Acid Fracturing: An Alternative Stimulation Approach in Carbonates
Ding Zhu, Texas A&M University
Background
Propped fracturing
Slide 2
Acid fracturing Matrix acidizing
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
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
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
Acid fracture scale Experimental scale
Intermediate scale
Scaling ProblemSlide 6
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
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
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
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
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
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
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
Simulation Results
• Straight acid
• Gelled acid
Slide 14
(Al Jawad, 2016)
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
Experimental Conductivity Evaluation
• Valuable tool for individual field treatment design
• Evaluate fluid/rock system• Identify etching pattern• Resultant conductivity
Slide 16
Acid Fracturing ProcedureSlide 17
Surface Characterization for
Dissolved Volume and Pattern
Slide 18
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
Etching Pattern: Channeling (Texas Chalk)
Les
s co
nta
ct t
ime
Les
s et
chin
g
Slide 20
Fractured Samples for Conductivity
21
(Newmann, et al., 2012)
Slide 22
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
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
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
Conductivity ComparisonSlide 28
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
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.%
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
Combined Acid and Proppant
(Thripathi and Pournik, 2015)
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
Thank You!
Questions?
Slide 31