Fuel 182 (2016) 227–235

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Fuel

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Full Length Article

Characterization of Marcellus Shale and Huntersville Chert before andafter exposure to hydraulic fracturing fluid via feature relocation usingfield-emission scanning electron microscopy

http://dx.doi.org/10.1016/j.fuel.2016.05.0610016-2361/Published by Elsevier Ltd.

⇑ Corresponding author.E-mail address: Barbara.Kutchko@netl.doe.gov (B. Kutchko).

Matthew Dieterich, Barbara Kutchko ⇑, Angela GoodmanU.S. Department of Energy, Office of Research and Development, National Energy Technology Laboratory, 626 Cochrans Mill Road, Pittsburgh, PA 15236, United States

a r t i c l e i n f o

Article history:Received 8 March 2016Received in revised form 11 May 2016Accepted 12 May 2016

Keywords:GeochemistryAppalachian BasinMarcellus ShaleScanning electron microscopyShaleFluid–rock interaction

a b s t r a c t

Two sets of experimental in situ fluid–rock interaction studies were implemented to understand theinteractions between hydraulic fracturing fluid and rocks of the Marcellus Shale gas play. MarcellusShale and Huntersville Chert core samples were exposed to synthetically prepared fracturing fluid andrecycled fracturing fluid from the field, respectively, and examined before and after in situ exposure usingsurface relocation techniques via high-resolution field-emission scanning electron microscopy (FE-SEM)to investigate chemical or physical alterations.Results indicate that in situ pressure promoted fracture growth along the sedimentological (horizontal)

bedding plane of the Marcellus Shale samples. Moreover, calcium carbonate (CaCO3) dissolution wasobserved and gypsum (CaSO4 ⁄ 2H2O) appeared to precipitate both on the surface and in the numerousfractures. Barite (BaSO4), strontianite (SrCO3), celestine (SrSO4), and apatite (CaPO4) formed a unique pat-tern of precipitates on the surface of the Huntersville Chert samples. Additionally, Rhenium and rareearth element (REE) Europium were identified in minerals which precipitated on the HuntersvilleChert surface identified by FE-SEM spectral analysis.

Published by Elsevier Ltd.

1. Introduction

The Appalachian Basin covers numerous states in eastern NorthAmerica including New York, Pennsylvania, Ohio, Maryland, WestVirginia, Kentucky, Tennessee, and Alabama. Overall, the Appala-chian Basin as a whole covers an aerial extent of 185,5002 miles,is 1075 miles long, and ranges from 20 to 310 miles wide [22].The hydrocarbon bearing Marcellus Shale Formation locatedwithin the Appalachian Basin spans 600 linear miles [6]. The lesslaterally extensive Huntersville Chert formation is located inwest-central Pennsylvania within the Appalachian Basin andunderlies the Marcellus Shale Formation [11]. In order to accessthe hydrocarbons stored in the Marcellus Shale directional drillingand hydraulic fracturing is implemented.

Hydrocarbon exploration of the Marcellus Shale has resulted inover 12,000 permitted wells in Pennsylvania between 2005 and2012 [27]. According to Vidic et al. [27] these 12,000 wells pro-duced between <0.1 and >20 million cubic ft/day of natural gas.Importantly, the Marcellus Shale can sustain the United States nat-

ural gas demand for approximately 15 years if usage remains thesame at 23 trillion cubic ft/year [23]. Due to the increase in drilledwells and high volumes of fluid utilized during hydraulic fractur-ing, experimental studies are required to determine whetherchemical and physical alteration of Marcellus Shale and confininggeologic formations occurs as residual fracturing fluid remains inthe subsurface. Marcellus Shale well stimulation which utilizes acomponent of recycled flowback water can benefit from under-standing the chemical and physical effects of fluid–rock interac-tions. For instance, determining alterations caused by thestimulation process with a recycled fluid component in MarcellusShale production may improve fracturing fluid recipes based uponwell-specific geochemistry to maximize hydrocarbon production.

Hydraulic fracturing of geologic formations has been utilized forthe production of hydrocarbons across the United States since the1940s [15]. Modern hydraulic fracturing techniques are applied toboth vertical and horizontal wells, with the majority being uncon-ventional horizontal wells in tight organic-rich shale formationssuch as the Marcellus and Utica Shales of the Appalachian Basinin the northeastern United States. In order to successfully hydrauli-cally fracture one horizontal Marcellus Shale well for hydrocarbonproduction, between 2 and 7 million gallons of water is required[12].

