Pore Scale Control of Gas and Fluid Transport at Shale Matrix-Fracture InterfacesA.D. Jew1,2, Q. Li1,2, A. Allali3, A.L. Harrison1,2, K. Maher1, Anthony Kovscek, G.E. Brown Jr.1,2,3, M. Zoback4, and J.R. Bargar2

1SLAC National Accelerator Laboratory, Stanford Synchrotron Radiation Lightsource2Stanford University, Department of Geological Sciences

3Stanford University, Department of Geophysics4Stanford University, Department of Energy Resource Engineering

Acknowledgements: Funding for this work was funded by NETL toSLAC under Contract #DE-AC02-765F00515. SSRL is a national userfacility supported by the DOE Office of Basic Energy Sciences. Part ofthis work was done at the Stanford Nano Shared Facilities.

Geochemical controls overbarite formation:

METHODS

METHODS RESULTS

RESULTS

CONCLUSIONS

• 0.1 mM BaCl2/Na2SO4 (I.S. = 0.6 mM)

• Organics (concentration set to literaturevalues, I.S. ~0.6 mM): Ethylene glycol,polyethylene glycol, methanol, acetate,kerosene, guar gum, citrate,glutaraldehyde, benzene, ammoniumpersulfate, Marcellus-derived bitumen

• pH: 2, 3, 4, 5, 6, 7 (adjusted with HCl)

• I.S.: 0.6 mM, 0.01 M, 0.1 M, 1 M, 2.6M (adjusted with NaCl)

• 80 oC incubation

• Constant mixing using end-over-endtumbler

• Incubation time 1 week with samplingevery 24 hours

• Filter size 0.02 mm

• Ba concentrations measured with ICP-OES

y = -1113ln(x) + 8244.2R² = 0.9844 y = 6463.9x-0.253

R² = 0.9929

y = 3794.1x-0.375

R² = 0.96430

2000

4000

6000

8000

10000

12000

14000

0 50 100 150 200

BaCo

ncen

trat

ion

(mg/

L)

Time (hrs)

pH Variation pH 2

pH 3

pH 4

pH 5

pH 5.75

pH 7

Calculated Ba concentration usingthermodynamic data for pH = 2

Calculated Ba concentration usingthermodynamic data for pH = 7 6.0E-06

1.6E-05

2.6E-05

3 4 5 6 7

Rate

(mol

e/s)

pH

Ba precipitation rates vs pH (24 hrs)

y = -638.6ln(x) + 5180.5R² = 0.98

y = -544.3ln(x) + 8738.1R² = 0.825

0

2000

4000

6000

8000

10000

12000

0 50 100 150Ba

Conc

entr

atio

n(m

g/L)

Time (hrs)

I.S. Variation

0.6 mM0.01 M0.11 M0.99 M2.26 M

4.0E-06

9.0E-06

1.4E-05

0 50 100 150

Rate

(mol

e/s)

Ionic Strength (mM)

Ba precipitation rates vs I.S. (24 hrs)

y = -633ln(x) + 5316.8R² = 0.963

0

2000

4000

6000

8000

10000

12000

0 50 100 150 200

BaCo

ncen

trat

ion

(ug/

L)

Time (hrs)

Selected Organics Control

Acetate

Citrate

Ammonium Persulfate

• pH and Ionic Strength have a strong influenceon barite precipitation

– ≤ pH 2 and High I.S. (≥0.99M) lowerhalts precipitation

• Ethylene glycol (anti-scaling agent) has noeffect on barite scale production

• Citrate, guar gum, glutaraldehyde, andpolyethylene glycol slow precipitation

• Marcellus-derived bitumen, acetate, benzene,and methanol enhance precipitation

• Ammonium persulfate significantly enhanceprecipitation with ~2/3 of total Ba precipitatedin 6 minutes

• Core scale experiment show that barite scaleformation precipitation was most obvious inshales with high pH buffering capacity

• Scale formation on the shale surface inhibits Feleaching from shale matrixes.

• Permeability measurements for shale matrixesbefore and after reaction are in progress

Fluid-Shale Permeability Controls

• Alteration in porosity, diffusivity, andpermeability of shale matrix can affect theefficiency of hydrocarbon production

• A few studies on chemical reactions withshale samples were conduced usingfractured cores and shale sands, focusingon fracture surface alteration

• We aim at examine chemical reactions inshale matrixes, and seed answers to severalquestions:o How deep the reactions penetrate

into the matrix? Is it in mm or μmscale?

o Does porosity alter in nanoscale ormicroscale?

o What are the effects on diffusivityand permeability of the matrix?

o How would mineralogy of the shaleaffect the results?

o How barite scale formation affectalteration of the shale matrix?

• Whole cores of Marcellus and EagleFord were reacted at 80 oC and 77 barfor three weeks at both dissolution- andprecipitation-favorable conditions.

Frac

.Flu

id

Seal ends

Pressure Vesselliquid/solid = ~1580 oC, 77 bar

• Cross sectionsof the pre-reaction andpost-reactioncores were cutfor analyses.

1 cm

Mineralprecipitationalong transect

θ

SAXS

XRM

Porosityalongtransect

Crosssection

• Micro-CT images willbe collected for pre-and post-reactioncores.

Dissolution-favorable(Frac. Fluid)

Precipitation-favorable(Frac. Fluid + SI (barite) = 1.3)

Marcellus(Pennsylvania):Carbonate-Poor

Post-rxn fluid

Fe ppt.

Pyrite Post-rxn fluidNo Fe ppt.

Precipitation onsurface and in matrix

Eagle Ford:Carbonate-Rich

pH 2→ 5.5 pH 2 → 5.5

• Barium is ubiquitous in hydraulicfracturing systems

o > 1 g/kg oil/gas shaleso > 10 g/kg drilling mudo > 5 g/L produced water

• Depending on the shale play, bariteprecipitation is highly problematic

• Barite has low solubility for sulfates(Ksp = 10-9.34)

• Numerous sources of Ba:o Bariteo BaCO3o Ba sorbed to clayso Ba-infused drilling mud

• Unknown if organic additives in fracturefluid inhibit or enhance barite precipitation

pH 2→ 4 pH 2 → 4

Dissolution-favorable(Frac. Fluid)

Precipitation-favorable(Frac. Fluid + SI (barite) = 1.3)

Helium Pulse-Decay Permeability Measurement Set-up