By Charlie McClean, Bill Myers, Didhiti
Talapatra, Mark Sloan and Stephen
Zuklic, Baker Hughes
Originally appeared in World Oil® SEPTEMBER 2014 issue, pgs 45-49. Posted with permission.
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Ultra-HPHT perforating system opens access to untapped reservoirs
1. Originally appeared in World Oil® SEPTEMBER 2014 issue, pgs 45-49. Posted with permission.
Ultra-HPHT perforating system opens
access to untapped reservoirs
World Oil® / SEPTEMBER 2014 45
HIGH PRESSURE/HIGH TEMPERATURE
In deploying the industry’s
first ultra-HPHT perforating
system in the Gulf of
Mexico, one operator was
able to access a previously
inaccessible formation.
System components were
qualified and suitable for use
at 35,000 psi and 470°F,
for the duration and
temperature ramp-up
specified during testing.
ŝŝCHARLIE McCLEAN, BILL MYERS, DIDHITI
TALAPATRA, MARK SLOAN and STEPHEN
ZUKLIC, Baker Hughes
As the global demand for energy con-tinues
to grow, operators are faced with
accessing reserves that, until recently, were
classed as uneconomical or operationally
risky. Much of this risk stems from the fact
that many of these reserves are in remote
reservoirs with exceedingly high tempera-tures
and pressures.
However, with continued advance-ments
in technology, and a better un-derstanding
of extreme well conditions,
operators are more willing to take on
ultra-high-pressure/high-temperature
(U-HPHT) projects. A Gulf of Mexico
operator, for example, expressed such a
willingness to extend the industry’s techni-cal
limits when drilling a well with a mea-sured
depth (MD) in excess of 29,000 ft.
Downhole temperature and pressure were
expected to reach up to 470°F and 35,000
psi, including the applied pressure required
to fire a perforating system.
This article reviews the detailed process
that the operator undertook with Baker
Hughes to develop and deliver a tubing-conveyed
perforating (TCP) system that
could successfully deploy in this U-HPHT
environment. This would not only enable
access to a previously untapped reservoir,
but it would mark the first-ever perforation
operation at 34,500 psi.
SYSTEM REQUIREMENTS
Perforating guns, which contain mul-tiple
explosive charges that create holes in
the casing string to allow communication
between the formation and the wellbore,
vary in diameter and length, but they typi-cally
share several main components in
common, Fig.1:
• A steel strip to hold shaped charges in
place
• A detonating cord that connects to
the back of each shaped charge
• Boosters, crimped to each end of the
detonating cord
• A hollow carrier into which the as-sembly
fits
• A connecting sub (tandem).
The gun system for any U-HPHT
project must withstand not only ambient
downhole pressures, but it must also hold
up to any additional applied pressure re-quired
to operate the firing system. The op-erator
and service provider worked closely
to develop design criteria for each compo-nent
of the perforating system, such that it
would work reliably, and safely, with the ex-treme
downhole conditions expected.
Steel. While the past few years have seen
considerable improvements in the perfor-mance
capabilities of high-strength, low-alloy
steel, the conditions expected in the
Gulf of Mexico well required a higher level
of scrutiny in steel quality during the manu-facturing
process. Any detected anomaly in
the material specification must be identified
as early as possible, so that corrective action
can be implemented and, if necessary, an-other
run of the material can be produced.
Gun system. While selecting a gun sys-tem
for any U-HPHT project, consider-ations
must be made for the pressure rat-ing
and for gun reaction after detonation.
Initial calculations were based on the for-mulas
provided by API for the collapse
of tubing (API Bulletin 5C3), as well as
supplemental proprietary formulas devel-oped
for the scalloped gun body geometry.
Finite-element analysis showed that the
raw material for the guns must have in-creased
minimum yield strength over stan-dard
gun body material, to withstand an
environment of 35,000 psi and 470°F. The
U-HPHT gun system must also include
a tandem sub connector that contains an
internal fluid bypass, enabling the gun sys-tem
to be fully compatible with automat-ic-
release systems, and other devices that
function by way of gun gas.
With the initial system requirements in
place, Baker Hughes performed a series of
simulation models to verify the product’s
initial design. The system was designed to
exceed the 35,000-psi operational pressure
with a safety factor, Fig. 2.