228 M. Dieterich et al. / Fuel 182 (2016) 227–235

Horizontal well stimulation of the hydrocarbon bearing Appala-chian Basin Middle Devonian Marcellus Shale involves large vol-umes of hydraulic fracturing fluid that interacts with formationwater and mineral surfaces within the subsurface [19,9]. Duringstimulation, between 47% and 91% of the fracturing fluid remainsin the subsurface while 9–53% returns out of the wellbore as flow-back water [8,27]. Fracturing fluid remaining in the subsurface canpossibly alter petrophysical characteristics including surface area,porosity, and mineralogy of the host formation that may inhibithydrocarbon permeability [19]. Captured flowback water is typi-cally recycled into additional fracturing fluid for well stimulation[21,2]. Fracturing fluid is created by mixing high volumes of waterwith additives such as biocides, friction reducers, and proppants.The fluid is subsequently forced downhole under high pressureand into the target geologic formation. Once the target formationhas been hydraulically stimulated, the fracturing fluid can returnto the Earth’s surface as flowback water for containment, but a por-tion of the fracturing fluid remains in the subsurface. On average10% of the fracturing fluid used to stimulate a horizontal MarcellusShale well returns as flowback [27], which means millions of gal-lons of fluid remain in the subsurface. The chemical and physicaleffects of the fracturing fluid remaining in the subsurface is uncer-tain, although shale absorption of the fluid through imbibition isone proposed hypothesis [19,9].

Flowback water from Marcellus Shale wells represents an envi-ronmental concern due to high salinity, total dissolved solids(TDS), and leachates from naturally occurring radioactive material(NORM) [12,7,1,5,10,3,24]. As a result of the complex chemistryand environmentally hazardous nature of flowback water, costlytreatment was previously conducted via transportation to wastewater facilities capable of handling high TDS fluids. Currently, ser-vice companies are recycling the flowback water for subsequentwell stimulation [21,2]. This process of utilizing recycled fluid forhydraulic fracturing requires additional research to understandthe effect of elevated TDS fluids on the target formation.

The objective of this study was to identify chemical and physi-cal alterations of solid core from the Marcellus Shale and Hun-tersville Chert (Onondaga Limestone) after interaction withfracturing fluid, as determined via feature relocation using fieldemission-scanning electron microscopy (FE-SEM). Both short-term (effects of fracturing fluid initially entering the subsurface)and long-term (effects of fracturing fluid remaining in the subsur-face) scenarios were examined. Investigating fluid impact on Hun-tersville Chert in addition to the Marcellus was performed becauseMarcellus Shale wells in Pennsylvania are commonly drilledthrough a segment of the underlying chert/limestone in order toacquire ‘‘conservation well” status. Therefore, Huntersville Chertunderlying the Marcellus Shale can be exposed to hydraulic frac-turing fluid during well stimulation. Experiments were conductedto determine whether the rock structure changes upon interactionwith fracturing fluid via mineral dissolution, precipitation, orchemical etching. This study also aimed to characterize chemicalprecipitates which formed during fluid contact.

2. Materials and methods

2.1. Rock

Rock core utilized during this experiment was retrieved inSeptember 2008 from a well site located in Greene County, Penn-sylvania [4]. The main core sample was stored under atmosphericconditions until 2014. Subset samples of Marcellus Shale and Hun-tersville Chert were collected from the middle of the core in orderto remove material directly in contact with the atmosphere. Thesamples were then immediately placed inside a nitrogen desiccator

to prevent atmospheric alteration. Throughout the experimentsutmost care was taken to limit atmospheric exposure of the rocksamples.

2.1.1. Marcellus ShaleThe Marcellus Shale sample was obtained from a depth of

7801 ft (2378 m) and is classified as a grayish black shale. Theshale was split into two pieces with a Buehler IsoMet low speedrock saw with a diamond blade. The samples were only exposedto the atmosphere long enough for cutting and analysis to takeplace. For storage between analyses the samples were placed in anitrogen desiccator. One piece of the core was cut parallel to thesedimentary bedding plane denoted as ‘‘Marcellus Shale parallelcut” (Fig. 1), and the second piece was cut transverse to the sedi-mentary bedding plane to expose an edge-on facies denoted as‘‘Marcellus Shale transverse cut” (Fig. 2). Cutting the core sampleinto two bedding planes allowed detailed electron microscopystudies to be conducted for both mineralogical surface orientationswhich encounter fracturing fluid during stimulation. Both Marcel-lus Shale samples had the internal rock face polished to allow forhigher resolution FE-SEM images. For the parallel and transversesamples, 5 locations were captured per sample via FE-SEM (10 sitesin total).

