Acid Fracturing

(1)

1Acid 1Acid

Principles of Acid

Fracturing

(2)

(3)

2Acid 2Acid

Acid Fracturing Basics



 Acid is injected above fracturing pressureAcid is injected above fracturing pressure –

– A hydraulic fracture is A hydraulic fracture is createdcreated 

 Limestone and DolomiteLimestone and Dolomite 

 FFracture faces are dissolved racture faces are dissolved and etcheand etchedd –

– Conductive channels are cConductive channels are createdreated 

 Length of etched fractureLength of etched fracture –

– Determined by acid Determined by acid type volume strength lea!o"type volume strength lea!o"

parameters reaction rate and spending rate#

parameters reaction rate and spending rate# 

 $"ectiveness determined by$"ectiveness determined by –

– Fracture lengthFracture length

–

(4)

%Acid %Acid

Candidates for Acid

Fracturing



Cleaner limestone and Dolomite

forma

tions

–

– &ust have good &ust have good fracturfracture containment to generate e containment to generate lengthlength



Dirty carbonate roc!s '( )*+ solubility in

,Cl-

,Cl-are poor candidates

are poor candidates

–

– Acid etched channel .ill be Acid etched channel .ill be impairedimpaired

–

– //elease of elease of insoluble material .ill plug insoluble material .ill plug the channelthe channel



Chal! formations may not be suitable

–

– 0oft unable to retain conductivity after closure0oft unable to retain conductivity after closure



ot applicable to

sandstone formations

–

– ,Cl even ,F .ill ,Cl even ,F .ill not adeuately etch sandstonenot adeuately etch sandstone



 fracture fracture faceface

–

– &aterials released through dissolution .ill plug the&aterials released through dissolution .ill plug the

fracture

(5)

3Acid

Acid 4 5ropped Fracturing

Comparison

Acid Fracturing 5ropped Fracturing

 $uipment67reatment Less complicated &ore complicated

 5roppant problems o 8es

 5roppant 7ransport 5roblem o 8es

 Fluid Loss Control 5oor Better

 _Candidates _{Carbonates 9nly} _{Carbonates :}

0andstones

 /esidual Damage in Fracture o 8es

(6)

;Acid

Factors <n=uencing Fracture Length 4

Conductivity

 Acid type strength and volume

– A"ects etched .idth and fracture lengths

 Acid lea!o"

– Additional complication due to gel ca!e erosion and .ormhole development

 Acid viscosity

– >overns fracture .idth and acid transport along length

 <njection rate  Formation type

(7)

?Acid

Consideration of Acid Fracturing

Design



Fracture propagation to the desired length



Acid is capable of dissolving large amount

of reservoir roc!



/etain adeuate length and conductivity

after closure



/apid cleanup of treatment =uid

(8)

)Acid

Acid Fracture &echanics

Acid Leako

Acid Reaction

(9)

@Acid

Acid /eaction

0trength LB CaC9% Dissolved

7ype of Acid '+- 61*** gal acid at 1**F ,ydrochloric 1; 1@%% 2* 2;1; 2@ %??2 Formic  )2? Acetic 1* 322



,igher strengths and higher volumes

(10)

Acid

Di"erent Acid 7ypes and

0trengths

(11)

1*Acid

Acid Lea!o"

 Acid lea!o" cause the decline in treating

pressure during pumping

– Fracture etension becomes impossible

– Conventional lter ca!es are destroyed by the acid

 atural Fissures and Fractures

– Fissures get .ider as more acid is introduced – Limit the fracture propagation

 Eormholes

– Divert larger volume of acid a.ay from the primary fracture

(12)

11Acid

Eormhole Development

 &ajor source of lea!o"

limiting penetration

 Eormholes also

reduce fracture .idth

 Form in the porosity of

(13)

12Acid

Eormhole Development

$"ect of 7emperature

(14)

1%Acid

Eormhole Development

$"ect of Acid Concentration

(15)

13Acid

Eormhole Development

$"ect of <njection /ate

(16)

1;Acid

Controlling Acid Lea!o"

 Acid 0.ellable 5olymers

– sed to control .ormhole early during treatment

 9il 0oluble /esins

– Limited commercial application

 >elled .ater pad ahead of acid or .ithin stages

of acid

– <ncreased penetration due to reduced acid reaction rate

 >elled acid

(17)

1?Acid

Controlling Acid Lea!o"

 >elled .ater pad ahead of acid or .ithin stages of acid

 >elled acid

(18)

1)Acid

Controlling Acid Lea!o"

 >elled .ater pad ahead of acid or .ithin stages of acid

 >elled acid

(19)

1@Acid

Controlling Acid Lea!o"

5olymeric pad G Acid stages

 /educed lea!o" due to

.ormhole plugGup

 Accelerated lea!o"

follo.ing gel ca!e erosion

(20)

1Acid

Controlling Acid Lea!o"

DuoFrac <<

 Alternating stages of

acid and gel

 <ncreased eHciency

(21)

2*Acid C t Cv x Cv y Cv z z D C z x y z e _  advection, convection x y z  diusion

