Pore Scale Control of Gas and Fluid Transport at Shale Matrix-Fracture Interfaces A.D. Jew 1,2 , Q. Li 1,2 , A. Allali 3 , A.L. Harrison 1,2 , K. Maher 1 , Anthony Kovscek, G.E. Brown Jr. 1,2,3 , M. Zoback 4 , and J.R. Bargar 2 1 SLAC National Accelerator Laboratory, Stanford Synchrotron Radiation Lightsource 2 Stanford University, Department of Geological Sciences 3 Stanford University, Department of Geophysics 4 Stanford University, Department of Energy Resource Engineering Acknowledgements: Funding for this work was funded by NETL to SLAC under Contract #DE-AC02-765F00515. SSRL is a national user facility supported by the DOE Office of Basic Energy Sciences. Part of this work was done at the Stanford Nano Shared Facilities. Geochemical controls over barite formation: METHODS METHODS RESULTS RESULTS CONCLUSIONS • 0.1 mM BaCl 2 /Na 2 SO 4 (I.S. = 0.6 mM) • Organics (concentration set to literature values, I.S. ~0.6 mM): Ethylene glycol, polyethylene glycol, methanol, acetate, kerosene, guar gum, citrate, glutaraldehyde, benzene, ammonium persulfate, 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, 2.6 M (adjusted with NaCl) • 80 o C incubation • Constant mixing using end-over-end tumbler • Incubation time 1 week with sampling every 24 hours • Filter size 0.02 mm • Ba concentrations measured with ICP- OES y = -1113ln(x) + 8244.2 R² = 0.9844 y = 6463.9x -0.253 R² = 0.9929 y = 3794.1x -0.375 R² = 0.9643 0 2000 4000 6000 8000 10000 12000 14000 0 50 100 150 200 Ba Concentration (mg/L) Time (hrs) pH Variation pH 2 pH 3 pH 4 pH 5 pH 5.75 pH 7 Calculated Ba concentration using thermodynamic data for pH = 2 Calculated Ba concentration using thermodynamic data for pH = 7 6.0E-06 1.6E-05 2.6E-05 3 4 5 6 7 Rate (mole/s) pH Ba precipitation rates vs pH (24 hrs) y = -638.6ln(x) + 5180.5 R² = 0.98 y = -544.3ln(x) + 8738.1 R² = 0.825 0 2000 4000 6000 8000 10000 12000 0 50 100 150 Ba Concentration (mg/L) Time (hrs) I.S. Variation 0.6 mM 0.01 M 0.11 M 0.99 M 2.26 M 4.0E-06 9.0E-06 1.4E-05 0 50 100 150 Rate (mole/s) Ionic Strength (mM) Ba precipitation rates vs I.S. (24 hrs) y = -633ln(x) + 5316.8 R² = 0.963 0 2000 4000 6000 8000 10000 12000 0 50 100 150 200 Ba Concentration (ug/L) Time (hrs) Selected Organics Control Acetate Citrate Ammonium Persulfate • pH and Ionic Strength have a strong influence on barite precipitation – ≤ pH 2 and High I.S. (≥0.99M) lower halts precipitation • Ethylene glycol (anti-scaling agent) has no effect on barite scale production • Citrate, guar gum, glutaraldehyde, and polyethylene glycol slow precipitation • Marcellus-derived bitumen, acetate, benzene, and methanol enhance precipitation • Ammonium persulfate significantly enhance precipitation with ~2/3 of total Ba precipitated in 6 minutes • Core scale experiment show that barite scale formation precipitation was most obvious in shales with high pH buffering capacity • Scale formation on the shale surface inhibits Fe leaching from shale matrixes. • Permeability measurements for shale matrixes before and after reaction are in progress Fluid-Shale Permeability Controls • Alteration in porosity, diffusivity, and permeability of shale matrix can affect the efficiency of hydrocarbon production • A few studies on chemical reactions with shale samples were conduced using fractured cores and shale sands, focusing on fracture surface alteration • We aim at examine chemical reactions in shale matrixes, and seed answers to several questions: o How deep the reactions penetrate into the matrix? Is it in mm or μm scale? o Does porosity alter in nanoscale or microscale? o What are the effects on diffusivity and permeability of the matrix? o How would mineralogy of the shale affect the results? o How barite scale formation affect alteration of the shale matrix? • Whole cores of Marcellus and Eagle Ford were reacted at 80 o C and 77 bar for three weeks at both dissolution- and precipitation-favorable conditions. Frac. Fluid Seal ends Pressure Vessel liquid/solid = ~15 80 o C, 77 bar • Cross sections of the pre- reaction and post-reaction cores were cut for analyses. 1 cm Mineral precipitation along transect θ SAXS XRM Porosity along transect Cross section • Micro-CT images will be collected for pre- and post-reaction cores. Dissolution-favorable (Frac. Fluid) Precipitation-favorable (Frac. Fluid + SI (barite) = 1.3) Marcellus (Pennsylvania): Carbonate-Poor Post-rxn fluid Fe ppt. Pyrite Post-rxn fluid No Fe ppt. Precipitation on surface and in matrix Eagle Ford: Carbonate-Rich pH 2→ 5.5 pH 2 → 5.5 • Barium is ubiquitous in hydraulic fracturing systems o > 1 g/kg oil/gas shales o > 10 g/kg drilling mud o > 5 g/L produced water • Depending on the shale play, barite precipitation is highly problematic • Barite has low solubility for sulfates (Ksp = 10-9.34) • Numerous sources of Ba: o Barite o BaCO3 o Ba sorbed to clays o Ba-infused drilling mud • Unknown if organic additives in fracture fluid 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