquired using a Signature
gauge to deliver a clear interpretation of the pressure data.
High-quality pressure data (middle, green) result in a pressure
derivative curve (red) that is easily discernable and from which
reservoir engineers can identify various pressure regimes during
buildup. A low-resolution measurement (bottom) may deliver an
uninterpretable dataset.
100
10
1
0.1
0.01
0.0010.0001
Pres
sure
, psi
0.001
10,000
1,000
00.0001
Pres
sure
, psi
Time, s
0.04
0.03
0.02
0
0 10 20 30 40 50 60 70 80 90 100 110 120
0.01
Pres
sure
, psi
Time, h0.001 0.01 0.1 1 10 100
Time, h0.01 0.1 1 10 100
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Autumn 2014 37
changes quickly, and the sampling rate is rela-tively slow when
this occurs, separating high-fre-quency noise from measurements is
difcult. A similar situation arises if phase segregation of small
quantities of water and gas in the well efu-ent occurs.
With the introduction of quartz gauges, the parameters of
pressure gauge metrology were improved signicantly. However,
experts recog-nized that the value of well tests was often impacted
by the fact that data were inaccessible until after the tests were
complete. To address this shortcoming, they developed a system that
allows operators to monitor the progress of a well test as the test
proceeds by delivering the down-hole pressure and temperature data
to the sur-face in real time. With insights provided by these data,
coupled with real-time downhole control, operators would then be
able to alter ongoing tests to meet their objectives.
Real-Time Data, Real-Time DecisionsTo reduce the uncertainty
associated with some well and reservoir parameters, engineers
typi-cally begin a well test design by dening the objectives of the
test (above). The acquisition of wireless real-time bottomhole
pressure and tem-perature data gives operators the ability to
man-age both the well and reservoir uncertainties, make adjustments
during the test and exercise a measure of control over operational
and cost challenges associated with traditional DSTs.
The sequence and duration of well test opera-tions are based on
initial data obtained from vari-ous sources, including
petrophysical logs and core analysis. Historically, well tests are
based on a design-execute-evaluate cycle, in which techni-cians
design and execute the tests to acquire downhole data for
evaluation and capture uid samples for laboratory analysis.
Downhole data are most frequently acquired using electronic
gauges in memory mode, which do not provide operators with
real-time feedback to validate pretest assumptions, to verify that
objectives are being achieved or to modify the tests during
execution. As a consequence, techni-cians typically execute the
well test program regardless of reservoir response. This can result
in unnecessary steps, prolonged tests, missed opportunities and
even damage to the reservoir. That the pretest assumptions are
wrong or the test is failing to meet objectives is often realized
only after the test has been concluded and the memory data have
been analyzed.
The industry has made attempts to correct this shortcoming by
using surface readout (SRO) systems. These SRO systems deploy
electric line tools to recover downhole data from electronic memory
gauges that are run as part of the DST toolstring. The data
download is typically per-formed toward the end of the test, which
limits any modication of the operation to managing the remainder of
the well test operation and does little to improve the overall
operational sequence.
The practice of deploying electric line tools has become
increasingly unpopular with opera-tors in expensive deepwater
projects. Operators are concerned that the electric line cable may
become snagged or part when it crosses valves. The efciency of
managing well test operations through electric line data
acquisition is also lim-ited because it is typically performed only
during nonowing periods; electric line toolstrings are at risk of
being forced up the hole when the well is owing.
To address these limitations, Schlumberger engineers developed
the Muzic downhole wire-less system (left). The Muzic system is
designed
6. Onur M and Kuchuk FJ: Nonlinear Regression Analysis of
Well-Test Pressure Data with Uncertain Variance, paper SPE 62918,
presented at the SPE Annual Technical Conference and Exhibition,
Dallas, October 14, 2000.
> A downhole reservoir testing system enabled by Muzic
wireless telemetry. A network of acoustic repeaters, attached to
the tubing using a system of clamps, enables remote interrogation
of downhole gauges or tools with feedback via computer terminal at
the rig. Two repeaters installed in each numbered node supply
horizontal redundancy; one repeater is always on standby. Vertical
redundancy is provided by repeaters able to communicate across
twice the normal spacing between repeaters, which is usually 305 m
[1,000 ft].
