Equilibrium Acid Fracturing

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


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


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


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---+---+--+--+---

Documents

Equilibrium Acid Fracturing