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A NEW SYSTEM FOR CONTROLLING THE CORSSLINKING

RATE OF BORATE FRACTURING FLUIDS

K.E. CAWLIZEL J.L. ELBEL

this article begins on the next page FF

PETROLEUM SOCIETY OF CIM/SOCIETY OF PETROLEUM ENGINEERS PAPER NO. CIM/SPE 90-91 THIS IS A PREPRINT - SUBJECT ToCORRECnON A NEW SYSTEM FOR CONTROLLING THE CROSSLINKING RATE OF BORATE FRACTURING FLUIDS BY K.E Cawlezel J.L. Elbel Domil ScMumbwgor PUBUCATION RIGHTS RESERVED THIS PAPER IS TO BE PRESENTED AT THE INTERNATIONAL TECHNICAL MEETING JOINTLY HOSTED BY THE PETROLEUM SOCIETY OF CIM AND THE SOCIETY OF PETROLEUM ENGINEERS IN CALGARY, JUNE 10 TO 13,1990. DISCUSSION OF THIS PAPER IS INVITED. SUCH DISCLOSS ION MAY BE PRESENTED AT THE MEETING AND WILL BE CONSIDERED FOR PUBLICATION IN CIAND SPE JOURNALS IF FILED IN WRITING WITH THE TECHNICAL PROGRAM CHAIRMAN PRIOR'RO THE CONCLUSION OF THE MEETING. ABSTRACT for field optimization of 'crosslink times'. Using this technique, delayed borate crosslinked fluids can be custom designed tor Borate crosslinked fracturing fluids have boon suocessftilly each iridmdual well configuration. utilized In fracturing operations for several years. These fluids provide exoslient rheological, fluid loss, and fracture conductivity properties over fluid temperatures up to 225OF

[1070C]. The mechanism of bomte crosslinking Is an equilibrium process which can produce very high fluid viscosities under conditions of low shear. While thls property Is very desirable once the fluid enters the fracture, lt can cause high fdcuonal pressure to be developed system has been developed which allows the crosslinking rate of these fluids to be rewly and easily controlled. Using this system, crosslinkin g rates can be controlled to give low tricuon pressure in the wellbore and yet maximum viscosity in the fracture for improved proppant transport. Field case histories

demonstrate the successful use of the delay ed borate crosslinked fluids and include resu lts of an injecuon procedure used to determine @on pressu re at different pump rates. The tdcbon pressure data also profiles the chan ges that occur as the fluid Is crosslinking at different stag es In the tubing. Results indicate that different crossl inking rates can be recognized by their 'friction pre ssure signature' and that the crosslinkin g rate can be easily altered during a treatment to reduce total treat ing pressure. This technique with its resulting frictio n pressure profiki has proven to be an

extremely useful tool References and lllustratons at end of paper. 91-1 Data from laboratory development, full scale friction tests, and fracturing treatments are Included which illustrate the properties of the fluid and the ability to couple fluid performance with treatment design. in the tubulars under certain flow oondrdons. A cross link acbvat or INTRODUCTION Field usage of borate crosslinked fbilds has been limited in some tubing/pump rate situations due to the large frictional pressure developed in the wellbore prior to entry into the fracture. A reliable method to delaythe crosslinking of these fluids would have the advantages of providing 1) low friction pressure In the wellbore, 2) increased fluid efficiency, and 3) maximum viscosity in the fracbjre for improved proppant transport capabilities. A slurried delayed borate crosslink activator system has been developed that will predictably crosslink borate fluids in the 0.5 to 4 min range. T he crosslinking rate is vaded by changing the conoentrabon of acbvalor in the fluid. This is accomplished in field operations by changing the metering rate of the acbvator. As the concentration of crosslink

activator slurry is increased the crosslinking rate increases. Crosslinking rate can be custom designed to conform to a specified 'crosslink time' or individual well configuration.

