06-acidfracturing

7/30/2019 06-Acidfracturing

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1 Acid Fracturing

Principles of Acid Fracturing


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2 Acid Fracturing

Acid Fracturing Basics Acid is injected above fracturing pressure

- A hydraulic fracture is created

Limestone and Dolomite

Fracture faces are dissolved and etched

- Conductive channels are created

Length of etched fracture

- Determined by acid type, volume, strength, leakoffparameters, reaction rate and spending rate.

Effectiveness determined by

- Fracture length- Fracture conductivity


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3 Acid Fracturing

Candidates for Acid Fracturing

Cleaner limestone and Dolomite formations

- Must have good fracture containment to generate length

Dirty carbonate rocks (< 70% solubility in HCl) arepoor candidates

- Acid etched channel will be impaired- Release of insoluble material will plug the channel

Chalk formations may not be suitable

- Soft, unable to retain conductivity after closure

Not applicable to sandstone formations- HCl, even HF will not adequately etch sandstone

fracture face- Materials released through dissolution will plug the fracture


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6 Acid Fracturing

Consideration of Acid FracturingDesign

Fracture propagation to the desired length

Acid is capable of dissolving large amount ofreservoir rock

Retain adequate length and conductivity afterclosure

Rapid cleanup of treatment fluid

Cost effective


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8 Acid Fracturing

Acid Reaction

Strength LB CaCO3Dissolved

Type of Acid (%) /1000 gal acidat 100F

Hydrochloric 15 1,833

20 2,515

28 3,662

Formic 9 726

Acetic 10 422

Higher strengths and higher volumes will createmore fracture width


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9 Acid Fracturing

Different Acid Types andStrengths


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10 Acid Fracturing

Acid Leakoff

Acid leakoff cause the decline in treatingpressure during pumping- Fracture extension becomes impossible- Conventional filter cakes are destroyed by the

acid

Natural Fissures and Fractures- Fissures get wider as more acid is introduced- Limit the fracture propagation

Wormholes- Divert larger volume of acid away from the

primary fracture


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11 Acid Fracturing

Wormhole Development

Major source of leakofflimiting penetration

Wormholes alsoreduce fracture width

Form in the porosity of

rock matrix


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12 Acid Fracturing


Effect of Temperature


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13 Acid Fracturing


Effect of Acid Concentration


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14 Acid Fracturing


Effect of Injection Rate


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15 Acid Fracturing

Controlling Acid Leakoff

Acid Swellable Polymers- Used to control wormhole early during treatment

Oil Soluble Resins

- Limited commercial application Gelled water pad ahead of acid or within stages

of acid- Increased penetration due to reduced acid reactionrate

Gelled acid- Reduced leakoff by increasing filtrate viscosity


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16 Acid Fracturing



Oil Soluble Resins





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17 Acid Fracturing



Oil Soluble Resins





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18 Acid Fracturing


Polymeric pad - Acid stages

Reduced leakoff due

to wormhole plug-up

Accelerated leakofffollowing gel cakeerosion


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19 Acid Fracturing


DuoFrac II

Alternating stages

of acid and gel

Increased efficiencyand fracture length


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20 Acid Fracturing

Acid Transport

Transport from the center of the fracture to thefracture walls, i.e. diffusion.

Transport along the fracture length. Effects dueto pressure and density differences, i.e.advection and convection

C

t

Cv

x

Cv

y

Cv

z zD

C

z

x y z

e

advection, convection

x

y

z

diffusion


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21 Acid Fracturing

Acid Diffusion

Fluid Leakoff

Rock Etching

Acid Diffusion

Acid transport due

to concentrationdifferences

Affects acid reactionrate, and hence fracturegeometry


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22 Acid Fracturing

Parameters Controlling Acid

Diffusion Reduce fluid turbulence: Addition of viscosifiersand wider fractures.

Reduce acid leakoff: Limited particle velocity tothe fracture walls.

Increase fracture width: More time for particle

transport.

Reduce temperature: Cooldown fluids


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23 Acid Fracturing

Acid Transport along Fracture

Length

Governed by fluid pressure,density differences and

gravity.

Used to promote longeretched fracture lengths

due to viscous fingering.


