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8/10/2019 Equilibrium Acid Fracturing
1/8
Equilibrium Acid Fracturing: A
New
Fracture Acidizing Technique for
arbonate Formations
S J Tinker SPE, Shell Western E P Inc.
Summary
The equilibrium-acid-fracturing technique was developed to stimulate wells in the Wasson San Andres Denver Production
Unit. This new treatment technique maximizes acid contact time with the fracture faces while allowing control of the created fracture
dimensions. Maximum acid contact time is essential to create highly conductive etched pathways on the fracture faces
of
cool dolomite
formations that react slowly with acid. Control of fracture dimensions is important in the San Andres Denver Unit because fractures
tend to grow uncontained in at least one vertical direction and the oil column
is
bounded by permeable gas-bearing intervals above
and permeable water-bearing intervals below.
With this technique, a fracture of desired dimensions is created by injection of acid at fracturing rates. The volume of acid required
to create the desired fracture dimensions is determined by a 2D fracture-geometry program with design parameters determined from
fracture field testing and laboratory testing. Injection is then continued at reduced rates that maintain equilibrium with the fluid leakoff
rate from the created fracture faces. Maintaining equilibrium between injection and leakoff allows the created fracture to be held open
without significant further fracture extension. Equilibrium is achieved in the field by maintaining the injection pressure below the frac
ture extension pressure but above the fracture closure pressure determined by fracture field testing.
This paper presents the background and theory
of
this technique along with design procedures, field examples, results, and conclu
sions. Results of the equilibrium-acid-fracture treatments and other acid stimulations performed in the Denver Unit are also compared.
Introduction
The Denver Unit is one
of
several production units in the Wasson
San Andres field in the west Texas counties of Gaines and Yoa
kum (Fig.
1).
The target interval
of
the San Andres formation, a
Permian dolomite, is at about 5,000 ft. The Wasson San Andres
field was discovered in 1936. Waterflooding in the Denver Unit
began in 1964, when the unit was formed; CO
2
flooding began in
1984, and expansion is ongoing today.
The average permeability in the Denver Unit is about 5 md. 1,2
The pay-quality rock is split up into two major divisions. The first
porosity zones, in the upper part of the reservoir, are underlain
by the main pay zones (see Fig. 2 for a type log). The reservoir
temperature is about 105F. An original gas cap and an inactive
aquifer exist. The oil column is bounded below by pay-quality water
bearing rock in all parts of the unit and bounded above by pay
quality gas-bearing rock in most
of
the unit.
1,2
An aggressive workover program made it possible to continue
efforts to improve the effectiveness
of
well stimulations. The
equilibrium-acid-fracturing technique was developed to improve the
stimulation results achieved with other techniques. The typical acid
formulation used for most acid stimulations in the Denver Unit,
including equilibrium acid fractures, is 28 HCI. Many acid frac
turing and acidizing techniques, ranging from matrix acid treatments
to viscous fingering
of
acid through a gelled fluid, have been used
throughout the industry. The equilibrium-acid-fracturing technique
is significantly different from the other fracture acidizing techniques
because it maximizes acid contact time with the fracture faces while
allowing for control of the created fracture dimensions. A fracture
of desired dimensions is created by injection of acid at fracturing
rates. The volume of acid required to create the desired fracture
dimensions is determined by the fracture-geometry program
ENERFRAC3 with design parameters obtained from fracture field
testing. After the fracture is created, the acid injection rate is re
duced until it matches the fluid leakoff rate from the fracture. When
these rates match, an equilibrium is established and the created frac
ture can be held open without significant further extension. In prac
tice, equilibrium is obtained by adjusting the injection rate to
maintain the injection pressure below the fracture extension pres
sure but above the fracture closure pressure (minimum in-situ stress)
determined from fracture field testing. Equilibrium acid fracturing
is used to obtain maximum oil stimulation without stimulating the
adjacent water
or
gas zones outside the oil column. This is particu
larly important in carbonate formations where such properties as
Copyright
1991
Society of Petroleum Engineers
SPE Production Engineering February 1991
Young s modulus, Poisson s ratio, minimum in-situ stress, and prop
agation pressure are fairly uniform and few barriers to vertical frac
ture extension exist. The extended acid contact time is desirable
in the relatively cool [105F bottomhole temperature (BHT)] Was
son San Andres dolomite. The equilibrium-acid-fracturing technique
was used successfully to stimulate wells in the Wasson San Andres
Denver Production Unit.
