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PETROLEUM SOCIETY
OF
CIM
PAPER NO. 78 - 29
11
r
/
.
THIS
IS
A PREPRINT - SUBJECT TO CORRECTION
SINGLE-STAGE
CHEMICAL TREATMENT
PROVIDES
STIMULATION
AND CLAY CONTROL IN SANDSTONE FORMATIONS
by
R.L. Thomas and C W_ Crowe. Members SPEAIME
Dowell Division t Dow Chemical U.S.A. Tulsa. Oklahoma
and
W Chmllowskl. Member CIM
Dowell
01
Canada. DivIsion of Dow Chemical
01
Canada Ltd. Calgary. Alberta
PUBLICATION RIGHTS RESERVED
THIS PAPER
IS TO BE
PRESENTED AT THE 29TH ANNUAL TECHNICAL MEETING
OF
THE PETROLEUM SOCIETY OF CIM IN
CALGARY. JUNE 13 - 16. 1973. DISCUSSION OF THIS PAPER
IS
INVITED SUCH DISCUSSION MAY BE PRESENTED AT THE
29TH ANNUAL MEETING AND WILL BE CONSIDERED FOR PUBLICATION IN
CIM JOURNALS IF FILED IN WRITING WITH THE
TECHNICAL PROGRAM CHAIRMAN PRIOR TO THE CONCLUSION OF THE
MEETING
ABSTRACT
A
new
chemical
treatment
has
been
developed
to
stimulate
problem
sandstone
formations. This treatment is
designed
to
overcome
a commonly observed problem in
which wells in i t ia l ly respond to matrix
acidizing
but
show rapid
production declines
following
treatment. These rapid declines
are generally attr ibuted to migration of
clays
and other
f ines.
The
t reating fluid,
which slowly
generates hydrofluoric acid
(HF),
reacts
mare
slowly
than conventional
Mud Acid and thus penetrates a greater
distance into the formation before spending.
The fluid also acts to s tabi l ize clays and
other fines by chemically fusing them to
the
sand
grains.
The
chemistry of
this
system is
ent i re ly dif ferent
from that
previously
reported
in
the
l i t e ra ture
The effect iveness of this s t imulation and
clay control method is
demonstrated
by
core
tes t studies_ The control mechanism
is i l lus t rated by
scanning
electron micro
scope studies and x-ray analyses of t reated
clays. Field
tes t
results are
also
pre-
sented.
[NTRODUCTl
ON
Sandstone matrix
acidizing has
long
been
used as a means of improving
produc
tion
of
oil
and gas by removing
formation
damage and increasing permeability of the
zone immediately around
the ~ e ~ l b o q e
Laboratory
and
field studies
have
demonstrated the effect iveness of this
type
treatment.
In spi te of
widespread
use,
however,
many formations do not
respond sat is factor i ly
to conventional
hydrochloriC
acid
HCl
)/HF acid Mud
Acid)
treatments. This is normally at t r ibuted
to rapid spending of H near the
wellbore.
Some
wells
in i t ia l ly show good
stimulation
but
la ter experience an unusually rapid
decline
in
production rate_
Such
product
ion decl
ines
are commonly Observed
in
wells producing from both consolidated and
unconsolidated
sand.
The
declines are
usually
at t r ibuted
to
plugging
by
migratory
clays and other fines
similar
to those
shown in Fig. 1 and indicated by Table 1.
Figure
1 is a
scanning electron microscope
SEM)
photograph
of a Blue Sky
formation
sand.
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Plugging by Migratory Fines
Formation plugging
by
mi%ratory fines was
demonstrated
by
Krueger
et
al in both laboratory
and
field
studies. Core test
results
revealed
that
high flow rates caused dislodgment of fines
with resulting loss of permeability. Krueger also
reported
field
studies
in which the production
decline rate was accelerated by flowing wells
above
an
optimum
rate,
As
the production
rate
\'laS
increased,
a corresponding increase in inorganic
solids content
of
the produced oil was also
observed. These observa tions strongly support the
theory
of
formation damage
from
fines migration.
Migratory fines are apparently released
first
by
exposure to
strang
acid and
later by
mechanical
forces resulting from the increased flow of
produced fluids. Various treatments and production
techniques have
been
devised in an effort
Sa
control 0
6
minimize this problem. Krueger and
Templeton described
treating
techniques in which
wells, fallowing stimulation. were returned to
production at a gradually increasing rate in order
to minimize
ffnss
migration. Variou: c l ~ y stabili
zation agents have
also been
applled ln an
effort
to
control
m o v e m e ~ ~ o f
fines_ Delayed
acting acidizing
systens have
been
developed to
provide deeper live acid penetration and thus
remove damaging fines some distance from the
\'Iell bore.