2.1.2. Huntersville Chert (Onondaga Limestone)The Huntersville Chert sample (Fig. 3) was obtained from a

depth of 7909.7 ft (2410 m) and is classified as a dark gray calcare-ous/argillaceous chert. The chert was split into two pieces with aBuehler IsoMet low speed rock saw with a diamond blade. Thesamples were only exposed to the atmosphere long enough for cut-ting and analysis to take place. For storage between analyses thesamples were placed in a nitrogen desiccator. One piece of the corewas cut parallel to the sedimentary bedding plane and the secondpiece was cut transverse to the sedimentary bedding plane toexpose an edge-on facies. Cutting the core sample into two bed-ding planes allowed detailed electron microscopy studies to beconducted for both mineralogical surface orientations. Both Hun-tersville Chert samples had the internal rock face polished to allowfor higher resolution FE-SEM images and 5 locations per samplewere captured via FE-SEM (10 sites in total).

2.2. Fracturing fluids

Two fracturing fluids were used during this study (1) a syn-thetic fracturing fluid for the Marcellus Shale experiments and(2) a field collected recycled fracturing fluid for the HuntersvilleChert experiments. The Huntersville Chert experiments were con-ducted first and resulted in a layer of sulfate precipitate the rocksurface, which obscured FE-SEM re-analysis. As our study aimedto investigate physical changes to the rock surface before and afterfluid contact, we created a synthetic fracturing fluid leaving outsulfate and barium in order to leave the Marcellus Shale’s rock sur-face unobscured for FE-SEM analysis.

2.2.1. Synthetic fracturing fluidSynthetic fracturing fluid was used for Marcellus Shale fluid–

rock interaction. The recipe is detailed in Table 1. The chemicalcomposition of the synthetic fracturing fluid was modeled by Liu[18] after analyzing samples of fracturing fluids collected from awell site in Greene County, Pennsylvania (samples were collectedby the U.S. Department of Energy) seen in Table 2. Three separatefracturing sites were sampled from the well-pad to provide aver-age elemental concentrations for the synthetic fluid after induc-tively coupled plasma-optical emission spectroscopic (ICP-OES)and ion chromatographic (IC) analysis by Liu [18].

Fig. 1. Marcellus Shale parallel cut core sample prior to FE-SEM initial character-ization (3.40 g).

Fig. 2. Marcellus Shale transverse cut core sample prior to FE-SEM initialcharacterization (6.97 g).

Fig. 3. Huntersville Chert parallel cut core sample (left) and transverse cut coresample (right). These images were taken after fluid–rock interaction with recycledfracturing fluid.

1 Recycled fracturing fluid was obtained by Rick Hammack of NETL from theblender on a well pad in Greene Co. Pennsylvania.

M. Dieterich et al. / Fuel 182 (2016) 227–235 229

Reagent grade salts and deionized water (Thermo Barnstead E-Pure system) were combined to generate 2 liters (L) of the syn-thetic fracturing fluid with a pH of 5.7. Salts used were calciumchloride dehydrate (CaCl2 ⁄ 2H2O), magnesium chloride hexahy-drate (MgCl2 ⁄ 6H2O), sodium chloride (NaCl), potassium chloride

(KCl), strontium chloride hexahydrate (SrCl2 ⁄ 6H2O), sodium bro-mide (NaBr), sodium bicarbonate (NaHCO3), sodium sulfate (Na2-SO4). Br- is not an additive in hydraulic fracturing fluid, but wasadded in the form of NaBr- as microgram quantities of Br- havebeen observed in Marcellus Shale leaching experiments and canpotentially be removed from the shale during fluid–rock interac-tion [26]. The synthetic fracturing fluid was stored in a Nalgenebottle and placed inside a nitrogen desiccator prior to use in orderto prevent atmospheric alteration. Comparison of the Liu [18] fieldcollected fracturing fluid to this study’s lab synthesized fluid can beseen in Table 2.

2.2.2. Recycled fracturing fluidRecycled fracturing fluid,1 which contains formation water from

previously stimulated Marcellus Shale wells was used directly forHuntersville Chert fluid–rock interaction. The fracturing fluid wasstored in a Nalgene bottle inside a nitrogen desiccator to preventatmospheric degradation. Elemental concentrations for recycledfracturing fluid collected from the same well as our fracturing fluidin Greene County, PA is seen in the left column of Table 2.

2.3. Rock characterization

2.3.1. Field emission-scanning electron microscopy (FE-SEM)Characterization of the polished Marcellus Shale and Hun-

tersville Chert samples was conducted on a FEI Quanta 600 FEGenvironmental-scanning electron microscope. The FE-SEM wasequipped with energy dispersive X-ray spectroscopy (EDX).Although EDX does not identify mineral phases, relative abundanceof elemental data was used to infer minerals present in shale andchert samples before and after fluid exposure. For each cut coreof the Marcellus Shale and Huntersville Chert samples, 5 miner-alogic sites were characterized (20 locations in total) via FE-SEMprior to fluid exposure. The 20 sites were then relocated afterfluid–rock interaction and recharacterized. In order to relocatethe initial image locations, detailed FE-SEM stage coordinates (Xand Y positions) were noted in addition to capturing sequentialwide field to high magnification images (e.g. magnification of500�, 5000�, and 10,000�) that aid in the relocation process[16]. Both secondary electron and backscatter electron imageswere collected at a working distance of 10 mm and beam spot sizeof 4 with voltages of 10 kV and 20 kV to accommodate samplecharging.