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PETROLEUM SOCIETY OF CIM/SOCIETY OF PETROLEUM ENGINEERS

THIS

IS A

PREPRINT

- SUBJECT TO

CORRECTION

PAPER NO

CIMISPE

9 91

A NEW SYSTEM FOR CONTROLLING

THE CROSSLINKING RATE OF BORATE

FRACTURING FLUIDS

BY

K.E. Cawlazel

J L Elbal

Dowell Schlumb.rglilr

PUBLICATION RIGHTS RESERVED

THIS PAPER IS TO BE PRESENTED AT THE INTERNATIONAL TECHNICAL MEETING JOINTLY HOSTED BY THE

PETROLEUM SOCIETY OF CIM AND THE SOCIETY OF PETROLEUM ENGINEERS IN CALGARY, JUNE

10

TO

13 1990

DISCUSSION OF THIS PAPER IS INVITED. SUCH DISCUSSION MAY BE PRESENTED

T

THE MEETING AND WILL BE

CONSIDERED FOR PUBUCA1l0N

IN C IM

AND SPE JOURNALS IF FILED IN WRITING WITH THE TECHNICAL PROGRAM

CHAIRMAN PRIOR TO THE CONCLUSION OF THE MEETING.

.,

.

ABSTHACT

Borate crosslinked fractUring

rlurds

have been successfully

ulilized in fracturing operations for several years. These fluids

provide excellent rheological, fluid loss, and fracture conductivity

propartias

ovar

fluid

tamparaluras up

to

225F [107C]

Tha

mechanism of borate crosslinking is an eqUilibrium process which

can

produce

very hIgh fluid Viscosities under

condlllons of

low

shear. While this property is very desirable once the

wId

enters

the fracbJre, It can cause high fricUonal pressure to be developed

in the tubulars under certain now condftions. A crosslink activator

system has been developed which allows the crossllnking rate of

these nuids to be reliably and easIly controlled. Uslng this system,

crossllnklng rates can

be

controlled to give low friction pressure In

the

wellbore

and ye t maxImum

viscosity

in the

fracture

fo r

improved proppant transport.

Field case hIstories demonstrate the successful use

of

the

delayed borate crosslinked fluids and include

results

of

an

Injection procedure used

to

determine friction pressure at diHerent

pump rates. The friction pressure data also profi les the changes

that

occur

as the rluid Is crossl lnking

at

different

stages In the

tubing. Results indicate that different crosslinklng rates can be

recognized by

their

friction pressure sIgnature and that the

crossllnking

rate

can

be

easi ly altered during

a treatment

to

reduce total treating pressure. This technique with its resulting

friction pressure profile has proven

to

be an extremely useful tool

References and Illustrations at end of DaDElr

for field optimization

or

crossl ink times. Using this technique,

delayed borate crosslinked fluIds can be custom designed for

8ach Individual well configuration.

Data from laboratory development, full scale rriction tests.

and

f racturing treatments

are

Included which i llust rate

the

propertlas of tha lIuld and Ihe ability to coupla fluid performance

with b eatment design.

INTRODUCTION

FIeld usage of borate crosslinked fluIds has been limited in

some tubing/pump

rate situations due

to the large frictional

pressure developed in the wellbore prior to entry Into the fracture.

A rel iable method to delay the crosslinklng of these fluids would

have the advantages

of

providing 1 low friction pressure in the

wellbore, 2 increased fluid efficIency, and 3 maxImum viscosity

in the fracture for improved proppant transport capabilities.

A slurried delayed borate crosslink activator system has

been developed that will predictably crosslink borate fluids in the

0.5 to 4 min range. The crossllnking rate is varied by changing the

concentration of activator in the fluid. This is accomplished in field

operatrons by changing the metering rate of lhe activator. s the

concentrat ion of crosslink

activator

slurry

is

increased the

crosslinking rate increases.

Crosslinking rate can be custom designed to conform lo a

soecified -crosslink time- or Individual well confiauration.

91 1

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Laboratory

development

the

delayed borate

crosslinked

fluids locused

on developing an activator

system

which could

reliably

controllhe crossllnklng

rate in

tubulars

and

also

produce

excellent rheological properties under the temperature

and shear

conditions found in

the

fracture.

Full

sca le f ie ld tes ts were

performed

to determine the

friction pressure

of

the delayed borate crosslinked fluid at different

pump

rates and

to compare changes In Irlclion pressure that

occurred

in

the

the

tubing to crossl ink l imes determined

by

sampling fluid

al

the missile.

EXPERIMENTAL

Chemlslry

The guar/borate

Ion

crosslinking

mechanism

consists

of

three steps. Ini tial ly boric add dissociates In water producing the

borate

ion.

Then,

In

alkaline media,

the

borate

Ion reacts with a

cis-hydroxyl

group

on

guar

to

form

a

monodlol

complex

(diester

boric acid) which fur ther reacts with

a

cis-hydroxyl group from

another

guar

molecule to

form

the

3-dimensional

diol

complex,

and thus the crossnnked

guar

fluid.