No operational problems were ob-served
during the modeling of the pro-posed
equipment layout. However, the up-front
modeling, alone, was not considered
sufficient for qualifying the TCP system
under real-world conditions. The operator
and service provider, therefore, collabo-rated
on a suite of real-life, full-scale quali-fying
tests for both individual components
and the TCP system, as a whole.
Firing system. To reduce the risks of
operational downtime, it was determined
that a redundant firing system would be
deployed. Because the firing system had
to be fully redundant, the service provider
Fig. 1. Perforating gun.
2. HIGH PRESSURE/HIGH TEMPERATURE
and operator decided on two absolute
pressure-activated firing heads that would
be conveyed in a shroud assembly. The fir-ing
heads used rupture discs with a higher
level of accuracy than the shear pin firing
heads more commonly used in the indus-try,
which is critical in ultra-deep, high-pressure
wells to reduce the amount of
pressure required for application. This fir-ing
head arrangement still provided a “last-on,
first-off,” top mounting arrangement,
for further operational safety.
Although they are run in parallel, the
firing heads are fully independent of each
other, with the explosives train coming to-gether
through a device that has two bal-listic
inputs and one ballistic output. Fur-thermore,
as one of the explosives trains
would always reach the TCP device first,
or if one of the explosives trains failed to
initiate, the device would still initiate the
gun string below. This serves to back-det-onate
the other explosives train, to ensure
that the firing head is not pulled out of the
hole with live explosives.
Explosives. The project’s high-tempera-ture
requirement made hexanitrostilbene
(HNS) the best choice for explosive. Expo-sures
to high temperatures for more than
the rated duration can cause the explosives
46 SEPTEMBER 2014 / WorldOil.com
to decompose, thus reducing the perfor-mance
of the shaped charges and leading
to a stop fire along the explosive train. De-pending
on the rate of heating, deflagra-tion—
a subsonic combustion process that
propagates through heat transfer—is also
possible. Deflagration can cause explosives
to gas out and build enough pressure inside
the gun body to rupture it.
QUALIFYING QUALITY
Because quality is one of the opera-tor’s
fundamental business goals, it was
of the utmost importance that testing be
conducted to the highest standard. As a re-sult,
the operator was fully involved in the
content of the quality plan for developing
and delivering a U-HPHT TCP system
that would reliably function in the extreme
reservoir conditions expected. The service
company’s quality assurance manager was
responsible for writing the quality plan.
This manager was also integrated fully with
the ballistics-engineering group, and had
direct access to the TCP product line team.
The project-specific quality plan fo-cused
on critical testing for hardware,
third-party components and shaped
charges. Scrutiny, over and above a stan-dard
quality plan, included increased third-party
witness and inspection participation,
and quality control inspection rates of sub-components
and documentation.
For hardware, this included:
• Lot acceptance testing of raw
materials
• 100% inspection of items deemed
critical to the success
• Increased levels of non-destructive
testing.
The aspects for ballistic devices and
shaped charges included:
• Batch qualification testing at well
conditions
• Increased levels of non-destructive
testing.
Inspection was a critical part of the
quality plan, and as such, operator repre-sentatives
were allowed to freely observe
manufacturing and testing activities, per
the following guidelines:
• A Monitor Point is an optional in-spection
point, during which an op-erator
representative, without notice
and without constraining the activity,
may monitor any applicable activity.
• A Witness Point is also an optional
inspection point, in which the ser-vice
company notifies the operator
representative. If available, the repre-sentative
may witness the inspection
point, without constraining the activ-ity.
The company or subcontractor’s
work will not be held or repeated.
• A Hold Point is a mandatory cus-tomer
inspection point. The service
provider will notify the operator rep-resentative
when any hold point is
reached, and work will not proceed
until the applicable inspection activ-ity
has been completed, or a written
waiver is received from the operator.
Gun body qualification. The steel
comprising the gun body was tested dur-ing
the manufacturing process, once the
service company and the steel vendor
defined the correct chemistry. The mill
run material was then tested by multiple
third-party labs and was ultimately ac-cepted,
only after the service company,
operator and steel vendor confirmed that
the material met the requirements.
Complete first-article gun bodies were
then machined and pressure tested for
qualification. The gun body was sealed
with bull plugs on both ends and placed
inside a pressure vessel. Pressure and tem-perature
were then increased to 35,000 psi
at 470°F and held for 4 hr. No signs of leak-age,
plastic deformation or collapse were
Fig. 2. Applied pressure was 50,000 psi, with a maximum principal stress of 137,130 psi.