2.3.2. X-ray diffraction mineralogyX-ray diffraction (XRD) characterization (Table 3) of the Marcel-

lus Shale and Huntersville Chert core used in this study was per-formed to identify bulk rock mineralogy prior to fluid–rockinteraction. XRD mineralogy of the Marcellus Shale identifiedquartz and illite as major minerals (>25%); minor minerals werepyrite and chlorite (10–25%); and gypsum, calcite, and dolomitewere trace minerals (<10%). Huntersville Chert XRD containedquartz, calcite, and dolomite as major minerals (>25%); minor min-eral was illite (10–25%); and pyrite, microcline, and albite weretrace minerals (<10%).

2.4. Experimental procedure

The two Marcellus Shale core samples were set inside separate500 mL Teflon lined containers, submerged in 100 mL of syntheticfracturing fluid, and subjected to 4000 psi (27.6 MPa) and 77 �C(based upon well logs) under an atmosphere of argon gas for 6 days

Table 1The synthetic fracturing fluid (pH of 5.7) was created with the above salts mixed in 2 L of ultrapure water to replicate the concentrations from recycled fracturing fluid collected inGreene County, PA. The final concentrations based on stoichiometric calculations are reported in the far right column of this table.

Component Target conc. ofcomp. (mg/L)

Salt Mass of saltadded (g)

Atomic Mass ofcomp. (g/mol)

Molecular wt.of salt (g/mol)

Molesof salt

Moles ofcomp.added

Mass ofcomp. added(g)

Volume ofsolution (L)

Comp.conc.(mg/L)

Ca2+ 2100 CaCl2 ⁄ 2H2O 15.4028 40.08 147.02 0.1048 0.105 4.199 2 2100Mg2+ 200 MgCl2 ⁄ 6H2O 3.3468 24.31 203.27 0.0165 0.016 0.400 2 200K+ 160 KCl 0.6114 39.10 74.55 0.0082 0.008 0.321 2 160Na+ 7260 NaCl 36.1755 22.90 58.44 0.6190 0.635 14.547 2 7273Sr2+ 200 SrCl2 ⁄ 6H2O 1.2184 87.62 266.65 0.0046 0.005 0.400 2 200Br� 140 NaBr 0.3613 79.90 102.89 0.0035 0.004 0.281 2 140HCO3

2� 260 NaHCO3 0.7166 61.02 84.01 0.0085 0.009 0.520 2 260Cl� – – – 35.45 – – 0.879 31.154 2 15,577SO4

2� 100 Na2SO4 0.2965 96.06 142.04 0.0021 0.002 0.201 2 100

Chargebalance

TDS

0.000 26,012pH 5.7

Table 2Chemical species comparison between recycled fracturing fluid from Greene County,Pennsylvania determined by Liu [18] via ICP-OES (left column) to synthetic fracturingfluid (right column, stoichiometric calculation) utilized during the Marcellus Shaleexperiment.

Chemical species Fracturing fluid fromGreene, Co., PA (mg/L)(used by Liu [18])

Synthetic fracturing fluid(mg/L) (this study)

Chloride (Cl�) 15,901 15,577Sodium (Na+) 6607 7273Sulfate (SO4

2�) 811 100Magnesium (Mg2+) 219 200Calcium (Ca2+) 2207 2100Potassium (K+) 141 160Bicarbonate(HCO3

�) – 260Bromide (Br�) – 140Strontium (Sr2+) 441 200pH 6.2 5.7

230 M. Dieterich et al. / Fuel 182 (2016) 227–235

(144 h). Argon was selected as the autoclave atmosphere to reduceseal leaks and to serve as an inert environment. The experimentaltime of 6 days was chosen for the Marcellus Shale core samples tomimic short-term effects of fracturing fluid initially entering thesubsurface. Temperature and pressure values were selected basedupon in situ lithostatic pressure and temperature gradients fromwell completion reports for a Marcellus Shale well in GreeneCounty, PA. After 6 days, the pressure in the vessels was slowlyreleased over 2 days to prevent rapid gas release from damagingthe structural integrity of the Marcellus Shale samples and wereallowed to air dry for 15 min. Following the 15 min of drying thesamples were placed inside a nitrogen dessicator. Both sampleswere characterized via FE-SEM after fluid contact to determinethe extent of chemo-physical alteration.