IRDV valve
Gauge carrier,Muzic wirelesssystem withSignature gauges
Gauge carrier,Muzic wireless
system withSignature gauges
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
192021
Repeaters
Seabed
SCAR sampler
Tubing
Flowhead Reeler
Interface box
Surface PC
Hanger
CERTISisolation system
> Types of well tests, test objectives and acquired data. Two
types of testsdescriptive and productivityprovide a variety of
downhole data. Descriptive tests seek information about well and
reservoir characteristics, whereas engineers typically use
productivity tests to understand the producing capacity, extent and
drive mechanism of a reservoir. Both types require bottomhole
pressure, bottomhole temperature and surface ow rates. Sequence and
duration of individual ow periods differentiate the test types.
Type of Test Test Objectives Acquired Data
Descriptive Well characteristics Bottomhole pressure and
temperature
Productivity Reservoir extent and drive mechanism Bottomhole
pressure and temperature
Inflow performance ratio (combined well and reservoir) Surface
flow rate
Reservoir characteristics (average reservoirpressure,
permeability thickness, storativity ratioand interporosity flow
coefficient)
Surface flow rate
Communication between wells and reservoirs(interference and
multizone tests)
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38 Oileld Review
to be embedded into the Quartet DST string. The system
interfaces with the Quartet reservoir test-ing system to facilitate
interactive well testing operations in which the operator has
direct access to downhole data in real time and is able to control
downhole tools through wireless com-mands. The distributed digital
wireless telemetry system uses an acoustic wave generated in the
test string to transmit information.
The acoustic network is composed of a series of tools clamped on
the outside of downhole test tub-ing (left). Each tool acts as a
repeater and can transmit or receive an acoustic signal as well as
allow control of downhole tools through wireless commands. By
initiating real-time changes to the proposed testing program,
operators can derive the maximum value from each testing
operation.
Digital data are relayed from one repeater to the next in either
direction on their way to their nal destination. In the bottomhole
assembly, the network interfaces either with downhole pressure
gauges for data acquisition or with downhole tester tools (tester
valve, circulating valve and sampler) to issue commands and verify
tool status. This interactive platform also opens the possibility
to expand the scope of reservoir testing to access previously
inaccessible parts of the well for instrumentation and tool
control.
The signal processing techniques used for downhole digital data
transmission are similar to methods employed in other wireless
communica-tions. However, successful wireless transmission is
affected by many things, including pipe or tub-ing effects, ambient
noise and electronics and battery limitations.
For acoustic propagation, tubing is a complex medium; its
effectiveness in propagating acoustic waves is hampered by noise,
attenuation and dis-tortion. For example, each time an acoustic
wave goes through a tubing connection, it generates an echo. The
series of echoes generated by crossing multiple joints are canceled
by advanced signal processing techniques to achieve point-to-point
communication. In addition, because the wireless telemetry system
relies on acoustic propagation, any increase in ambient noise
conditions down-hole can adversely impact transmission.
Additional engineering challenges arise from the low-power
electronics required for long dura-tion battery operation. This
low-power require-ment limits the choice of downhole processors and
impacts the available processing power. To address these
challenges, a specic network pro-tocol was developed that manages
and optimizes communication through a repeater network.
>Network architecture of the Muzic wireless system. The Muzic
wireless network is based on acoustic clamp-on style repeaters
(left ) attached to tubing. The transducer generates an acoustic
signal (red) encoded with digital information. Bidirectional
acoustic energy travels the length of the pipe and is transmitted
from each repeater to adjacent repeaters until the signal reaches
the user at the surface. With such a series of repeaters, a network
architecture (right ) can be established in which transmitting
nodes (R) send and receive information from transmitting hubs and
sensing or actuating end nodes (E). End nodes are points of
interest for the surface user and include sensors to acquire
measurements or actuators to control devices.