Fig. 3. Gun tube, fully loaded with the selected shaped charges (left), and the condition
of the gun body after the ballistic survival test (right). The gun body did not crack or split
during the ballistics test.
3. HIGH PRESSURE/HIGH TEMPERATURE
World Oil® / SEPTEMBER 2014 47
noted on the gun body after the test.
Gun body ballistic qualification. To
qualify the gun body for ballistic survival,
the 7-ft gun body was fully loaded with se-lected,
HNS, hard rock shaped charges, and
shot wet under ambient pressure and tem-perature.
The maximum diametrical swell
was 3.544 in., or 1.52% of original OD,
which is well below the customer’s specified
drift diameter of 3.625 in. The gun body
was also drifted, using a 3.625-in. ID drift
gauge (18 in. long) after the test and with-out
any issues. No visible splits or cracks
were noted on the gun body, Fig. 3. The
3.500-in. OD HPHT gun body was, thus,
qualified for survival in wet application us-ing
the selected HNS shaped charges.
EXPLOSIVES QUALIFICATION
Due to the job’s high-temperature
requirements, all explosives were batch-qualified
for use at 470°F. The explosive
powder was also ampule-tested, which
was considered the thermal stability test
that most closely mimics the conditions
to which explosives are exposed in oilfield
operations. In this test, a sample of the ex-plosive
is sealed in a glass ampule and then
placed in a heating block, Fig. 4. The block
was ramped up to the desired temperature
of 470°F over a 2-hr period, and the block
containing the ampules was then held at
that temperature for an additional 2 hr.
The results of these tests gave a time-ver-sus-
temperature chart, based on the specif-ic
batch of HNS explosives, which would,
in turn, be used on the planned project.
Shot tests were also conducted with
shaped charges held at ambient tempera-ture,
and at reservoir temperature for a pe-riod
of 2 hr. Shots were made, each at tem-perature
and ambient, to gather sufficient
data to make a comparison. The shaped
charges were shot into API-specified QC
concrete targets, and performance was
measured on the basis of the size of the exit
holes on the scallops, casing and penetra-tion
in the target. The results of this testing
confirmed that the explosives in the shaped
charge did not decompose or deflagrate,
and the drop in performance was negligible
in relation to the exit holes and marginal
from the target penetration. This marginal
drop in performance was deemed accept-able,
and all shaped charges were qualified
for use at 470°F for the expected duration.
System integrity test (SIT). A final test
was performed to qualify the system for
use, as per the customer-specified well con-ditions,
which consisted of the U-HPHT
firing head system connected to two 1-ft
gun bodies and a tandem sub, with detona-tion
cord and booster-to-booster transfer.
All items used in the test were from the
same batches that were previously qualified
for use in the well. The purpose of the test
was to validate the successful ballistic train
activation of the firing heads, and the deto-nating
cord booster-to-booster transfer at
these elevated temperatures and pressures.
The entire assembly was lowered into
the pressure vessel, which was then sealed
and brought to temperature and pres-sure
under controlled conditions meant
to simulate running in hole. Temperature
was increased in a linear ramp from 72°F
to 470°F (approximately 11°F every hour),
while pressure was increased from 0 to
28,000 psi, over a period of 38 hr. This
length of time approximated the time that
it would take to get the assembly to target
depth in the well.
The tool was then soaked at 28,000 psi
and 470° F for 2 hr, followed by increasing
the pressure to activate the time-delayed
firing system. A loud mechanical noise of
the hammer sub shifting at 32,400 psi con-firmed
the successful rupture of the discs
and initiation of the time delay.
Four minutes and 28 sec later, the time
delay of the firing head system successfully
detonated. A 100-psi increase in pressure
inside the vessel—caused by expanding
gases released during detonation—con-firmed
the successful detonation of all ex-plosives.
The vessel was depressurized and
allowed to cool for the rest of the day. The
tool was retrieved from the vessel the next
day and carefully disassembled to retrieve
the witness plate. The uniform explosive
imprints along the plate confirmed a suc-cessful
test and high-order detonation of
all explosives.
TAKING TO THE FIELD
All qualifying tests for the U-HPHT
gun system were successful, with full func-tion
of the U-HPHT firing head at the
desired pressure and temperature. There
was high-order detonation of the explosive
Fig. 4. Glass ampules containing the powdered explosive (left) were placed in a heating
block ramped-up to 470°F, and held at that temperature for 2 hr.
Fig. 5. Time–temperature data for the duration of the test.