American Oil and Gas Reporter - January 2017 - 110

Tech Trends
the seat or the same degree of leadin/lead-out, which can influence pressure
drop. If fracture ports are not pinned to a
higher pressure than the pressure drop
across the seat, the sleeve can open prematurely.
Ball seat erosion typically has been
considered a concern at the beginning of
the frac treatment, where seats are the
smallest, and at the end of a job, where
seats have seen the most proppant. In reality, ball seat erosion is most evident in
middle stages. The smallest seats tend to
see higher velocities, but have not had
enough volume pumped across them to
create an issue.
The largest seats at the end of the
completion have seen the most volume
of fluid/proppant across their seats, but
at lower rates. The calculations in Figure
3 depict the difference in total erosion

for a 20-stage system when pumped at
constant and controlled rates using 1⁄16inch increments.
During a stimulation treatment, the
ball should land and shift a ball seat to
the open position, which is confirmed at
surface with a spike in the pressure chart
at the beginning of each stage. After realizing that not all stages were being
stimulated, the operator changed his
OHMS system. Using the new OHMS
system, sleeve efficiency in the uncemented completion systems increased
from 67 percent to 97 percent.
To address potential sleeve failure mechanisms, uncemented wells sometimes are
completed using the plug-and-perf method,
similar to cemented liner completions. In
the STACK, perforations are shot between
packers and are isolated with composite
plugs between each stage, with a plug and

FIGURE 3
Total Erosion for 20-Stage System using 1⁄16-inch Ball Seat Increments
Pumped at Constant (Top) and Controlled (Bottom) Rates
0.014

120.0

0.012

100.0

0.010

60.0
0.006

Rate (bpm)

Erosion (in)

80.0
0.008

Isolation & Production Analysis

40.0
0.004
20.0

0.002
0.000

0.0
1

Total Erosion

10 11
Stage

Erosion Limit

Rate

0.014

120.0

0.012

100.0

0.010

60.0
0.006
40.0
0.004
20.0

0.002
0.000

0.0
1

Total Erosion

10 11
Stage

Erosion Limit

110 THE AMERICAN OIL & GAS REPORTER

16
Rate

Rate (bpm)

Erosion (in)

80.0
0.008

perforating charge conveyed between each
stage. The plug is set and multiple perforation intervals then are shot.
Some operators use a plug with a ball
seat as a contingency for a failed perforating run in this application. The ball is
dropped and seated after the conveyance
string (typically wireline) is out of the
well.
Plugs can have failure mechanisms
similar to packers and sleeves, depending
on the type of plug deployed. In the case
of plugs that incorporate a ball seat and
ball as the internal isolation, the failure
mechanisms are similar to fracture ports,
and stage isolation failures often are
related to ball tolerance and seating practices.
Figure 4 shows example tracer results
from a cemented well in which the bridge
plug failed to provide internal isolation.
Perforations were shot for the subsequent
stage, and each stage was traced with a
unique near-wellbore proppant tracer.
Two of the four original perforation
clusters were stimulated in stage 13, as
shown from the iridium (red) tracer. Stage
14 showed minimal coverage, as shown
by the scandium (yellow) tracer. Stage
13 shows scandium in all four clusters,
as opposed to the two clusters that were
taking treatment during this stage. This
is the result of plug failure between stages
13 and 14.
Spectral gamma ray logs were evaluated to categorize open-hole packer isolation. Production analysis was performed
on the same set of wells that was evaluated
for isolation and stimulation effectiveness.
The production results then were compared
with the percentage of packers/stages that
maintained integrity throughout the treatment.
In addition to comparing the isolation
failures with production analysis, the
wells were further evaluated based on
known parameters during stimulation
treatment. The average differential pressure, downhole temperature conditions,
type of completion, and wellbore trajectory
all were studied as part of the evaluation.
Special consideration should be given
when looking at spectral gamma ray data
on proppant-traced wells. Initially, the
log should be analyzed macroscopically

American Oil and Gas Reporter - January 2017

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