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B a c k g r o u n d

Propellants can break through

formation damage created by
perforating
James Barker, Jet Research Center, a division of Halliburton Energy Services
John Hardesty, Jet Research Center, a division of Halliburton Energy Services
Phil Snider, Marathon Oil Company

P erforation of the well casing and its cement

sheath has been effectively and economically
cal energy per unit volume. The pressure related
to this large energy induces liner collapse. It is the
carried out for many years with conical shaped impact pressure of the liner that causes formation
charges releasing large amounts of stored chemi- damage, reducing the permeability. The following

Casing
Carrier Cement
Explosive

Case

Conical
Booster
Liner

Fluid Gap Formation

(a) DP Shape charged perforator (b) DP charge prior to initiation

Jet Penetrates Carrier

Later Stages of Liner Collapse Produce

Liner Collapses to Form Jet Slower-Moving Slug

(c) DP charge at start of initiation (d) DP charge penetrating the carrier

Jet

Stretching Jet Penetrates Formation

with Millions of PSI of Pressure
(e) DP charge perforating jet beginning to extend (f) DP charge penetrating into the formation

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Compacted Zone
(with Damaged Permeability
from Perforating)
Casing Grain Fracturing Zone
Pulverization Zone

Charge and Core Debris

Open Perforation

Cement

Microscopic photograph of undisturbed sand grains prior to Microscopic photograph of damaged sand after perforating.
perforating.

graphic illustrates how a deep penetrating (DP) significant amounts of funding and resources on com-
shaped charge penetrates the wellbore components. bining perforating and flow testing in stressed perme-
On a microscopic basis, the formation materials able core samples. These tests clearly show that
subjected to this very high load and load rate (mil- because a shaped charge penetrates a significant dis-
tance into the formation, it does not mean the entire
lions of psi in microseconds at the jet center) shatter
perforation tunnel will allow fluid flow. Shaped charge
the individual formation sand grains, as well as reduce
design, overburden stress, rock type and permeability,
the cement particles to a very fine powder. This
and underbalance levels all have an impact the effec-
diminished particle size creates a surrounding filter
tive length of the perforation tunnel. It is common for
cake, reducing permeability and inhibiting effective
a perforation that penetrates 15 in. (38 cm) into the
subsequent fluid injection. Because this new filter cake
formation to have only a flow contribution from the
is so compacted, it is not easily removed by flushing first half of the created tunnel. The following three fig-
or underbalanced perforating operations. ures show the same charge design, shot into the same
Shaped charge manufacturing companies, such as formation permeability at underbalanced, overbal-
Halliburton’s Jet Research Center are now spending anced, and balanced conditions (Figures 1a, b, c). It

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B a c k g r o u n d

(a) Figure a shows the effective length of the perforation, when a stressed Castlegate sand-
stone core was shot with 3500 psi (22 MPa) overbalance pressure and subsequently flow test-
ed. The effective length of the perforation which would flow was only 5.5 in. (14 cm),
although the perforation penetrated 12 in. (30 cm). In all three examples, the fluid in the
sandstone’s pore space was odorless mineral spirits and the permeability of the rock is
approximately 1000 md. The decrease in total target penetration of this specific test is also
due to a higher overburden stress.

(b) Figure b shows the effective length of the perforation, when a stressed Castlegate sand-
stone core was shot at balanced conditions and subsequently flow tested. The effective length
of the perforation which would flow was only 7 in. (18 cm), although the perforation pene-
trated 14 in. (35 cm).

(c) Figure c shows the effective length of the perforation, when a stressed Castlegate sandstone
core was shot with 3500 psi (22 MPa) underbalance pressure and subsequently flow tested. The
effective perforation would flow almost its entire length of 15 in. (38 cm). It is important to
note that 3500 psi (22 MPa) underbalance, generally considered an extremely high level, in this
high permeability sandstone with liquid filled pore space, and cannot always be achieved for
operational reasons.
Figure 1 – Stressed Castlegate sandstone cores perforated at various pressure differential conditions.

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can clearly be seen that underbalanced conditions can into the wellbore. Many of those individuals intimate-
improve the perforation’s effective length, but there ly familiar with perforating believe the industry as a
still is a significant length of the perforation tunnel that whole will be moving towards more flow testing work
does not contribute to flow. and tailored perforating charges specifically designed
for the operator’s formation characteristics, as well as
To solve this problem, the StimGun™ assembly, and
the use of propellant technology to enhance inflow
its family of products, use a high energy gas pulse,
performance. Operators may very well move away
with a significantly lower pressure loading rate – from API charge penetration data and more towards
thousands of psi in milliseconds – to break through a focus on perforating for flow. The next article, does
this hard filter cake and create a pathway into the for- an excellent job of showing some of the productivity
mation, enabling further fluid injection or enhancing enhancements which can result in not only improved
the well’s inflow performance. The force of the pulse perforating performance, but also using the propel-
also removes materials plugging the perforations and lant to break down the perforations with very short
redistributes them farther into the fractures or back fractures of varying conductivity.

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