The significant aspect of this technique is the continued etching
of the fracture faces for extended periods
of
time while the frac
ture is open without further fracture growth after the initial frac
ture dimensions are created. Other fracture acidizing techniques
usually consist of high-rate continuous injection of either acid alone
or
alternating stages of acid and various gelled fluids. Often the
total fluid volumes for these other methods are quite high and the
stimulations
m yor
may not be designed with regard to the ulti
mate created fracture dimensions. When fracture growth is uncon
tained in at least one vertical direction, as in the Wasson San Andres
field, the ultimate created fracture dimensions become important.
The created fracture dimensions become extremely critical to the
overall success
of
the stimulation when the oil column is bounded
by productive gas zones above and water zones below. Stimula
tion of pay-quality zones outside the oil column usually result in
excessive water and/or gas production, both of which negatively
affect stimulation. Out-of-zone stimulation can also have detrimental
effects if the field has secondary or tertiary recovery potential.
Another fracture acidizing technique consists of creating a frac
ture with acid and/or other fluids, etching the fracture with acid,
allowing the fracture to close, and finally injecting acid into the
closed fracture at pressures below the closure pressure.
4
The con
cept of injecting acid into a closed fracture is almost opposite that
of
equilibrium acid fracturing. The equilibrium technique holds the
fracture open while acid continues to etch its faces without signifi
cant further fracture extension and allows live acid to reach the frac
ture tip in cool dolomite formations. Injection into a closed fracture
tends to concentrate the stimulation effects very near the wellbore
because
of
the slow rates required to maintain a closed fracture.
This paper does not present detailed data and discussion
on
the
reactivity
of
the San Andres dolomite formation with acid because
that topic is thoroughly covered elsewhere. 5
The effectiveness of the equilibrium-acid-fracturing technique was
proved by field application in a heterogeneous, layered carbonate
formation. Larger production increases at lower costs were obtained
with the new technique than with the other stimulations in the same
field.
25
8/10/2019 Equilibrium Acid Fracturing
2/8
Fig.
1 Locatlon
of Denver Unit within the Wasson San Andres
field.
heory
The equilibrium-fracture-acidizing technique maximizes acid con
tact time with slow-reacting hydrocarbon-bearing carbonate for
mations without fracturing into adjacent water- or gas-bearing zones.
Because
ofthe
uncontained fracture growth and st iff rock (Young's
modulus
of
6,000,000 psi), very small volumes
of
fluid can create
large fractures.
6
These properties render impractical the use of
large-volume, continuous, high-injection-rate acid fracture treat
ments. The alternative
of
a short, small-volume acid treatment is
also unsatisfactory because
of
the San Andres dolomite's slow reac
tion rate. In the Denver unit, the acid
flow-by or
contact time
is an important factor that affects stimulation response. The Was
son San Andres' low BHT causes the dolomite to react very slow
ly, even with 28% HCI. The heterogeneous nature
of
the rock causes
the differential etching required to create a conductive flow path
way, but the slow reaction rate requires extended acid contact time
to create an effective fracture. f he fracture is not held open for
extended contact time with live acid, the acid leaks off into the
matrix, where its reaction has much less effect on the stimulation
than it would
if
it spent on the fracture faces. The equilibrium tech
nique provides a way to extend the acid contact time with the frac
ture faces without extending the fracture dimensions.
The equilibrium technique takes advantage of the difference be
tween the fracture propagation pressure (the pressure at which a
fracture extends) and the minimum in-situ stress (the pressure at
which a fracture opens
or
closes). To extend the acid contact time
after the desired fracture dimensions are created by high-rate in
jection (at
or
above fracture propagation pressure), the injection
rate is reduced to match the leakoff rate from the fracture faces.