Clamp
Surface repeaterS
RepeaterR
End nodeE
Bidirectionalacoustic message
Piezoelectrictransducer
Acoustic message
Productiontubing
S
R R
R R
R R
R R
R R
R R
R R
E EE E
R R
R R
R R
R R
E EE E
E
>Comparing Signature gauge real-time data with memory data.
Pressure data obtained by a Signature quartz gauge and transmitted
wirelessly in real time are a nearly perfect match with data
downloaded from memory during a pressure transient well test
offshore Indonesia for Total E&P. The quartz gauges transmitted
real-time bottomhole pressure and temperature data to the surface
without interruption for almost seven days. These data allowed
pressure transient analysis to be performed in real time and
facilitated the validation of the ongoing well test operations
versus the Total E&P Indonesia test objectives.
Pres
sure
and
pre
ssur
e de
rivat
ive
Time
Pressure
Pressurederivative
MemoryReal time
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Autumn 2014 39
The Muzic system makes possible a new work-ow for real-time
testing operations. A decision tree within this workow includes
risk assess-ment, test planning, data validation, quality assurance
and quicklook validation of data dur-ing the execution phase. This
process allows real-time decisions and adjustments to the testing
plan while the test is underway.
A Real-Time Interpretation Workow In traditional well testing
operations, engineers design, prepare and execute the test and
inter-pret the acquired data in sequence. In this post-mortem
approach to reservoir characterization, insight obtained during
data analysis does not impact the original design or execution
phases, and the interpretation usually takes place after operations
are concluded.
The availability of downhole data and tool sta-tus information
in real time from technologies such as Muzic wireless telemetry
represents a signicant shift from the sequential approach. Feedback
from the reservoir is immediate and available during the execution
phase, allowing the operator to modify the test sequence and
operation while the test string is still in the well. Real-time
information about the condition of the wellbore and status of
downhole tools consider-ably impacts operational efciency and gives
the operator condence in the validity of the mea-surements (above
right).
Introduction of real-time monitoring into the standard well test
workow reduces overall costs and rig time because the process is
driven by actual reservoir responses and not by generally accepted
practices and estimates (right). Any erroneous operational steps
can be immediately identied and remedied, eliminating
uncertain-ties and the costs of repeat operations as a result of
inconclusive operational data.
Total E&P planned an exploration test of a 45 deviated well
offshore East Kalimantan, Indonesia. The target zone was at 3,200 m
[10,500 ft] MD with a bottomhole pressure of 25,000 kPa [3,600 psi]
and a bottomhole temperature of 118C [244F].
The operators test objectives were to analyze the downhole
pressure transient data and obtain initial estimates of key
reservoir properties such as pressure, skin, permeability thickness
and boundaries. A solution was designed around Muzic wireless
telemetry interfacing with high-resolution Signature pressure
gauges. The gauges, which proved to provide data that matched
nearly perfectly with data gathered using memory gauges,
transmitted downhole pressure and tem-perature for almost seven
days (previous page, bottom). This continuous ow of data
allowed
engineers to optimize ow and maintain reservoir conditions below
depletion during testing. The reservoir engineer was also able to
perform real-time interpretations of pressure transient data and
thus validate that test objectives were being
met. Because the engineers were able to deter-mine the test
objectives had been achieved as the test was proceeding, they could
shorten the ow-ing period without fear of losing valuable pressure
transient data.
> A real-time dataset overlaid on a memory dataset. In this
example, data captured in memory mode (green) and real-time data
(red) track perfectly. Data captured in memory mode can be accessed
only when they are downloaded after the test is ended.
Wireless-enabled reservoir testing, however, allows operators to
observe pressures in real time and make decisions accordingly.
Information that operators may derive from real-time test data and
use to make decisions include tubing conditions while running in
the hole (1), underbalance before perforation (2), connectivity
after perforation (3), progress of cleanup and owing periods (4)
and buildup (5, blue shading). The ow rate (blue curve) is visible
in real time throughout the test. Real-time measurements ceased
when the operator began to pull out of the hole after almost seven
days.
