American Oil and Gas Reporter - January 2017 - 109

Tech Trends

5,000

100

4,000

80

3,000

60

2,000

40

1,000

20

0

0

D
EH
1.3
75
1.5
00
1.6
25
1.7
50
1.8
7
2. 5
00
0
2.
12
2. 5
25
2. 0
37
5
2.
50
0
2.
62
5
2.
75
0
2.
87
5
3.
00
0
3.
12
3. 5
25
0
3.
37
5
3.
50
0
3.
62
5

Pressure Drop Across Seat (psi)

120

P Drop

Rate

500
450
400
350
300
250
200
150
100
50
0

120
100
80
60
40

Rate (bpm)

20

68
7
2. 5
75
0
2. 0
81
2
2. 5
87
5
2. 0
93
75
3.
00
3. 00
06
2
3. 5
12
5
3. 0
18
75
3.
25
0
3. 0
31
3. 25
37
5
3. 0
43
7
3. 5
50
0
3. 0
56
25
3.
62
5
3. 0
68
75

2.

25

50

62
2.

56

-

0
2.

When a packer loses its seal against
the wellbore, isolation between two stages
is lost. Isolation is critical to the success
of a treatment, since it prevents stages
from being under- or overstimulated. In
multiple packer failure scenarios evaluated
in the STACK study:
* There was excessive differential
pressure across the packer (the pressure
exerted during stimulation treatment minus
the pressure acting on the element from
the alternate sides: reservoir pressure on
the uphole side, and reservoir pressure
plus the pressure from the previous stage's
treatment on the downhole side). If differential pressure is not properly calculated,
the element could fail and stage isolation
will be lost.
* Thermal contraction can cause significant pipe movement (on the order of
feet in formations with significant cooling).
Unlike SEPs, HSMPs are not susceptible
to thermal contraction failure, and maintain
pressure against the formation during
stimulation because force from the body
lock ring prevents the packer from relaxing.
* Pressure cycling results in fatigue.
A packer in an open-hole multistage system (OHMS) must hold pressure from
both sides. For example, while treating
stage A, the toe side of the packer will be
at a higher pressure. Once stage A is
treated and stage B is being stimulated,
the pressure across the packer reverses.
It is possible to lose isolation because of
that pressure reversal.

6,000

Ball Seat Size (in)

Pressure Drop Across Seat (psi)

Packer Failure Mechanisms

FIGURE 2
Pressure Drop Calculations for 20-Stage System using 1⁄8-inch (Top) And
1
⁄16-inch (Bottom) Ball Seat Size Increments

Rate (bpm)

into the rubber element, which causes
the rubber to begin degrading and expanding. The rubber continues to expand
until it contacts and seals against the
wellbore.
Fracturing ports have an outer housing
with an inner sliding sleeve and a millable
ball seat attached to the sliding sleeve.
The inner sliding sleeve is pulled over
the windows of the fracturing port and
shear pins are placed to close the port. A
seal is created by pumping a fracturing
ball down the liner to a corresponding
ball seat. The force from fluid pressure
breaks the shear pins and shifts the sleeve
to expose the windows to provide reservoir
access so stimulation treatments can be
pumped.

Ball Seat Size (in)
P Drop

Internal Isolation Failure
Initially, the study looked at packer
failures as the main cause of stage isolation
issues in uncemented completion systems.
It became apparent, however, that isolation
issues inside the completion system/liner
with either fracturing ports or bridge
plugs also could lead to a loss of isolation.
Multiple scenarios for loss of internal
isolation were evaluated and summarized
in three categories: ball tolerances/working
limits, fracture port functionality, and
ball seat erosion.
One potential failure point is actuation
balls, which have a small contact area
with fracture port ball seats. With most
systems using 1⁄8- or 1⁄16-inch increments
between ball/ball seat sizes, the tolerances
with machining the balls and seats becomes ever-smaller and more precise.
For example, in a 1⁄8-inch increment
system, a 2.0000-inch seat and a 2.1250inch ball leaves a 0.0625-inch contact
area on either side of the ball with the
seat. In a 1⁄16-inch increment system, a

Rate

2.0000-inch seat and a 2.0625-inch ball
leaves a 0.03125-inch contact area. The
balls must be spherical within their tolerances and the ball seats must be round
and true. If any of these dimensions or
configurations is incorrect, the ball may
not seal.
Fracture ports that open prematurely
essentially create a "leak" in the system.
Premature sleeve shifts can occur because
of improper completion operations or
poor sleeve design. Because of the contracted diameter of ball seats, the force
of the fluid alone can shear the pins and
open the port. Shear pins for the port
sleeves should be pinned above pressure
drop calculations, which take into account
seat size, pump rate, and other treatment
variables.
Figure 2 shows the calculated pressure
drop across each ball seat in a 20-stage
system at a pump rate of 100 barrels a
minute. The pressure drop should be tailored to the actual seats being used. Most
ball seats have a leading edge into the
seat, but not all seats have a tail-out of
JANUARY 2017 109
American Oil and Gas Reporter - January 2017

Table of Contents for the Digital Edition of American Oil and Gas Reporter - January 2017