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24 Acid Fracturing


LengthViscous fingering

Occurs when viscous fluid is displaced by less

viscous fluid

Three positive effects:- Acid velocity is increased- Acid etched length is increased

- Acid leakoff area is decreased

A DUOFRAC II treatment also experience viscousfingering



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25 Acid Fracturing

Acid Transport along FractureLength

Standard Analysis vs. 3-D NumericalAnalysis

0 100 200

Fracture Half-Length - ft

4896

4916

4936

4956

4976

4996

5016

WellDepth-ft

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

WellDepth-ft 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

One dimensional Three dimensional


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26 Acid Fracturing

Acid Reaction Rate

The number of acid molecules with carbonaterock per unit of time

Controlling mechanism

- Diffusion and reaction kinetics

The diffusion and kinetic mechanism can bereduced- By decreasing the temperature- By increasing the viscosity of acid mixture


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27 Acid Fracturing

Acid Reaction Rate

M

tK C Cacid

r wall eqm

m

Macid = moles of acid at fracture wall

Kr = Reaction rate constant

Cwall = Acid concentration at fracture wall

Ceqm = Equilibrium acid concentration

Depends on detailed chemical composition ofspecies involved


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28 Acid Fracturing

M

tK C C C C vacid

g wall wall L

Macid = moles of acid at fracture wall

Kg = Diffusion constant

Cwall = Acid concentration at fracture wall

C = Average acid concentration

vL = Leakoff velocity

Acid Reaction Equilibrium

Acid concentration at surface balanced by thattransported through diffusion


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29 Acid Fracturing

Acid Reaction Equilibrium

Diffusion limited acid fracturing:

- Extremely fast reaction rate. Etching limited by diffusionacid transport

Kinetic limited acid fracturing:

- Rapid acid transport. Limited acid - rock reaction.


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30 Acid Fracturing

Optimizing Conductivity & EtchedFracture Length

No theoretical limitation of conductivity value

- A matter of pumping more acid to widen the etched width

Maximum stimulation ratio achieved

- Corresponds to the case of infinite conductivity fracture

Optimum acid fracture penetration

x k wk

f

f

50


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31 Acid Fracturing

Fluids for Deeper AcidPenetration

Leakoff control is imperative

Decreasing leakoff through natural fissures

- 100 mesh resin/sand, or fine salt

- LCA and viscous pads

Decreasing leakoff due to wormholes

- LCA

- Viscosified acid (DGA)

- DUOFRAC II

Decreasing leakoff through fracture walls

- Viscous fluid bank

f


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32 Acid Fracturing







- LCA


- DUOFRAC II



Fl id f D A id


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33 Acid Fracturing







- LCA


- DUOFRAC II




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34 Acid Fracturing

Cooldown

Cooldown in Acid Fracturing- Controls diffusion and surface reaction rates

BHST > 200F, rapid reaction with HCl

- Acid etching is limited to a flow test

Best fluid for cooldown

- High leakoff fluids

- Affected by volume, rate and fluid invasion to theprimary porosity

Requirement in a fissured reservoir

- Initiate cooldown once the leakoff to fissures has beenlimited


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35 Acid Fracturing

Retarded Acid

Acid with a reduced reaction rate

Penetrates more deeply into the fracture

Fracture width is decreased

The degree of retardation is defined byretardation factor (RF)

- HCl, RF = 1

Retarded Acid, RF > 1


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36 Acid Fracturing

Retardation Factor

Base values RF

- HCl, DGA and LCA 1

- DAD 2

- Surfactant retarded with

F98 2

- Organic acid 4

- SXE 10


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37 Acid Fracturing

Retardation Factor-Static & DynamicConditions


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38 Acid Fracturing

Acid Fracturing DesignFundamentals

Achieving acid fracture penetration

Maximize acid fracture length- BHST < 200F, use fluid and lowest leakoff- BHST > 200F, use cooldown + acid retardation

Acid fracture length should be limited- Where no barrier to limit height growth- To prevent communication with water or gas zones- Fracture length = 1/2 thickness of producing interval

(radial)

Maximize the injection rate- Deeper penetration- Dictated by maximum allowable wellhead pressure

06-acidfracturing

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