Pres
sure
, psi
Rate
, bbl
/dTime, d
8,000
7,000
6,000
5,000
4,000
3,000
2,000
1,000
1,250
2,500
0
00 1 2 3 4 5 6 7 8 9
Memory gaugeReal-time pressure
15
2
3
4
> A workow for integrating the test design, execution and
interpretation sequence in real time. Muzic wireless telemetry and
InterACT collaboration software enable real-time interpretation and
analysis for use in updating the geologic model and rening the
transient analysis and eventual nal reservoir model. The
integration process includes information from the geologic model
(1) used in test equipment selection (2) and test design (3).
Because real-time bottomhole data are available during the test
(4), the test results are continuously compared with the initial
design expectation, and this output (5) helps in rening the nal
interpretation (6). This process continues iteratively for each ow
period and results in a model with least uncertainty for the
reservoir engineer. (Adapted from Kuchuk et al, reference 3.)
Geologic model
Test design
Real-time wellsite or remote-site interpretation
Final interpretation andvalidation model, verification
and uncertaintyOperation and
data acquisition
Hardwareselection
12
3
4 6
5
-
40 Oileld Review
Petrobras engineers working in a presalt envi-ronment in the
Santos basin offshore Brazil sought to obtain real-time data at the
surface during a deepwater well test and to eliminate the wireline
run typically required to acquire such data. Schlumberger and
Petrobras engineers chose to deploy wireless-enabled Signature
gauges in the well, which is in 2,000 m [6,600 ft] of water 250 km
[155 mi] off the coast of Brazil. The Muzic wireless telemetry
system and pres-sure and temperature gauges enabled for wire-less
communication were run in the well. This conguration permitted
engineers to receive data during ow and shut-in periods, to monitor
cleanup efciency in real time and to obtain key reservoir
information before the end of the test (left). As a consequence,
reservoir engineers were able to observe the pressure transient
after perforation gun detonation to conrm dynamic underbalance.
Petrobras and Schlumberger engineers were also able to conrm
downhole valve status, com-pute productivity as the well was owing,
conrm that sufcient data were acquired during the ini-tial and main
buildup periods, eliminate a wire-line run and establish the
reservoir pressure after the initial postperforating ow period
(below left).
A common challenge in well test operations is managing the
duration of the buildup period. Test operators often calculate a
buildup period as an integer multiple of the owing period duration.
By accessing the actual downhole pressure response in real time
during the buildup period, engineers are able to determine that the
desired reservoir response has been achieved and vali-dated sooner
than would be the case using the multiple, thus saving the operator
hours of rig time. Conversely, if the reservoir response objec-tive
has not been met, the test can be extended.
The overall efciency of the operation is improved because
downhole tool status can be veried at each step of the program.
Important decisions about the progress of the test can be made with
clear understanding of the reservoir response from downhole
pressure conditions, which makes the overall operation safer. Using
wireless tool activation also takes less time and requires fewer
operational steps than do tradi-tional pressure activation methods.
Real-time data are important for characterizing the reser-voir with
the least possible uncertainty. The Muzic system enables remote
interpretation through data sharing and collaboration software.
Based on a geologic model, the well test is designed and gauges
and DST tools are selected to meet certain operational and
acquisition criteria.
> Real-time productivity index mapping during well testing.
Using the Muzic system, the operator tracked the productivity index
during ow on several choke sizes.
Prod
uctiv
ity in
dex
Cleanup Second flow Third flowFirstflow
Firstbuildup
Secondbuildup Ch
oke
size
Time, d0 1 2 3 4 5 6 8 9 107
Productionlogging
toolrigup
Real-time productivity index Choke size
> Obtaining critical data in real time. The overlap of
real-time and memory data demonstrates the accuracy of real-time
data and their capability to provide sufcient insight into
operational events, even though the real-time data sampling is less
dense than memory mode sampling. An inset from a separate test
shows TCP gun detonation data (left ); the sharp decrease followed
by a sharp increase in pressure conrms in real time the
postperforation ow of reservoir uid into the wellbore. An inset
from a separate test showing pressure response during the main
pressure transient test (right ) demonstrates that the volume of
data captured is adequate for detailed analysis, such as
productivity index determination and pressure transient analysis,
during ow and buildup periods.
