Petrophysical considerations in evaluating and producing shale gas resources

1

C.H. Sondergeld1, K.E. Newsham2, J.T. Comisky2, M.C. Rice2, and C.S. Rai1

1Mewbourne School of Petroleum and Geological EngineeringUniversity of Oklahoma

2Apache Corp

Petrophysical Considerations in Evaluatingand

Producing Shale Gas Resources

SPE Unconventional Gas Conference, 23-25 Feb, Pittsburg Pennsylvania

2

Increasing Scale

ParticleMotion

Storage

Capacity

Sorption Diffusion Darcy Pipe =SlippageBrownian

Nano-Porosity Interparticle-Porosity Wellbore =Fracture Porosity

Flow

Capacity

ElectrochemicalGradients

Random vibration

Viscous

Flow

Type

Pore

Type

1 micron

Hydraulic

Fracture

10 0.1 0.001Free molecularflow

ContinuumFlow

SlippageFlow

TransitionFlow

KnudsenFlow

Regime

Exploitation of Gas Shales Involves Gas Flow at Many Scales

Ultimate supply is governed by flow at the smallest scales.

3

Pore sizes from NMR and SEM imaging

Ion-milling- SEM backscattered image

Moncrieff, 2009

2

1 3S

T V r

0.05 m ms

free

capillary

claybound

3 100r nm to nm

NMR and SEM agree!

4

Parameter Desired Result

Dehydration Effects (Sw) < 40% SwDepth Shallowest Depth in Dry gas Window

Fracture Fabric and Type Vertical vs. horizontal orientationOpen vs. Filled with silica or calcite

Gas Composition low CO2, N, and H2SGas-Filled Porosity (Bulk volume gas) > 2% Gas Filled Porosity

Gas type (biogenic, thermogenic, or mixed) Thermogenic

Internal Vertical Heterogeneity Less is betterMineralogy > 40% Quartz or Carbonates

< 30% ClaysLow expandablilty

Biogenic vs. detrital silicaOGIP (free and sorbed) > 100 BCF/section

Permeability > 100 nanoDacryPoisson's Ratio (static) < 0.25

Pressure > 0.5 psi /ftReservoir Temperature > 230 F

Seals Fracture Barriers Present Top and BaseShows High gas Readings-ProductionStress < 2000 psia Net Lateral Stress

Thermal Maturity Dry gas window > 1.4 RoThickness > 30 m

Total Organic Content (and Type) > 2%Wettability Oil prone wetting of kerogen

Young's Modulus > 3.0 MMPSIA

Desirable Gas Shale Characteristics

5

Compositional Variation in Shales (limey system)

TransmissionFTIR

16 minerals

6

Compositional Variations in Shales

FTIR predict more clay and less quartz! Better agreement with logs andpoint counting.

We

igh

t%

7

TOC from Logs: Modified Passey Method

• Passey method under predictsTOC in mature and over-maturegas shale

• Use Multiplier ‘C’

• This example …

Vro = 2.2, LOM = 14.5, max ∆logR = 1.0

Predicted TOC =

∆ logR x 10(2.297 – 0.1688 x LOM) x C

TOC∆ logRLOM

Un-Modified Passey Passey with C = 4

8

Brittleness from Composition and Velocities

Vp&Vs EMatched Mineralogy

Brittleness Index= (Qtz / (Qtz + Carb + Clay)

BRITTLENESS INDEX= (Ebrit + brit)/2

Ebrit = ((E-1)/(8-1))*100brit= ((-0.15)/(0.4-0.15))*100

(Rickman et al. 2008)

Brittleness fromcomposition similar to

that from sonic logs

9

Intrinsic properties should not dependon sample size.

1.00E-06

1.00E-05

1.00E-04

1.00E-03

1.00E-02

1.00E-01

0.1 1 10 100

K,

md

particle size, mm

sh-12 sh-05

Shale

Cui et al. 2009Luffel et al. 1993

nd

d

TGA_FTIR Data for Shales

• Equilibration time iscomposition dependent but<300 minutes

• Only water detected

100 oC27 oC

100 oC

100 oC

42 oC

72 oC

100 oC 1 hr

3 hr

6 hr

9 hr

H2O

Removes residual hydrocarbonsand water but retains TOC,matrix, and clay bound water.Useful in determining heattreatment before porosity andpermeability measurements.

11

Porosity Comparison

2435.00

2440.00

2445.00

2450.00

2455.00

2460.00

2465.00

2470.00

2475.00

2480.00

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0

De

pth

,m

As Received Core Porosity,%

Lab 1 Lab 2 Lab 3

2435.00

2440.00

2445.00

2450.00

2455.00

2460.00

2465.00

2470.00

2475.00

2480.00

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0

De

pth

,m

Dry Core Porosity,% BV

Lab 1 Lab 2

“As Received” Comparison Dry Comparison

Wide variation in simple properties among labs, greater than a factor of 2 onas received samples. Real or procedural?

