Abstract
Laboratory
tests
were
conducted
in
a
triaxial
load
frame
with
acoustic
emission
and
transmission
capability
to
investigate
mechanisms
that
might
be
initiating
the
microseismicity
experienced
in
CO
2
injection
operations.
Although
often
related
to
reactivation
of
mapped
faults
or
local
fracturing
due
to
reduced
injectivity,
the
case
of
the
Illinois
Basin
–
Decatur
Project
is
used
here
to
illustrate
the
need
for
better
understanding
of
what
triggers
microseismic
events
in
relatively
large
permeability,
good
reservoir
candidates.
There,
microseismicity
has
oc-
curred
in
the
CO
2
storage
target
formation,
the
Mt.
Simon
sandstone,
as
well
as
in
the
underlying
Precambrian
basement.
The
microseismicity
in
the
Mt.
Simon
sandstone
occurred
ahead
of
CO
2
plume
arrival
and
at
relatively
low
injection
pressure
conditions,
well
below
the
fracturing
pressure
at
the
injection
well.
A
hypothesis
is
suggested
for
the
occurrence
of
such
events
in
the
field,
whereby
critically
stressed
planes
are
activated
by
the
passage
of
the
pressure
front
at
injection
start;
these
faults
are
small
and
thus
not
visible
in
the
seismic
survey.
In
order
to
test
this
hypothesis,
sandstone
plugs
were
prepared
by
two
different
methods
to
incorporate
a
fracture
plane,
which
we
attempted
to
reactivate
by
pore
pressure
pulses.
The
reactivation
was
successful
at
low
pressure
for
a
fracture
created
in
the
laboratory
at
reservoir
conditions
but
was
unsuccessful
except
at
a
much
higher
pore
pressure
in
a
saw-cut
artificial
fracture.
The
results
suggest
that
tortuous,
rough
stress-induced
fractures
may
be
easier
to
reactivate
because
of
the
higher
probability
that
sections
are
already
favorably
oriented
with
respect
to
critical
shear
stress
at
a
low
pore
pressure
increase.
Saw-cut
fractures
may
close
completely
under
isotropic
stress
loading
and
may
be
difficult
to
activate
unless
exactly
oriented
with
respect
to
critical
shear
stress
at
a
low
pore
pressure
increase.
Acoustic
emission
accompanying
fracture
reactivation
was
also
recorded
and
analyzed.
This
revealed
a
di
fferent
event
distribution
energy
between
creating
and
reactivating
the
fracture
tests
were
conducted
in
a
triaxial
load
frame
with
acoustic
emission
and
transmission
capability
to
investigate
mechanisms
that
might
be
initiating
the
microseismicity
experienced
in
CO
2
injection
operations.
Although
often
related
to
reactivation
of
mapped
faults
or
local
fracturing
due
to
reduced
injectivity,
the
case
of
the
Illinois
Basin
–
Decatur
Project
is
used
here
to
illustrate
the
need
for
better
understanding
of
what
triggers
microseismic
events
in
relatively
large
permeability,
good
reservoir
candidates.
There,
microseismicity
has
oc-
curred
in
the
CO
2
storage
target
formation,
the
Mt.
Simon
sandstone,
as
well
as
in
the
underlying
Precambrian
basement.
The
microseismicity
in
the
Mt.
Simon
sandstone
occurred
ahead
of
CO
2
plume
arrival
and
at
relatively
low
injection
pressure
conditions,
well
below
the
fracturing
pressure
at
the
injection
well.
A
hypothesis
is
suggested
for
the
occurrence
of
such
events
in
the
field,
whereby
critically
stressed
planes
are
activated
by
the
passage
of
the
pressure
front
at
injection
start;
these
faults
are
small
and
thus
not
visible
in
the
seismic
survey.
In
order
to
test
this
hypothesis,
sandstone
plugs
were
prepared
by
two
different
methods
to
incorporate
a
fracture
plane,
which
we
attempted
to
reactivate
by
pore
pressure
pulses.
The
reactivation
was
successful
at
low
pressure
for
a
fracture
created
in
the
laboratory
at
reservoir
conditions
but
was
unsuccessful
except
at
a
much
higher
pore
pressure
in
a
saw-cut
artificial
fracture.
The
results
suggest
that
tortuous,
rough
stress-induced
fractures
may
be
easier
to
reactivate
because
of
the
higher
probability
that
sections
are
already
favorably
oriented
with
respect
to
critical
shear
stress
at
a
low
pore
pressure
increase.
Saw-cut
fractures
may
close
completely
under
isotropic
stress
loading
and
may
be
difficult
to
activate
unless
exactly
oriented
with
respect
to
critical
shear
stress
at
a
low
pore
pressure
increase.
Acoustic
emission
accompanying
fracture
reactivation
was
also
recorded
and
analyzed.
This
revealed
a
di
fferent
event
distribution
energy
between
creating
and
reactivating
the
fracture
