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

3

213

1 Ccf CCwk

cf CCwk ln213

1

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 31 2 , 3 4 5 , 60

1 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 'n

beff 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