Time
Tubing-conveyed perforating (TCP)gun detonation
Main pressure transient test
BHP
Real-time bottomhole pressureMemory bottomhole pressureReal-time
annulus pressure
Memory annulus pressureReal-time bottomhole temperatureMemory
bottomhole temperature
-
Autumn 2014 41
During the operation, the downhole pressure and surface rate
data acquired by the system are vali-dated in real time, and QA/QC
can be performed immediately. Engineers can use these data for
quicklook interpretations and to determine well and reservoir
parameters. The initial reservoir model may then be updated in real
time with the information from the well test to generate a new
interpretation model, veried with less uncer-
tainty. The process is multidisciplinary and dynamic; results
from interpretation and analysis can be used to modify earlier
assumptions in an iterative fashion and continuously generate a
clearer picture of the reservoir.
Maersk Oil drilled an exploration well offshore Luanda to
acquire data that would conrm the presence of hydrocarbons in the
target formation. The well was drilled into oil-bearing
sandstones;
the primary target was at a depth of approxi-mately 5,000 m
[16,000 ft] in water depth of 1,462 m [4,797 ft].
Downhole gauges enabled by Muzic wireless telemetry transmitted
data successfully through-out the test. The operator was able to
verify the underbalance prior to perforating, establish initial
reservoir pressure after perforating, verify the sta-tus of the
downhole tools during the test, optimize the cleanup period by
monitoring sandface pres-sure, reduce duration of buildup and conrm
that samples were being taken in ideal conditions.
The RT Certain real-time test collaboration service brought
reservoir experts at the rig in Luanda and in Copenhagen, Denmark,
together in a virtual environment. A software platform enabled
wellsite data transmission and interpre-tation tools that allowed
experts to make the right decisions on site and from remote
locations. This integrated system also helped ensure sufcient data
were collected to complete a successful pres-sure transient well
test.
The wireless downhole testing system saved 28 hours of rig time,
about US$ 1.5 million in rig spread costs, while acquiring sufcient
data for key reservoir property estimation (left). A com-parison of
memory data from the gauges retrieved at the surface with the
real-time data used for interpretation during the test validated
the deci-sions made during the operation.
The Future of Well TestingEngineers have long recognized the
value of DSTs but in certain circumstances have had to make
compromises between quality data, costs and risk. Real-time
wireless telemetry addresses those compromises by providing a means
to cap-ture real-time data throughout the test, remotely activate
downhole tools and isolate zones of interest efciently without
permanent packers and the need to collect reservoir uid samples at
specied times. Most importantly, unlike in the past, engineers can
be certain they have achieved test objectives before the test is
ended.
The future of real-time well testing goes beyond transmitting
data to include the actuation of multiple devices in the DST string
using this same wireless backbone. The immediate reward for these
expanded capabilities will be measured in saved time, saved capital
and improved ulti-mate hydrocarbon recovery as a result of
develop-ment designs and production schedules informed by
high-quality data and accurate knowledge of reservoir
characteristics. RvF
> Real-time decision making. A well test, as planned, would
have taken nearly ve days (top). Using the wireless-enabled
downhole reservoir testing system, engineers at Maersk Oil were
able to monitor reservoir parameters and make decisions in real
time, which shortened the well test by more than a day. Real-time
data (middle) allowed the operator to obtain necessary downhole
information with which to characterize the reservoir and meet its
test objectives in 28 fewer hours than was called for in the
original test plan (bottom ).
Time, d0 1 2 3 4 5
Initialflow
Initialbuildup
Cleanup
Main flow Main buildup
Plan
Secondbuildup
Samplingflow
Pres
sure
Rate
Time, d
28 hours saved
0 1 2 3 4 5
Pres
sure
Rate
Initialflow
Cleanup
Main flow Main buildup
Actual
Secondbuildup
Samplingflow
Initialbuildup
Initial flowInitial buildupCleanup flowSecond buildupMain
flowMain buildupSampling flowTotal
0.52121224488
106.5
0.52.49.910.521.722.710.878.5
Flow Period Plan, h Actual, h