As the injection and leakoff rates come into equilibrium, the pres
sure drops below propagation pressure.
f the pressure in a frac
ture is maintained above the minimum in-situ stress (or closure
pressure) but below the fracture propagation pressure, the fracture
will
be
held open without significant further extension. The pres
sure difference between fracture propagation and closure is called
net fracture pressure. The fracture overpressure is the difference
between the corrected instantaneous shut-in pressure (ISIP) and the
minimum in-situ stress. The corrected ISIP is measured shortly af
ter shut-in when the fracture remains open and nearly ceases
to
prop
agate.
3
,6,7 A typical overpressure in the Denver Unit is 500 psi.
Keeping the fracture open throughout the low-rate injection por
tion
of
the equilibrium treatment allows the fracture faces to be
stimulated vigorously. In cool dolomite formations, live acid
pumped into the open fracture under equilibrium conditions reaches
the fracture tip. Because the stimulation is kept
in
zone and live
acid is allowed to reach the entire fracture area, the equilibrium
technique optimizes acid use
in
both vertical and lateral placement.
26
G MM
RAY
SONIC
LOG
I--t-----Ir-::::B '; 4700
I--+-r---H.. . . . . - j 4800
FIRST POROSITY'
MARKER
- - - 4 ~ ; . - - - - - I H - + ~ - - : ; i F - - - 1
I-+--- f.iit--+--l 4900
: : ~ .
.
MAIN
PAY M RKER
- - - 4 f - - : ; , : - + + ~ - ~ ~ - - i
SCALE
IN
FEET
[
50
o
1
{
1--+------1 ,,:::+--1 5000
I - - - - ~ - - l
5100
1--+-1- ': , .-+--15200
Fig.
2 Denver
Unit San Andres type log.
Estimation
of
Leakoff
Rate.
After the fracture
of
the desired di
mensions is created, the injection rate must be reduced to match
the leakoff rate from the fracture, which can be estimated with a
well-known equation.
3
.
8
Eq. 1 can be used to estimate the total
leakoff rate at any time
t
after the fracture has been created:
qt t) =r
o
t
da (1)
o .Jt-TD a)
where
A
= fracture area (ft2),
Ao
=A to),
c
t
= total in-situ leakoff
coefficient
ft/minl/z),
qt(t)=leakoff rate at time
t
(ft3
Imin),
t=time (minutes), t = time to create the fracture (minutes), and
TD a)=dimensionless
time function.
By
knowing the leak-off rate at any time after the fracture has
been created, one can calculate the volume
of
fluid that leaks off.
This calculation enables the treatment designer to determine the equi
librium pump-rate schedule and the total acid volume required for
the desired total acid contact time.
Fracture
Field Testing. Fracture field (minifracture) testing plays
an important role in the design and execution
of
fracture treat
ments.3.68 Minifracture testing determines the in-situ fracturing
parameters, such as the minimum in-situ stress, fracture propaga
tion pressure, overpressure, and in-situ total leakoff coeffi
cient.
3,6,7
Fig.
3 is
an example
of
a fracture field test for a Denver
Unit well. A predetermined volume
of
brine was pumped into the
SPE Production Engineering,
February
1991
8/10/2019 Equilibrium Acid Fracturing
3/8
40
5000
OJ
0
-i
35
-i
0
c
-
PUMP RATE
:
J:
e
. BOTTOMHOLE PRES.
- 4500
0
30
r
IT1
0
-0
QI
::n
25
IT1
I f )
( f I
( f I
::>
c
20
- 4000
::n
~ ' . ' ' ' ' ' ' ' ' '
IT1
W
: : -
ISIP = 3910
PSI
,
,
,
15
~ , \
T
a::
-
.
,
::Ii
.
::>
10
3500
::l
a.