12

Grain Volumes measured by two labs

2.00

2.20

2.40

2.60

2.80

3.00

2.00 2.20 2.40 2.60 2.80 3.00

La

b2

AR

Bu

lkD

en

sity,

gcc

Lab 1 AR Bulk Density, gcc

2.40

2.50

2.60

2.70

2.80

2.90

2.40 2.50 2.60 2.70 2.80 2.90

La

b2

AR

Gra

inD

en

sity,

gcc

Lab 1 AR Grain Density, gcc

“As Received” Bulk Density “As Received” Grain Density

Bulk volume measurement is consistent, whereas grainvolume measurement is different. This produces differencesin reported porosities.

13

1.00E-06

1.00E-05

1.00E-04

1.00E-03

1.00E-02

1.00E-01

0.1 1 10 100

K,m

d

particle size, mm

sh-12 sh-05

1.0E-12

1.0E-09

1.0E-06

1.0E-03

1.0E+00

0 5 10

Porosity, %

k,

md

crushed

barnett

crushed2

BC-CA

marcellus

Y_gs

syn

Gas Shale Permeability

After Cui et al. 2009

Strong particle size dependence

After Wang and Reed, 2009

Wide range in measured permsreflecting techniques and sampling ?

nd

d

pd

d

nd

14

Composite of TRA permeabilities and porositiesfor a number of gas shales.

Very limited dynamic range. Shales from Canada, Illinois, Texas and Arkansas

0.001

0.01

0.1

1

10

0 5 10 15

k,m

d

Porosity, %

TRA-GasShale

hr1

b1

b2

b3

b4

sws

swgh

swgm

ofr

rl1

d

15

Pressure dependence of shale permeability:

0.0001

0.0010

0.0100

0.1000

1.0000

10.0000

100.0000

0 10 20 30 40 50 60

k,m

d

Pconf, MPa

A B C Y1 y2 y3 y4 y5 wel pier

K

d

nd

16

Pressure dependence suggest microcrack influence

Walsh’s theory (1981) predicts linear dependence in this variable space. Singlesmooth plane in Al2O3 is the upper bounding red line. All other plugmeasurements including “whole” plugs and fractured shales fall below this.

13 2

1 lno o o

k h P

k a P

fracturedsurfaces

17

0

10

20

30

40

50

60

70

80

90

0 10 20 30 40

Yo

un

g's

Mo

du

lus

,E

,G

Pa

Pressure, MPa

v

h

45

Zun

Pathi

Enz

h1

h2

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0 10 20 30 40

n

Pressure, MPa

v

h

45

Zun

Pathi

Enz

h1

h2

Elastic Properties (Young’s modulus, Poisson’s ratio)

Wide range in mechanical properties driven by anisotropy and composition.

Horiz45o

Horiz45o

18

Anisotropy

p-wave anisotropy

Both approach 30-50% in gas shales!

2 2

22

p _h p _v

p _v

v v

v

2 2

22

s _h s _v

s _v

v v

v

Consistent with mechanical properties, Young’s modulus anisotropy!

Symmetry TI to Orthorhombic

0

0.1

0.2

0.3

0.4

0.5

0 0.1 0.2 0.3 0.4 0.5

Barnett

Floyd

1:1

s wave anisotropy

horizontal

45o

vertical

19

0

1

2

3

4

5

0.5 1 1.5 2 2.5 3

Eh/Ev

nxy

/ nzx

FloydBarnettBaxter

Effect of Anisotropy on Closure Stress

xy =0.25, zx= 0.375 and Eh = 2Ev produces a h = v

Frac containment becomes difficult!

1H zx

h v

v xy

E

E

Austin Slate

0

100

200

300

400

500

600

700

0 20 40 60 80 100

Angle

Fa

ilu

reS

tre

ng

th,M

Pa

.

40k

30k

20k

10k

5k

20

2435.0

2440.0

2445.0

2450.0

2455.0

2460.0

2465.0

2470.0

2475.0

2480.0

0.0 20.0 40.0 60.0 80.0 100.0

De

pth

,m

AR Water Saturation

Lab 1 Lab 2 Lab 1 Sw Corrected to Lab 2 Porosity

Saturation changes with heating and time

Saturation estimates areaffected by accuracy ofporosity measurements

Mavor, 2009

21

Mineralogy varies considerably in a particular shale

TOC estimation by Passey et al. (1990) method works well with a multiplier

Logs require independent mineralogy calibration , FTIR fast and sufficient

Core handling and preparation needs to be standardized

Permeability on crushed samples reflect grain size more than matrix perms

Permeability measurements on cores display a strong crack component

Variable salinities render Archie saturation calculations questionable

Anisotropy is strong (30-50%) and influences closure stress estimatesand fracture containment

Recommend a committee to create standards and protocols for shale measurementsrevisit the GRI recommendations, we can’t afford to crush ¾ lb of shale!

Conclusions and Recommendations