Principles of Acid Fracturing
1
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, leakoff parameters, reaction rate and spending rate.
•
Effectiveness determined by - Fracture length - Fracture conductivity
2
Acid Fracturing
Candidates for Acid Fracturing •
Cleaner limestone and Dolomite formations - Must have have good fracture containment to generate length
•
Dirty carbonate rocks (< 70% solubility in HCl) are poor candidates - Acid etched channel will be impaired - Release of insoluble material will plug the channel
•
Chalk formations may not be suitable -
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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
3
Acid Fracturing
Acid & Propped Fracturing Comparison
4
Acid Fracturing
Propped Fracturing
Equipment/Treatment Equipment/Treatment
Less complicated
More complicated
Proppant problems
No
Yes
Proppant Transport Problem
No
Yes
Fluid Loss Control
Poor
Better
Candidates Candidate s
Carbonates Only
Carbonates + Sandstones
Residual Damage in Fracture
No
Yes
Fracture Length Achievable
Shorter
Longer
Acid Fracturing
Factors Influencing Fracture Length & Conductivity •
Acid type, strength and volume - Affects etched width width and fracture lengths
•
Acid leakoff - Additional complication due to gel cake erosion and wormhole development
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Acid viscosity - Governs fracture width and acid transport along length
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Injection rate
•
Formation type - Mineralogy, temperature, saturation, wettability
5
Acid Fracturing
Consideration of Acid Fracturing Design •
•
•
6
Fracture propagation to the desired length Acid is capable of dissolving large amount of reservoir rock Retain adequate length and conductivity after closure
•
Rapid cleanup of treatment fluid
•
Cost effective
Acid Fracturing
Acid Fracture Mechanics Acid Leakoff
Acid Reaction
Acid Transport
7
Acid Fracturing
Acid Reaction Strength
Dissolved Type of Acid at 100°F
/1000 gal acid
Hydrochloric
15 20 28
1,833 2,515 3,662
Formic
9
726
Acetic
10
422
•
8
(%)
LB CaCO3
Higher strengths and higher volumes will create more fracture width
Acid Fracturing
Different Acid Types and Strengths
9
Acid Fracturing
Acid Leakoff •
Acid leakoff cause the decline in treating pressure 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 -
10 Acid Fracturing
Divert larger volume of acid away aw ay from the primary fracture
Wormhole Development
•
•
•
Major source of leakoff limiting penetration Wormholes also reduce fracture width Form in the porosity of rock matrix
11 Acid Fracturing
Wormhole Development Effect of Temperature
12 Acid Fracturing
Wormhole Development Effect of Acid Concentration
13 Acid Fracturing
Wormhole Development Effect of Injection Rate
14 Acid Fracturing
Controlling Acid Leakoff •
Acid Swellable Polymers - Used to to control wormhole early early during treatment
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Oil Soluble Resins - Limited commercial application
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Gelled water pad ahead of acid or within w ithin stages of acid - Increased penetration due to reduced acid reaction rate
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Gelled acid - Reduced leakoff by increasing filtrate viscosity
15 Acid Fracturing
Controlling Acid Leakoff •
Acid Swellable Polymers - Used to to control wormhole early early during treatment
•
Oil Soluble Resins - Limited commercial application
•
Gelled water pad ahead of acid or within w ithin stages of acid - Increased penetration due to reduced acid reaction rate
•
Gelled acid - Reduced leakoff by increasing filtrate viscosity
16 Acid Fracturing
Controlling Acid Leakoff •
Acid Swellable Polymers - Used to to control wormhole early early during treatment
•
Oil Soluble Resins - Limited commercial application
•
Gelled water pad ahead of acid or within w ithin stages of acid - Increased penetration due to reduced acid reaction rate
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Gelled acid - Reduced leakoff by increasing filtrate viscosity
17 Acid Fracturing
Controlling Acid Leakoff Polymeric pad - Acid stages •
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Reduced leakoff due to wormhole plug-up Accelerated leakoff following gel cake erosion
18 Acid Fracturing
Controlling Acid Leakoff DuoFrac II •
•
Alternating stages of acid and gel Increased efficiency and fracture length
19 Acid Fracturing
Acid Transport C
Cv
t
x
x
Cv
y
y
Cv
z
z
z
D
e
y
C z
advection, convection diffusion •
•
x
z
Transport from the center of the fracture to the fracture walls, i.e. diffusion. Transport along the fracture length. Effects due to pressure and density differences, i.e. advection and convection
20 Acid Fracturing
Acid Diffusion Acid Diffusion •
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Acid transport due to concentration differences
Fluid Leakoff
Affects acid reaction rate, and hence fracture geometry Rock Etching
21 Acid Fracturing
Parameters Controlling Acid Diffusion •
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•
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Reduce fluid turbulence: Addition of viscosifiers and wider fractures. Reduce acid leakoff: Limited particle velocity to the fracture walls. Increase fracture width: More time for particle transport. Reduce temperature: Cooldown fluids
22 Acid Fracturing
Acid Transport along Fracture Length •
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Governed by fluid pressure, density differences and gravity. Used to promote longer etched fracture lengths due to viscous fingering.