HO 0 OH

/ /

B(OH) + 8 + 2H

2

0

.

/

HO

a OH

Crussllnklng

Rate

Crosslinking

rates

were obtained using

a

Waring

l it ted with a 1-l iter [ ~

blender

jar. The appropriate v

l Iuid containing the crosslinker was added to

the

ja r

blender speed control led at

1800 rpm.

The

activator w

added

to the

inner edge of

the

vortex

and mIxed for one

The

fluid was then

transferred

to a

Fann 35

vlscometer

The apparent v iscosi ty was measured at 100 rpm. V

increase

with elapsed

time

was

recorded.

Figure 2

shows the

crosslinking

rate

depende

activator

concentration.

Resul ts show tha t crossll

decreases

wtth

increasing activator

concentration

and

tha

min

crosslink

times

the viSttlslty remains

low for

this

Urn

and

then Increases

sharply

to

greater than

500 cps [0

rather than showing very gradual

viscostty

increases

wlih

RheoloQY

Reciprocat ing Capi llary V iscometers 1 were u

rheological characterization, because attempts to mea

properties

of

these thick

fluids

using rotational vlscomete

not successful due

to the

Weissenberg

Effect.

Recip

capil lary viscometers allow

long

l ime, high

temperabJre

data

to

be collected using capil lary tubes as the meas

device.

The

basic

principle

of

operation involves recipro

portion of

the

test

fluid

back and

forth

inside a capil lary t

measuring

the

resultant pressure

drop

as a

function of

f

temperature

and time_

By knowing pressure drop,

1I0w r

tubing dimensions,

the

shear stress/shear rate data

determined.

The

rate

of

crosslinking

can be

controlled by delaying Ihe

rate at

which

the pH

or

the sys tem is

increased.

A crosslink

activator

s lu rr y was deve loped

which

delays the onset 0 f

crosslinking by

slow

solubilization

an alkaline compound_

Figure 1 shows the increase i n f lui d

pH

that occurs as th e

concentration

of

the actlvator is increased,

a

oH

/ /

R 8

- /

-

a

oH

+

Rheological data indicated that the

fluid

behavior

and

the f low

consistency Index

K

change dramatically

fo

crosslinlted fluids depending on shear rate range tes t

shear

rate

range over

which

the

Power Law

coefficien

determined was 40160 sec-

1

. This

is reported

2

to

be I

actual shear

rate to

which the

fracturing nuid Is

expos

moving

through the rracture. Constan t shear belwee

ramps

was

8 sec

1

.

Apparent viscosity

data

lo r

a

borate ful ly crosslinked f lu id as

a

function of

rluld

temp

are given in Figure

3.

FIELD TEST METHODS

LABORATORY TESTS

The

materials

used in

laboratory testing were base

fluids,

borate

crosslinker,

and

crosslink activator

s lurry. All

base

fluids

consisled of 40

Ibm

hyd,orypropyl gua' (HPG)/l

,000 gal

2

KC I

wale'

[4.8

kg m

or 40

Ibm gua,/l,OOO gal 2

KCI

water

[4.8

kglm

3

j.

The borate

crosslinker

was

used

at 1 Ibm

4

Ibm/1000

gal

[0.12 to 0.48 fluid depending on the base

fluid. The

activator slurry

concentrat ions used were

from

0.5

ga l

to 8

ga l

,000 gal [0.5

dm

3

8

dm

base

fluid

depending

on

Ihe

crosslinking rate desired.

Laboratory

testing

concentrated

on

determining th e

crosslinking rate and the

meological

properties

of delayed

borate

fluid s. The ef fec ts of t he type of

gelling

agent, gel ling agent

concentra tion, temperature , mix ing energy, and

pH

on

crosslinking rates and

meological

properties were studied.

CroJsllnk TIme Detennlnatlon

Relating

crosslink tims of

fluid

samples

taken

allhe

s

increases

in friction pressure

or

nuid performance in the f

often unsuccessful because shear effecls during

crosslin

dramatically

change f lu id Viscosi ty . In the case

crosslinked

fluids, shear

degradation in

tubulars is

not

a

even

with fast

crosslinking

systems,

but because

the vis

the

Iluid

is

highly

dependent on

Ihe effective

shear in th

crossl ink t imes predicted from

surface

tesls should

be

u

starting place to se l

activator concenlralions. Final conce

can be adjusted depending on the resuhant friction pressu

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