The Huntersville Chert core samples were placed inside sepa-rate 500 mL Teflon lined containers, submerged with 85 mL ofrecycled (containing formation water) fracturing fluid, and sub-jected to 50 �C at 1500 psi (10.3 MPa) under an atmosphereof nitrogen for 89 days (2139 h). Nitrogen was selected as the

Table 3XRD mineralogy results of the Marcellus Shale and Huntersville Chert core samples utilirepresents >25%, minor represents 10–25%, and trace represents <10%.

Sample Quartz Muscovite Chlorite Pyrite

Marcellus Shale Major Major Minor MinorHuntersville Chert Major Minor – Trace

Major represents >25%.Minor represents 10–25%.Trace represents <10%.

autoclave atmosphere to serve as an inert environment since argonwas not available at the time of experiments. The experimentaltime of 89 days was selected for the Huntersville Chert to maxi-mize potential reactions and mimic long-term effects of fracturingfluid remaining in the subsurface. Temperature and pressure val-ues were selected based upon in situ lithostatic pressure and tem-perature gradients from well completion reports, althoughautoclave seal issues prevented full pressure of 4000 psi(27.6 MPa) from being reached to operate safely. After the 89 daysof fluid exposure, the pressure in the vessel was slowly releasedover 2 days to prevent rapid gas release from damaging the struc-tural integrity of the Huntersville Chert samples and were allowedto dry in a nitrogen desiccator for 1 day. Both samples were char-acterized via FE-SEM after fluid contact to determine the extent ofchemo-physical alteration.

3. Results

3.1. Marcellus Shale

Mineral dissolution and etching was observed after syntheticfracturing fluid interaction for 6 days primarily on carbonate loca-tions (light gray regions inside the black oval of Fig. 4) compared toclay (dark gray outside the black oval of Fig. 4) as determined byFE-SEM characterization. For example, Fig. 4 from the MarcellusShale transverse cut shows the morphological alteration of carbon-ate which was induced by the fracturing fluid. Foliated clay min-eral grains dominate the background matrix and appear to beunaltered by fluid contact. The red outlined regions in both thebefore and after images shows the carbonate region underwentdissolution which altered carbonate surface morphology. Addition-ally, gypsum precipitation occurred due to fluid contact on the car-bonate region circled in yellow, which might have precipitatedafter calcium was dissolved into solution. Pyrite seen as brightwhite circular framboids in Fig. 4 appear to be unaltered by fluidcontact. Comparing FE-SEM results of the transverse and parallel

zed during this experiment represented as semi-quantitative weight percent. Major

Calcite Dolomite Gypsum Microcline Albite

Trace Trace Trace – –Major Major – Trace Trace

Fig. 4. Marcellus Shale transverse cut before (left) and after (right) synthetic fracturing fluid exposure FE-SEM backscatter images at 1000� magnification. Gypsumprecipitation occurred (outlined in yellow) on the surface of a carbonate region. Chemical dissolution of the carbonate region also occurred as the morphology changedbetween the before and after images (outlined in red). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of thisarticle.)

Fig. 5. Marcellus Shale transverse cut before (left) and after (right) Marcellus Shale synthetic fracturing fluid exposure FE-SEM backscatter images at 500� magnification.Magnified view of the fracture growth in the after image is outlined in red. (For interpretation of the references to colour in this figure legend, the reader is referred to the webversion of this article.)

M. Dieterich et al. / Fuel 182 (2016) 227–235 231

cut Marcellus Shale samples does not provide conclusive evidencethat either grain orientation underwent higher or lower amountsof chemical and physical alteration.

The main fracture seen in the Marcellus Shale transverse cutoutlined in red of Fig. 5 prior to fluid–rock interaction increasedin size after interaction, which was likely caused by experimentalpressures of 4000 psi (27.6 MPa). Pyrite is seen as bright white cir-cular framboids in Fig. 5 while foliated dark gray in color clay min-erals dominate the background. There was no apparent chemical orphysical alteration detected via FE-SEM to the clay matrix or pyriteafter fluid contact. Fractures in the Marcellus Shale parallel cuttrapped synthetic fracturing fluid and promoted gypsum mineralprecipitation seen in Fig. 6. Clay minerals dominate the back-ground and appear unaltered after fluid contact. Pyrite seen asbright framboids in Fig. 6 appear to be unaltered after fluid contact.Although not all fractures in the experimental Marcellus Shalesamples had gypsum precipitates, there were multiple surfaceregions where gypsum formed.