23 Acid Fracturing
Acid Transport along Fracture Length Viscous 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 viscous fingering
24 Acid Fracturing
Acid Transport along Fracture Length Standard Analysis vs. 3-D Numerical Analysis 4896
4896 0.00 - 0.01
t f 4916 h 4936 t p e D 4956 l l e W 4976
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
t f 4916 h 4936 t p e D 4956 l l e W 4976
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
4996
4996 5016 0
100
Fracture Half-Length - ft
One dimensional 25 Acid Fracturing
200
5016 0
100
Fracture Half-Length - ft
Three dimensional
200
Acid Reaction Rate •
•
•
The number of acid molecules with carbonate rock per unit of time Controlling mechanism - Diffusion and reaction kinetics The diffusion and kinetic mechanism can be reduced - By decreasing the temperature - By increasing the viscosity of acid mixture
26 Acid Fracturing
Acid Reaction Rate •
Depends on detailed chemical composition of species involved M acid t
K r C wall
C eqm
m
Macid = moles of acid at fracture wall Kr
= Reaction rate constant
Cwall = Acid concentration at fracture wall Ceqm = Equilibrium acid concentration 27 Acid Fracturing
Acid Reaction Equilibrium •
Acid concentration at surface balanced by that transported through diffusion M acid t
K g C
C wall wal l
C
C wall v L
Macid = moles of acid at fracture wall Kg
= Diffusion constant
Cwall = Acid concentration at fracture wall
28 Acid Fracturing
C
= Average acid concentration concentr ation
vL
= Leakoff velocity
Acid Reaction Equilibrium •
Diffusion limited acid fracturing: - Extremely fast reaction rate. Etching limited by diffusion acid transport
•
Kinetic limited acid fracturing: - Rapid acid transport. Limited acid - rock reaction.
29 Acid Fracturing
Optimizing Conductivity & Etched Fracture Length •
No theoretical limitation of conductivity value - A matter matter of pumping more acid to widen the etched etched width
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Maximum stimulation ratio achieved - Corresponds to the case of infinite conductivity fracture
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Optimum acid fracture penetration
x
30 Acid Fracturing
k w f
f
50 k
Fluids for Deeper Acid Penetration •
Leakoff control is imperative
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Decreasing leakoff through natural fissures - 100 mesh resin/sand, or fine salt - LCA and viscous pads
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Decreasing leakoff due to wormholes - LCA - Viscosified acid (DGA) - DUOFRAC II
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Decreasing leakoff through fracture walls - Viscous fluid bank
31 Acid Fracturing
Fluids for Deeper Acid Penetration •
Leakoff control is imperative
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Decreasing leakoff through natural fissures - 100 mesh resin/sand, or fine salt - LCA and viscous pads
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Decreasing leakoff due to wormholes - LCA - Viscosified acid (DGA) - DUOFRAC II
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Decreasing leakoff through fracture walls - Viscous fluid bank
32 Acid Fracturing
Fluids for Deeper Acid Penetration •
Leakoff control is imperative
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Decreasing leakoff through natural fissures - 100 mesh resin/sand, or fine salt - LCA and viscous pads
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Decreasing leakoff due to wormholes - LCA - Viscosified acid (DGA) - DUOFRAC II
•
Decreasing leakoff through fracture walls - Viscous fluid bank
33 Acid Fracturing
Cooldown •
Cooldown in Acid Fracturing - Controls diffusion and surface reaction rates
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BHST > 200°F, rapid ra pid reaction re action with HCl - Acid etching is limited to a flow test
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Best fluid for cooldown - High leakoff fluids - Affected by volume, rate and fluid invasion to the primary porosity
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Requirement in a fissured reservoir - Initiate cooldown once the leakoff to fissures has been limited
34 Acid Fracturing
Retarded Acid •
Acid with a reduced reaction rate
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Penetrates more deeply into the fracture
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Fracture width is decreased
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The degree of retardation is defined by retardation factor (RF) - HCl, RF = 1 Retarded Acid, RF > 1
35 Acid Fracturing
Retardation Factor •
Base values
RF
- HCl, DGA and LCA
1
- DAD
2
- Surfactant retarded with F98
2
- Organic acid
4
- SXE
10
36 Acid Fracturing
Retardation Factor-Static & Dynamic Conditions
37 Acid Fracturing
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Acid Fracturing Design Fundamentals
Achieving acid fracture penetration
Maximize acid fracture length - BHST < 200° 2 00°F, F, use fluid fl uid and lowest low est leakoff leakof f -
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Acid fracture length should be limited -
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BHST > 200°F, 200°F, use cooldown cooldow n + acid retardation retardat ion
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 -
38 Acid Fracturing
Deeper penetration Dictated by maximum allowable wellhead pressure