Acid 7ransport



7ransport from the center of the fracture to

the fracture .alls i#e# di"usion#



7ransport along the fracture length# $"ects

due to pressure and density di"erences i#e#

advection and convection

(22)

21Acid

Fluid Leako

Rock Etching Acid Diusion

Acid Di"usion

 Acid transport due to

concentration di"erences

 A"ects acid reaction

rate and hence fracture geometry

(23)

22Acid

5arameters Controlling Acid

Di"usion



/educe =uid turbulenceI Addition of

viscosiers and .ider fractures#



/educe acid lea!o"I Limited particle

velocity to the fracture .alls#



<ncrease fracture .idthI &ore time for

particle transport#

(24)

2%Acid

Acid 7ransport along Fracture

Length

 >overned by =uid pressure density di"erences and gravity#  sed to promote longeretched fracture lengths due to viscous ngering#

(25)

23Acid

Acid 7ransport along Fracture

Length

Jiscous ngering



9ccurs .hen viscous =uid is displaced by

less viscous =uid



7hree positive e"ectsI

– Acid velocity is increased

– Acid etched length is increased

– Acid lea!o" area is decreased



A D9F/AC << treatment also eperience

(26)

2;Acid

Acid 7ransport along Fracture

Length

0tandard Analysis vs# %GD

umerical Analysis

0 100 200 Fracture Half-Length - ft 4896 4916 4936 4956 4976 4996 5016 W e l l D e p t h - f t 0.00 - 0.01 0.01 - 0.01 0.01 - 0.02 0.02 - 0.02 0.02 - 0.02 0.02 - 0.03 0.03 - 0.03 > 0.03 0 100 200 Fracture Half-Length - ft 4896 4916 4936 4956 4976 4996 5016 W e l l D e p t h - f t 0.01 - 0.02 0.02 - 0.02 0.02 - 0.03 0.03 - 0.04 0.04 - 0.05 0.05 - 0.06 0.06 - 0.07 > 0.07

(27)

2?Acid

Acid /eaction /ate



7he number of acid molecules .ith

carbonate roc! per unit of time



Controlling mechanism

– Di"usion and reaction !inetics



7he di"usion and !inetic mechanism can

be

– By decreasing the temperature

(28)

2)Acid

Acid /eaction /ate

M

t K C C

acid

wall eqm m

&_acid K moles of acid at fracture .all  _r K /eaction rate constant

C_.all K Acid concentration at fracture .all

C_em K $uilibrium acid concentration

Depends on detailed chemical composition of species involved

(29)

2@Acid

M

t K C C C C v

acid

wall wall L

&_acid K moles of acid at fracture .all  _g K Di"usion constant

C_.all K Acid concentration at fracture .all

C K Average acid concentration v_L K Lea!o" velocity

Acid /eaction $uilibrium

Acid concentration at surface balanced by that transported through di"usion

(30)

2Acid

Acid /eaction $uilibrium



Di"usion limited acid fracturingI

– $tremely fast reaction rate# $tching limited

by di"usion acid transport



inetic limited acid fracturingI

– /apid acid transport# Limited acid G roc!

reaction#

(31)

%*Acid x k w k f f 50

9ptimiMing Conductivity 4

$tched Fracture Length



o theoretical limitation of conductivity

value

– A matter of pumping more acid to .iden the

etched .idth



&aimum stimulation ratio achieved

– Corresponds to the case of innite

conductivity fracture

(32)

%1Acid

Fluids for Deeper Acid

5enetration



Lea!o" control is imperative



Decreasing lea!o" through natural ssures

– 1** mesh resin6sand or ne salt – LCA and viscous pads



Decreasing lea!o" due to .ormholes

– LCA

– Jiscosied acid 'D>A-– D9F/AC <<



Decreasing lea!o" through fracture .alls

(33)

%2Acid

Fluids for Deeper Acid

5enetration



Lea!o" control is imperative



Decreasing lea!o" through natural ssures



Decreasing lea!o" due to .ormholes

– LCA



Decreasing lea!o" through fracture .alls

(34)

%%Acid

Fluids for Deeper Acid

5enetration



Lea!o" control is imperative



Decreasing lea!o" through natural ssures



Decreasing lea!o" due to .ormholes

– LCA



Decreasing lea!o" through fracture .alls

(35)

%3Acid

Cooldo.n

 Cooldo.n in Acid Fracturing

– Controls di"usion and surface reaction rates

 B,07 N 2**F rapid reaction .ith ,Cl

– Acid etching is limited to a =o. test

 Best =uid for cooldo.n

– ,igh lea!o" =uids

– A"ected by volume rate and =uid invasion to the primary porosity

 /euirement in a ssured reservoir

(36)

%;Acid

/etarded Acid



Acid .ith a reduced reaction rate



5enetrates more deeply into the fracture



Fracture .idth is decreased



7he degree of retardation is dened by

retardation factor

'/F-– ,Cl /F K 1

(37)

%?Acid

/etardation Factor

Base

values

/F

G ,Cl D>A and LCA 1

G DAD 2

G 0urfactant retarded .ith

F@ 2

G 9rganic acid 3

(38)

%)Acid