A
new retarded
HF acid system (Clay Acid) has
proven extremely
effective
for matrix aCidizing
sandstone formations. The Clay Acid is usually
applied as an overflush to rlud Acid. Upon enter
ing the formation, the Clay
Acid
slowly generates
HF
thus providing greatly increased live acid
penetration. The system requires neither the use
of flammable organic esters nor multiple staging
of treating fluids for HF generation. In addition
to
providing deep
live
aCid.penp.tration. the
treatment
also stabilizes
any
undissolved clays
contacted.
In contrast to conventional clay
stabilizers, which
act by
ion exchange or adsorp
tion,
Clay Acid produces an actual chemical fusion
of
fines and clay platelets. Since fines are
physically cemented in place, they are much less
1 kely to be
disturbed by
the increased fluid flOl'1
resulting
from the
stimulation treatment. Labora
tory tests also
sho'.'l that treated clays are desen
sitized and are no longer swollen or dispersed by
contact
with incompatible fluids.
Clay Stabilization Studies
The effect of Clay
Acid
on water sensitive
formations can
be
demonstrated
by
conventional
core test techniques. The most widely used
method
of
determining water
sensitivity
consists
of
flowing a sodium chloride (MaCl) brine through the
care
fallowed by
distilled
water. The amount of
permeability loss during flow of the distilled
water indicates the degree
of
water sensitivity.
Fig_ 2 illustrates the
effect
of this type
test on an unconsolidated Frio sand. The test
sand contained 2.2 per cent kaolinite, 3.4 per
cent i l l i te and
2.5 per cent montmorillonite.
Almost total plugging of the core occurred during
flow of distilled water. Fig. 3
shows
the results
of a similar test on the
same
type of Frio sand
employing Clay Acid prior to the 6 per cent sodium
chloride (rlaCl) -
distilled
water sequence_ In
this
test, after
injection
of 5 pore
volumes
of
Clay Acid, the test was
shut-in for
18 hours at
150 F to allow the Clay Acid to spend
and
stabil
ize clays. Fallowing
this
treatment, the core
showed
no
evidence
of
water
sensitivity.
Similar clay stabilization tests
were
per
formed on various
other
water sensitive cores
including Berea and unconsolidated Miocene sands.
In
all
cases, treatment with Clay Acid provided
excellent
stabilization.
S E ~
studies
have proven extremely useful in
revealing
the
mechanism
of stabilization by Clay
Acid. The technique used in these s r ~ d i e s was
previously reported by Thomas
et
al. With
this
technique, the same clay platelets before and
after
chemical treatment
can
be photographed.
Fig. 4 shows kaolinite platelets before and after
exposure to Clay Acid. This treatment produces
obvious fusion
of
the individual
platelets.
Fig.
5
shows
another group of clay platelets before and
after treatment \'/ith spent Clay Acid. In these
photographs, actual fusion af the modified clay
mass to the sand grain is observed. This type
fusion
of
clay
platelets
should stabilize clays to
both ionic
and
mechanical shock. The spent Clay
Acid used in these studies was prepared
by
spend
ing the acid on an excess of bentonite
for
18
hours at 15D F_
The effect
of
Clay
Acid
on the cation exchange
properties
of clay
was
also
investigated.
Cation
exchange capacity (CEC) is defined as the number
of milliequivalents of
a cation
that
can ion
exchange with
100 grams
of a mineral.
For clays,
CEC's
can vary from 3 to 15
fOflkaolinite and
from
80
to
15D
for montmorillonite_ The
ability
of
clays to exchange cations on their surface plays a
very important role in the water
sensitivity
of
sandstone formations. Normally. exposure to
monovalent cations increases the tendency of clays
to
swell
or
migrate while exposure
to
polY'7 lent
cations decreases their \'1ater sensitivity.
The CEC of clays is p r i ~ a r i l y attributed
SUbstitution of
silicon C5i
)
by
aluminum (Al 0)
in+5he tetrahedral l a y e ~ a n d by
r e p l a c ~ ~ e n t of
Al with magnesium Mg ) or iron (Fe ) in the
octahedral
layer.
Broken
bands
around the edges
of
the
clay
may
also contribute
to the
CEC_
The
net negative charge formed
by this
isomorphous
ll
SUbstitution causes an attraction for cations.
Normally
the
cations on the surface or edges of
the
clay
are exchangeable but may became permanent
if the charge
is
high enough.
The
stabilization
of clays
by
polyvale?t
a
cations
has
b ~ ~ n r e p o r t ~ g in the l i terature .
Zirconium (Zr ) and Al
stabilize
clays
by
hydrolyzing to yield highly charged oligomers
