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Acid Fracturing: An Alternative Stimulation Approach in Carbonates
Ding Zhu, Texas A&M University
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Background
Propped fracturing
Slide 2
Acid fracturing Matrix acidizing
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Acid Fracturing
Pro• Easy to pump• Screenout free• Network building in natural fractured formation• Smaller scale compared with propped fracturing
Con• Depends on formation heterogeneity more critically • Only works for carbonate/carbonate rick formation• Conductivity declines fast as closure stress
increases
Slide 3
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Main Issues in Acid Fracturing
• Candidate selection• Optimization design (rate, volume)• Multi-stage/zonal isolation/diversion• Modeling of acid fracturing, fully numerical
models and empirical correlations• Conductivity testing procedures• Productivity predictions
Slide 4
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ModelingEmpirical Correlations for Fracture Conductivity• Nierode-Kruk (1973)• Gangi (1978)• Walsh (1981)• Gong (1993)• Mou-Deng (2013)
Numerical Modeling for Transport Simulation• Settari (1993)• Oeth (2013)
Slide 5
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Acid fracture scale Experimental scale
Intermediate scale
Scaling ProblemSlide 6
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Nierode and Kruk (1973) – Exponential function
Gangi (1978) – Power function
Walsh (1981) – Logarithmic function
Conductivity Correlations
32131 C
cf CCwk
cf CCwk ln2131
cf CCwk 21 exp
Slide 7
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Empirical Correlations by Mou-Deng
expf cwk
0.520.42.8
, ,00.22 0.01 1f D x D D z Dwk
414.9 3.78ln 6.81ln 10D E
9 3 1 2 , 3 4 5 , 601 2 3 4 5 6
4.48 10 1 ( ( )) ( ( )) ( 1)
1.82, 3.25, 0.12, 1.31, 6.71, 0.03
Df D x D zwk w a erf a a a erf a a e
a a a a a a
Slide 8
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Empirical Correlations for Conductivity
0.1
1
10
100
1000
10000
100000
0 1000 2000 3000 4000 5000 6000 7000
c
(psi
)
wkf (md-ft)
Nierode-Kruk model
Mou-Deng model
Slide 9
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Numerical Model: Etching Width Prediction
2D Solutions– Type curves to predict penetration
(Roberts and Guin, 1974)
– Early simulators based on finite difference
– Typically some average integrated across channel (Settari, 1993)
Settari (1993)
2
2
y
CD
y
Cv
x
Cu eff
1 'nbeff kCy
CD
Slide 10
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Settari (2001) modified 2D approach– No height dependence
– Analytical velocity solution applied
Romero (1998) 3D approach– Analytical velocity solution applied
Settari et al. (2001)
2
2
y
CD
z
Cw
y
Cv
x
Cu
t
Ceff
Numerical Model: Etching Width PredictionSlide 11
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3D Acid Transport Model (Oeth, 2013)
vLeakoff
Qinj
wid
th
dire
ctio
n
• Velocity profile for non-Newtonian fluid• Acid concentration in y-direction• Leakoff from fracture to formation
y
CD
yz
Cw
y
Cv
x
Cu
t
Ceff
Slide 12
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Mass Balance: Reaction of acid vs. volume of rock removed f = fraction of leakoff acid to react with the fracture surfaces
before entering the formation
Acid-Etched Width with Leakoff
y
CDCfv
MW
t
tzxyeffL
acid
1
),,(w
idth
di
rect
ion
Slide 13
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Simulation Results
• Straight acid
• Gelled acid
Slide 14
(Al Jawad, 2016)
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From Conductivity to Productivity
0
20000
40000
60000
80000
100000
120000
140000
160000
180000
0 0.1 0.2 0.3 0.4 0.5 0.6
Cu
mu
lati
ve
Pro
du
ctio
n (
ST
B)
permeability (md)
Straight Acid
Gelled Acid
Emulsified Acid
Slide 15
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Experimental Conductivity Evaluation
• Valuable tool for individual field treatment design
• Evaluate fluid/rock system• Identify etching pattern• Resultant conductivity
Slide 16
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Acid Fracturing ProcedureSlide 17
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Surface Characterization for
Dissolved Volume and Pattern
Slide 18
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Fracture Conductivity Apparatus
Side Piston
Load Frame
Side Piston
Force
N2
Load Frame
Core Sample
Mass Flow Controller Back Pressure Regulator
Pressure Transducers
Slide 19
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Etching Pattern: Channeling (Texas Chalk)
Les
s co
nta
ct t
ime
Les
s et
chin
g
Slide 20
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Fractured Samples for Conductivity
21
(Newmann, et al., 2012)
Slide 22
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Candidate Selection
Fact:Most wells that can be acid fractured are also candidates for propped fracture
Fiction:Hydraulic fracture with proppant is always better
Formation mechanical properties, rock mineralogy and reservoir parameters determine the appropriate stimulation method.
Slide 22
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Experimental Conditions
Acid Etching Test Acid Type 20% Gelled HCl AcidInjection rate 1 Liter /minContact Time 10 minutesTemperature 125°F, 150°F
Well Sample Proppant Type
Proppant Concentration, lb/ft2
1 A 30/50 mesh ceramic 0.12 B
C
30/50 mesh ceramic
30/50 mesh ceramic
0.1
0.13 D
E
20/40 mesh sand
20/40 mesh sand
0.2
0.2
Propped Fracture Conductivity Test
Slide 25
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Acid Etching Results
0.145 in3 0.241 in3
0.224 in3 0.414 in3
Sample A (Well 1) Sample B (Well 2)
Sample C (Well 2)Sample D (Well 3)
Slide 26
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Conductivity ComparisonSlide 28
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Observations
• Low unpropped fracture conductivity indicates that a stimulation treatment is required to improve well performance in the studied reservoir.
• Conductivity of propped fractures was higher than acid fracture conductivity under the closure stress of 7000 psi.
• For lower reservoir permeability, acid fracturing could be sufficient for well performance stimulation.
Slide 29
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Background: Eagle Ford ShaleSlide 27
Eagle Ford Outcrop with Zone Specification (Gardener et al., 2013)
• Eagle Ford shale is a potential acid fracturing candidate due to high carbonate content-Zone B averages 70 wt.%-Zone C averages 75 wt.%-Zone D average 83 wt.%
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Zone B Conductivity ResultsSlide 28
1
10
100
1000
0 1000 2000 3000 4000
Fra
ctu
re C
on
du
ctiv
ity
(md
-ft)
Closure Stress (psi)
B_1; 28 wt.% HCl 20 minB_2; 28 wt.% HCl 20 minB_3; 15 wt.% HCl 20 min
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Combined Acid and Proppant
(Thripathi and Pournik, 2015)
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Conclusions1. Better models, both empirical and numerical, have been developed
with geostatistical consideration. These models can help tounderstand the outcomes of acid fracture.
2. Identifying etching pattern and acid/rock compatibility in labexperimental investigation is recommended for each field/area.
3. The outcomes of acid fracturing depend on combination of formationrock properties, reservoir flow properties, field operation designparameters. Integrated study with production prediction helps toselect/design the simulation treatments.
4. Acid fracturing has potential in low perm, high carbonate contentreservoirs.
Slide 30
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Thank You!
Questions?
Slide 31