3.2. Huntersville Chert

The FE-SEM EDX results of the Huntersville Chert transverseand parallel cut core samples results indicated mineral precipitatesformed after the samples were exposed to recycled fracturing fluidfor 89 days. EDX elemental abundances showed the likely minerals

to precipitate included barite (BaSO4), and a fine frained matrixprecipitate including strontianite (SrCO3), celestine (SrSO4), andapatite (CaPO4). Mineral precipitation occurred in large quantitiesand obscured the Huntersville Chert’s surface (Fig. 7). The mineralprecipitation was also visible in the handsample core pieces(Fig. 3), appearing as white coating covering the originally darkgray in color parallel cut and transverse cut core samples. Fig. 7shows pyrite framboids before and after fluid contact. The imageon the right shows the strontianite, celestine, and apatite precipi-tation covering pyrite framboids. Mineral precipitation completelycovered four out of ten initial characterization sites and allowed foronly six to be re-characterized. Based on both morphology and ele-mental data, FE-SEM revealed that strontianite, celestine, and apa-tite formed as the crunchy and rice grained pattern precipitateacross all samples such as that seen in Fig. 8. Fig. 8 shows a mag-nified view of a pyrite grain before and after fracturing fluid expo-sure. The strontianite, celestine, and apatite precipitation coveredthe pyrite grain after fluid interaction. Comparing FE-SEM resultsof the transverse and parallel cut Huntersville Chert samples didnot provide conclusive evidence that either grain orientationunderwent higher/lower amounts of chemical and physicalalteration.

Barite was detected as large oval minerals seen in Fig. 9 acrossall Huntersville Chert transverse and parallel samples afterfluid contact. Fig. 9 shows two after fluid contact images of the

Fig. 7. Huntersville Chert transverse cut before (left) and after (right) recycled fracturing fluid exposure FE-SEM secondary electron images at 30,000� magnification. Thislocation shows pyrite framboids becoming encased in a precipitate believed to be apatite, strontiate, and celestine which was interpretted via EDX spectroscopy.

Fig. 6. Marcellus Shale parallel cut before (left) and after (right) synthetic fracturing fluid exposure FE-SEM backscatter images at 5000� magnification. Circled in yellow aregypsum crystals (CaSO4 ⁄ 2H2O). This magnified view shows gypsum precipitation occurred in the Marcellus Shale fracture after fluid exposure.

Fig. 8. Huntersville Chert parallel cut before (left) and after (right) recycled fracturing fluid exposure FE-SEM secondary electron images at 10,000�magnification. The beforefluid exposure image on the left shows a large oval pyrite grain and surrounding carbonate (light gray). Following fluid exposure this pyrite and carbonate region was coveredin a precipitate believed to be apatite, strontiate, and celestine, which was interpretted via EDX spectroscopy. Barite also precipitated and is seen as oval minerals in themiddle and top right of the after fluid exposure image.

232 M. Dieterich et al. / Fuel 182 (2016) 227–235

Huntersville Chert parallel cut showing mineral precipitation. Theimage on the left shows extensive barite precipitation as the whiteovals and apatite, strontianite, and celestine as the gray back-ground. The image on the right is a magnified view of barite crys-tals with the filamentous structure of the apatite, strontianite, andcelestine visible throughout the background. Additionally, Rhe-

nium and rare earth element (REE) Europium were detected viaFE-SEM EDX on minerals which precipitated on all of the Hun-tersville Chert transverse and parallel cut samples. Prior to contact-ing the Huntersville Chert, the pH of the fracturing fluid was 4.3with a conductivity of 60.3 lS/cm measured on a calibrated Met-tler Toledo Seven Multi 8603. Post fluid–rock interaction, the pH

Fig. 9. Huntersville Chert parallel cut after recycled fracturing fluid exposure FE-SEM backscatter electron images at 2410� (left) and 25,000� (right) magnification. Bariteprecipitated in the form of white ovals and spheres seen in both images. The filamentous background material is a precipitated layer believed to be apatite, strontiate, andcelestine, which was interpretted via EDX spectroscopy.

M. Dieterich et al. / Fuel 182 (2016) 227–235 233

of the fracturing fluid was 6.2 with a conductivity of 69.2 lS/cm.The increase in pH and conductivity may be indicative of carbonatedissolution due to the acidic fracturing fluid.