N
5
'0
,
'
0
3000
0 10 20
30
40
50
60
TIME
min)
Fig. 3-Fracture field test on Denver Unit Well 4130.
well at a rate sufficient to create a fracture. At shut-in, an ISIP of
3,910
psi (bottomhole) was observed. For a stationary fracture, the
ISIP approximates the fracture propagation pressure. The well pres
sure was monitored until enough data were collected for an evalu
ation
of
the total in-situ leakoff coefficient. A fracture-reopening
test at a low constant rate was also performed
6
7 to determine the
upper-bound estimate ofthe minimum in-situ stress. Fig. 4 shows
the results
of
the fracture-reopening test. The upper bound of the
minimum in-situ stress, the point at which the p r e ~ s u r e v s . t i m e
plot deviates from the compressibility-controlled straight line, was
found to be
3,540
psi (bottomhole). A lower bound was estimated
to be 3,400 psi from the flowback shown in Fig. 4. After the frac
ture was reopened, the well was flowed back at a relatively con
stant rate. The rate, however, was not recorded. A lower inflection
in the pressure-vs.-time plot was identified at about 3,400 psi (bot
tomhole).7 The lower-bound estimate
of
the minimum in-situ stress
was only slightly below the upper-bound estimate identified by the
reopening test. This
is
consistent with the indication
of
closure seen
from the extended pressure falloff in Fig. 3. The overpressure for
this well was thus estimated to be
370
psi
(3,910-3,540
psi). The
total in-situ leakoff coefficient was then calculated to be 0.0005
ft/(min)
h
with the local-pressure-match technique described in
Ref. 7. The in-situ measured fracture parameters are then used to
design the fracture stimulation treatment for the well.
esign rocess
The general procedure for designing an equilibrium treatment should
be adequate for most cases, but some minor modifications may be
required in special situations.
1.
Estimate the minimum injection rate required to create a frac
ture to ensure that a fracture is actually created.
2. Obtain fracture field test data and laboratory data to define
the in-situ fracture design parameters and rock deformation prop
erties,
3. Run an overpressure-calibrated fracture-geometry program to
determine the volume required to create a fracture of the desired
dimensions.
4. Establish treatment-pressure guidelines to prevent further ex
tension of the fracture during equilibrium etching and to ensure that
the fracture is open.
5. Using the leakoff equation, determine the expected pumping
schedule, total treatment volume, and required pumping equipment.
Fracture field test data should be obtained, preferably from a test
on the subject well.
I f
a fracture field test is not practical for that
SPE Production Engineering, February 1991
2 5 . - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - ~ ~ m
o
C
20
E
3800
o
3:
:r
o
r
-
6
co
n 15
2
3600
:0
v>
v>
C
::0
3200 N
o
2
TIME (min)
3000
5
Fig. 4-Fracture reopening test on Denver Unit Well 4130.
. 0 r - - - - - - - - - - - - - - - - - - . - - - - - - ~ ~ - - - - _ _
l
et
.0005 l t / . r . i ln
I. R :
80
ft
80
70
60
o
D
'
:>
50
- .0
w
'
" 30
-
.
::>
-
20
10
-
TOTAL
ACID VOL. :
6170
9
01
2000
TOTAL ACID CONTACT HUE E
240 Min.
FRAC
EXTENSION PRES.
1500
, - - h " " , , : : : ~ - - - - - - - - 1
4600
,:.:
_
NEUTRON
LIXi
; ~ ~ ~ ~ ~
: : : '
(POOOSITY)
4650
/ / / / / / / / GIIS On..
COHTflC.T
Fig. 6-Postfracture gamma ray/temperature log for Denver
Unit Well 6716.
cient and a dimensionless time function divided by the square root
of
time since the start
of
injection.
8
The estimated leakoff-rate information is used three ways. The
first use is for estimating the acid volume to be injected at low rates
after the desired fracture dimensions are created. This volume
is
determined by integrating the rate-vs.-time curve for a specified
total acid contact time. Second, the expected pumping schedule is
determined from the required acid volume and the leakoff rate as
a function
of
time. Finally, the leakoff-rate information
is
used to
determine the type
of
pumping equipment required to perform the
job. Because the leakoff rate declines with time, the pump rates
at the end of the treatment can, for some wells, be as low as 5
gal/min at the end
of
the job. The equipment must be capable
of
pumping at the rates needed to match the leakoff rates throughout
the entire equilibrium portion of the treatment.
