Acid Fracturing: An Alternative Stimulation Approach in Carbonates Ding Zhu, Texas A&M University
Propped fracturing Background Acid fracturing Slide 2 Matrix acidizing
Acid Fracturing Slide 3 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
Main Issues in Acid Fracturing Slide 4 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
Modeling Slide 5 Empirical 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)
Scaling Problem Slide 6 Intermediate scale Acid fracture scale Experimental scale
Conductivity Correlations Slide 7 Nierode and Kruk (1973) Exponential function k f Gangi (1978) Power function k w 1 3 C C C3 f Walsh (1981) Logarithmic function k w 1 3 C C ln f w 1 1 C 1 exp C 2 2 2 c c c
Empirical Correlations by Mou-Deng wk exp f 2.8 0.22, 0.011, wk f 0 D x D D z D 4 14.9 3.78ln D 6.81ln E 10 9 3 f 1 2 D, x 3 4 5 D, z 6 D wk 4.4810 w 1 a erf ( a ( a )) a erf ( a ( a )) ( e 1) 0 a 1.82, a 3.25, a 0.12, a 1.31, a 6.71, a 0.03 1 2 3 4 5 6 c 0.4 0.52 Slide 8
c (psi) Empirical Correlations for Conductivity Slide 9 100000 10000 1000 Nierode-Kruk model Mou-Deng model 100 10 1 0.1 0 1000 2000 3000 4000 5000 6000 7000 wkf (md-ft)
Numerical Model: Etching Width Prediction 2D Solutions Type curves to predict penetration (Roberts and Guin, 1974) Early simulators based on finite difference D C n' eff kc b y C C u v D x y eff 1 2 y C 2 Slide 10 Typically some average integrated across channel (Settari, 1993) Settari (1993)
Slide 11 Numerical Model: Etching Width Prediction Settari (2001) modified 2D approach No height dependence Analytical velocity solution applied Romero (1998) 3D approach Analytical velocity solution applied C t u C x C v y C w z D eff 2 y C 2 Settari et al. (2001)
width direction 3D Acid Transport Model (Oeth, 2013) Slide 12 Velocity profile for non-newtonian fluid Acid concentration in y-direction Leakoff from fracture to formation C t u C x v C y C w z y D eff C y Q inj v Leakoff
width direction Acid-Etched Width with Leakoff Slide 13 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 y( x, z, t) t MW C acid fvlc Deff 1 y
Simulation Results Slide 14 Gelled acid Straight acid (Al Jawad, 2016)
Cumulative Production (STB) From Conductivity to Productivity Slide 15 180000 160000 140000 120000 100000 Straight Acid Gelled Acid Emulsified Acid 80000 60000 40000 20000 0 0 0.1 0.2 0.3 0.4 0.5 0.6 permeability (md)
Experimental Conductivity Evaluation Slide 16 Valuable tool for individual field treatment design Evaluate fluid/rock system Identify etching pattern Resultant conductivity
Acid Fracturing Procedure Slide 17
Surface Characterization for Dissolved Volume and Pattern Slide 18
Fracture Conductivity Apparatus Slide 19 Force Mass Flow Controller Load Frame Side Piston Core Sample Back Pressure Regulator N 2 Side Piston Load Frame Pressure Transducers
Less contact time Less etching Etching Pattern: Channeling (Texas Chalk) Slide 20
Fractured Samples for Conductivity Slide 22 (Newmann, et al., 2012) 21
Candidate Selection Slide 22 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.
Experimental Conditions Slide 25 Acid Etching Test Acid Type 20% Gelled HCl Acid Injection rate 1 Liter /min Contact Time 10 minutes Temperature 125 F, 150 F Propped Fracture Conductivity Test Well Sample Proppant Proppant Type Concentration, lb/ft 2 1 A 30/50 mesh ceramic 0.1 2 B 30/50 mesh ceramic 0.1 C 3 D E 30/50 mesh ceramic 20/40 mesh sand 20/40 mesh sand 0.1 0.2 0.2
Acid Etching Results Sample A (Well 1) Sample B (Well 2) Slide 26 0.145 in 3 0.241 in 3 Sample C (Well 2) Sample D (Well 3) 0.224 in 3 0.414 in 3
Conductivity Comparison Slide 28
Observations Slide 29 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.
Background: Eagle Ford Shale Slide 27 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.% Eagle Ford Outcrop with Zone Specification (Gardener et al., 2013)
Fracture Conductivity (md-ft) Zone B Conductivity Results Slide 28 1000 100 B_1; 28 wt.% HCl 20 min B_2; 28 wt.% HCl 20 min B_3; 15 wt.% HCl 20 min 10 1 0 1000 2000 3000 4000 Closure Stress (psi)
Combined Acid and Proppant (Thripathi and Pournik, 2015)
Conclusions Slide 30 1. Better models, both empirical and numerical, have been developed with geostatistical consideration. These models can help to understand the outcomes of acid fracture. 2. Identifying etching pattern and acid/rock compatibility in lab experimental investigation is recommended for each field/area. 3. The outcomes of acid fracturing depend on combination of formation rock properties, reservoir flow properties, field operation design parameters. Integrated study with production prediction helps to select/design the simulation treatments. 4. Acid fracturing has potential in low perm, high carbonate content reservoirs.
Slide 31 Thank You! Questions?