4. Discussion

4.1. Effects of fracturing fluid on the Marcellus Shale

This research builds upon the work of Pagels et al. [19] by inves-tigating the fluid–rock impact of fracturing fluid on the MarcellusShale structure from a high resolution FE-SEM perspective underin situ conditions. Our analyses provided data on the physical(e.g. fractures) and mineralogical changes (e.g. dissolution or etch-ing) that can occur as a result of synthetic fracturing fluid contactto mimic short-term effects of fracturing fluid initially entering thesubsurface. According to Pagels et al. [19], fracturing fluid remain-ing in the Marcellus Shale can degrade the performance of fractur-ing fluid proppants (i.e. sand grains), which can become imbeddedinto the shale due to the loss of rock strength from fluid imbibition.The region of shale which contacts the fracturing fluid undergoessorption altering characteristics due to imbibition that can reducefracture connections and negatively affect hydrocarbon transport[19]. Pagels et al. [19] found that imbibition reduces hydrocarbonproduction in Marcellus Shale wells. Additionally, reduction offracture connectivity might negatively affect reservoir storagepotential during carbon dioxide (CO2) sequestration in tight shaleformations [28].

Dissolution and etching of carbonate minerals occurred in theMarcellus Shale core as seen in Fig. 4, while clay minerals appearedunaltered in all FE-SEM images. Clay minerals remained visiblyunaltered to FE-SEM characterization likely because illite and chlo-rite in the Marcellus Shale are not water sensitive [13]. Carbonatehas the potential to dissolve and become etched as the mineralstructure is less stable compared to that of clay during interactionwith acidic water based fracturing fluid. These short term effectsare consistent with those reported by Kaszuba et al. [14] wherethey noted that carbonates react and undergo dissolution quickerthan silicates when in contact with acidic fluids.

FE-SEM results indicate that previous fractures in the MarcellusShale samples widened and propagated post fluid contact possiblydue to in situ pressure/temperature conditions in the autoclave(Fig. 5). The alteration to the fractures in the transverse MarcellusShale core may be due to the autoclave pressure(4000 psi/27.6 MPa) overcoming the bedding plane strength. Whilethis experiment aimed to replicate in situ conditions of the Marcel-lus Shale, additional research is needed to better understand if

fracture widening would occur under confining lithostatic pressureand temperature found in subsurface conditions for fluid–rockinteraction.

Our experiments provided evidence of mineral precipitation inthe form of gypsum on the Marcellus Shale surface and in multiplefractures. Gypsum formed as a result of calcium interaction withwater and sulfate (SO4

2�). The precipitation of gypsum in fracturesobserved in Fig. 6 occurred under static laboratory conditions attemperature and pressure and requires further research in theform of core flow-through experiments at confining temperatureand pressures. Core flow-through experiments might identify ifgypsum would precipitate during Marcellus Shale hydraulic frac-turing under lithostatic forces. Importantly, the complex natureof Marcellus Shale reservoir mineralogy further complicatesfluid–rock geochemical reactions inside fractures. Potential reac-tions include dissolution and precipitation of fluids trapped in frac-tures as fluid chemistry alters by contacting the rock [14]. Kaszubaet al. [14] described that reservoir locations with high fluid flowwere likely to experience dissolution while regions with low fluidflow were likely to experience precipitation. Whether or not thesedissolution and precipitation reactions alter hydrocarbon flowthrough a reservoir like the Marcellus Shale requires furtherinvestigation.

4.2. Effects of fracturing fluid on the Huntersville Chert

Our analyses provided data on the physical (e.g. fractures) andmineralogical changes (e.g. dissolution or etching) that can occuras a result of exposure to recycled fracturing fluid to maximizepotential reactions of the Huntersville Chert and mimic long-term effects of fracturing fluid remaining in the subsurface. Precip-itation of barite (BaSO4) after fluid–rock interaction determined viaFE-SEM EDX characterization occurred on both Huntersville Cherttransverse and parallel cut samples. Barite precipitation was dueto the fact that drilling mud commonly contains barium and sul-fate as additives and combined into solution with the fracturingfluid. Strontianite (SrCO3), celestine (SrSO4), and apatite (CaPO4)also precipitated seen in Figs. 7–9. Marcellus Shale recycled frac-turing fluid commonly contains strontium, barium, and sulfate,which explains the precipitation of minerals bearing these ele-ments on the Huntersville Chert surface [1,12,7,5,10,3,25]. Mineralprecipitation occurred in high quantities and limited the relocationof six out of ten initial features to be identified via FE-SEM. Thesample coverage in mineral precipitation is likely the result of highTDS fluid–rock interaction over the long-term 89 days of exposure.Due to barite being a non-water soluble mineral the precipitatecoating was unable to be removed from the Huntersville Chert

234 M. Dieterich et al. / Fuel 182 (2016) 227–235

surface by simple rinsing before recharacterization. Removal of theprecipitate, which was not achieved, would have allowed for inves-tigation of surface alteration to the chert including possible chem-ical etching and dissolution features.