Example Design. Fig. 5 shows a typical example
of
an equilibrium
acid-fracture treatment for the Denver Unit. A Young's modulus
of
6,000,000 psi and a Poisson's ratio
of
0.3 were determined from
static-loading laboratory core tests. A totalleakoff coefficient e,)
of
0.0005 ft/(min)
'h,
an overpressure
of
500 psi, an n
of
1,
and
a viscosity
of
1 cp were also used as input data. The constant
n
is the flow-behavior index.
The desired pump rate of 2 bbl/min was used to create an un
contained radial fracture with an 80-ft radius (l60-ft total wellbore
height) by injection
of
2,940 gal
of
acid. This amounted to 35
minutes of acid contact time. The rate was then reduced so that
the pressure dropped below the fracture extension pressure but
stayed above the closure pressure. After the fracture was created,
an additional 3,230 gal of acid was pumped into the open fracture
at rates that matched the leakoff rate
of
the fracture. The pump rates
for this portion
of
the treatment ranged from 40 to 10 gal/min (Fig.
5). Acid was pumped for an additional 205 minutes after the frac
ture was created for a total acid contact time
of
240 minutes.
28
o
.a
.a
1 0 0 0 ~ - - - - - - - - - - - - r - - - - ~
1 0 ~ - - - - - - - - 4 - - - - 4 ~ - - - - ~
u
C
IL
D
U
Oil
W TER
TOTAL FLUID
1 ~ ~ ~ ~ ~
1981
1983 1985 1987
1989
YEAR
Fig. 7-Productlon curve for Denver Unit Well 6716.
Field-Application Considerations. Several items should be con
sidered during the planning stages to ensure that the job is a tech
nical and operational success. In cold dolomites like the Wasson
San Andres, it
is
desirable to maximize the acid contact time. To
tal pumping times for typical jobs in the Denver Unit range from
two to four hours. A maximum time of about 4 hours has been used
for several reasons. First, it
is
operationally favorable
if
the total
treatment time, including setup, production logging (if desired),
the actual treatment, and any posttreatment logging, can be com
pleted in daylight. Second, with the extended pump times, the re
quired pump rates can be very low. Rates on the order
of
4 to 5
gal/min have been experienced. Many service company pump trucks
cannot pump at these rates. One way to solve this problem is to
hook up a split-stream manifold, which allows part
of
the pump's
output to be injected into the well while the remainder
of
the fluid
is returned to the tank or transport. When a split-stream setup is
used, all the monitoring equipment should be installed between the
manifold and the wellhead so that accurate treatment pressure and
rate information can be modified.
Pressure guidelines play an important role in an equilibrium-acid
fracture treatment. The treater and/or foreman supervising the job
must know that the treatment pressure
is
actually within the
prescribed guidelines. The treatment-pressure guidelines for the low
rate or equilibrium portion do not include the tubing friction
or
any
other source
of
friction in the system. Although the friction is low
or negligible for many jobs,
in
some situations the equilibrium rates
are above 1 bbl/min and friction pressures can significantly affect
the surface tubing pressure. The surface
or
tubing pressure usually
is the only pressure that is monitored. One method of dealing with
the frictional effects is to use friction charts for the tubing size used.
Another method that was used successfully
in
the field (which also
removes all the friction in the system) is to shut down periodically
for 1 or 2 minutes. A brief shutdown to observe the real treatment
pressure should not adversely affect the treatment because, in most
cases, the fracture will not close in that short a time. In some situa
tions, it is not practical to shut down the pumps, so friction charts
must be used.
Field Examples
The first example discussed in this section shows what can occur
when uncontained fracture growth exists and is not taken into ac
count in the treatment design. The remaining three examples per
tain to the equilibrium technique: equilibrium rates and pressures
during pumping into a stationary open fracture, posttreatment tem
perature logs for a producer completed with an equilibrium acid
fracture, and the injection-profile performance
of a CO
2
injector
completed with the equilibrium technique. The field examples are
SPE Production Engineering, February 1991
8/10/2019 Equilibrium Acid Fracturing
5/8
l .5r - . - - - - - - - - - - - - - - - - - -
\
0.5
- ACTUAL PUMP
RATE
DESIGN PIJYP RATE
- .
CTU L TUBING P R E 5 ~
FRActURE EXTENSION
PRESSURE
(800 PSI)
1400
1200
1000
800
.....
_._._._._._._._._._._._.-._.
600
400
200
FRACTURE CLOStSfE
PRESSLRE
(JOO PSI)
~
________________ 40
o 20
040
60 80
tOO
120 140 16 18
Fig.
8-Equlllbrlum
rates and pressures for Denver Unit Wen
6431.
consistent with and representative
of
the overall results obtained
from the many treatments performed in the Denver Unit.
Evidence
of
Uncontained Fracture Growth. Denver Unit Well
6716 was treated with an acid fracture in 1985. A large volume
of
fluid was pumped at high rates, resulting in a fracture that grew
uncontained in the downward direction. Production results indicate
that the fracture extended well below the oil/water contact (OWC).
The treatment consisted
of
8,500 gal
of
crosslinked gel followed
by three alternating stages
of
gelled HCl (3,400 gallstage) and two
stages
of
a 40-lbm linear hydroxypropyl guar gel (2,000 gal/stage).
The final acid stage was flushed with 2,000 gal
of
brine. The total
treatment volume was 24,700 gal pumped at 5.5 bbl/min.
The treatment was tagged with radioactive material. A postfrac
ture gamma ray and temperature log (Fig. 6) was run to determine
the interval that was treated. This log showed that the treatment
went below the total depth (TD)
of the well. The top
of
the treated
interval appears to be at the top
of
the perforated interval, which
is also the top
of
the first-porosity zones. Above these zones, the
dolomite is very dense and has quite low porosity. The tight, dense
rock above the first-porosity zones appeared to contain the frac
ture at the top, but no lower containment
of
the fracture was ob
served.
Fig. 7 shows the production response to this stimulation. The
well was producing 50 BOPD plus 50 BWPD before the treatment.
The water production increased to a sustained rate between 500
and 600 BID oil production increased by only about 10 BID The
lO-fold increase in water production
is
clear evidence that the frac
ture extended below the OWC, which was 45 ft below the TD. We
can conclude from the production results that the major stimula
tion effect was in the water-bearing intervals below the OWC. From
fracturing-parameter data from the area, a conservative estimate
of the downward fracture growth indicates that the fracture extended
at least 100 ft below the OWC. The production-performance data,
posttreatment log, and estimate
of
ultimate fracture dimensions
based
on actual fracturing data in the area are evidence that fractures grow
uncontained
in
at least one vertical direction
in
this formation. Un
contained fracture growth was also observed
in
the Bennett Ranch
Unit
of
the Wasson San Andres field.
6
Equilibrium Rate
and
Pressure Data. Denver Unit Well 6431
was stimulated with the equilibrium technique. A prestimulation
fracture field test determined the minimum in-situ stress, total in
situ leakoff coefficient, overpressure, and fracture propagation pres
sure. The minimum in-situ stress was found to be 2,770 psi (bot
tomhole), fracture propagation pressure was 3,270 psi (bottomhole),
and overpressure was 500 psi. The calculated total in-situ leakoff
coefficient was 0.00125 ft/(min)
'h.
The treatment was designed to
create a radial fracture with a 50-ft radius using 1,050 gal
of
28
HCI at 2 bbIlmin followed by 2,460 gal of28% HCl at equilibri
um pump rates and pressures. The treatment was performed as it
SPE
Production Engineering,
February 1991
PERFS
- j : : = : : : j : : = ~
TRAVEL TIME
{HICRO-SEC/Fn
80 70
60
'50
H . . - ~
PCROSITY
CUTOFF
.: .
f - -_+\+-_+-- ,.
; . , t : , + j . . _ - + ~ -
g ~ T ~ b T
ACOUSTIC
f ' - ~ TR VEL
TIME
.
'
T E M E R A ~
-1---+---+--+--+---
Recommended