Rhenium and rare earth element (REE) Europium were likelyobserved with EDX elemental FE-SEM analyses and are believedto have either (1) leached out of minerals in the Huntersville Chertcore sample, or (2) dissolved out of subsurface formations and intothe recycled fracturing fluid. Middle Devonian aged dolomites havebeen found to contain REEs [20]. REEs have the potential to substi-tute in for Ca2+ and Mg2+ in carbonate minerals and are present inhigher concentration compared to freshwater sources [20]. Addi-tionally, REEs can be found in minerals such as quartz, feldspar,and clay minerals. The Huntersville Chert sample utilized duringthis study contained quantities of quartz, carbonate, illite, pyrite,and feldspar, which possibly contain Rhenium and Europium. Qingand Mountjoy [20] observed Middle Devonian limestone, dolomite,and calcite contained the REE Europium, although their study didnot report Rhenium. During this study, Rhenium and Europiumwere potentially leached from the cherts mineral structure duringlaboratory fluid–rock interaction and recrystallized on the chertsurface; or were dissolved into the fracturing fluid during subsur-face fluid–rock interaction. Due to the fact that recycled fracturingfluid contains formation water which underwent fluid–rock inter-actions in the subsurface, the source of REE might be from the recy-cled component of the fracturing fluid and not the HuntersvilleChert core sample mineral structure. Additional research investi-gating the Huntersville Chert elemental composition (total rockdissolution) and REE geochemistry of the recycled fracturing fluidmight help provide an origin of REE detected in the precipitate’smineral structure.

5. Conclusion

The preliminary results of this study show physical and chem-ical changes to Marcellus Shale and Huntersville Chert after expo-sure to fracturing fluid. Both short-term (effects of fracturing fluidinitially entering the subsurface) and long-term (effects of fractur-ing fluid remaining in the subsurface) scenarios were examined.Chemical and physical alteration of the Marcellus Shale and Hun-tersville Chert occurred after fluid–rock interaction at in situ condi-tions with fracturing fluid. After Marcellus Shale interaction withsynthetic fracturing fluid the dissolution of carbonate, fracturepropagation, and gypsum precipitation was observed based uponFE-SEM pre and post-fluid contact imaging. Huntersville Chertsamples interacted with recycled (containing a component of for-mation water) fracturing fluid and the precipitation of barite,strontianite, celestine, and apatite occurred as inferred from FE-SEM X-ray dispersive spectroscopy. Carbonate dissolution was alsobelieve to have occurred as pH changed from an initial pre-fluidrock interaction value of 4.3–6.2 after fluid–rock exposure. Rhe-nium and rare earth element Europium was identified in FE-SEMenergy dispersive X-ray spectroscopy of the Huntersville Chertsamples, which possibly leached from the Huntersville Chert struc-ture or was present in the initial recycled fracturing fluid and min-eralized on the chert’s surface.

This in situ temperature and pressure experiment investigatedthe chemical and physical effects of fracturing fluid, but there arenumerous limitations to consider. This study was not a coreflow-through experiment, i.e. the samples were unconfined[19,17], which might closely replicate fluid–rock interaction insubsurface geologic formations. The Marcellus Shale experimentdid not utilize fracturing fluid with additives (biocides, proppants,friction reducers, etc.). Once removed from the fluid MarcellusShale and Huntersville Chert samples were not immediately rinsed

with water, or another solution to remove excess fracturing fluid.Moreover, XRD analysis was not performed post fluid–rock interac-tion due to time constraints. We recommend that future studies beconducted under strictly controlled conditions with FE-SEM andXRd to further investigate and quantity these changes.

Disclaimer

This report was prepared as an account of work sponsored by anagency of the United States Government. Neither the United StatesGovernment nor any agency thereof, nor any of their employees,makes any warranty, express or implied, or assumes any legal lia-bility or responsibility for the accuracy, completeness, or useful-ness of any information, apparatus, product, or process disclosed,or represents that its use would not infringe privately ownedrights. Reference herein to any specific commercial product, pro-cess, or service by trade name, trademark, manufacturer, or other-wise does not necessarily constitute or imply its endorsement,recommendation, or favoring by the United States Governmentor any agency thereof. The views and opinions of authorsexpressed herein do not necessarily state or reflect those of theUnited States Government or any agency thereof.

Acknowledgements

This work was completed as part of the National Energy Tech-nology Laboratory (NETL) research for the Department of Energy’sComplementary Research Program under Section 999 of the EnergyPolicy Act of 2005.

We would like to thank Rick Hammack for the recycled fractur-ing fluid from the well pad in Greene Co. Pennsylvania and BrettHoward for X-ray diffraction characterization of the MarcellusShale and Huntersville Chert core. Also, we would like to thankthe reviewers who helped improve our paper. This research wassupported in part by appointments to the National Energy Technol-ogy Laboratory Research Participation Program, sponsored by theU.S. Department of Energy and administered by the Oak RidgeInstitute for Science and Education.

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