marcellus issue5

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Jointing and Fracturing in theMarcellus ShaleA discussion of natural fractures, or joints, present in the Marcellus Shaleand the hydraulic fractures that are induced during unconventional gasdrilling to extract natural gas.

P A L E O N T O L O G I C A L R E S E A R C H I N S T I T U T I O N

T H E S C I E N C E B E N E A T H T H E S U R F A C E

M A R C E L L U S S H A L E I S S U E N U M B E R 5 A U G U S T 2 0 1 1

1259 Trumansburg Road

Ithaca, New York 14850

www.museumoftheearth.org/marcellusshale

MUSEUMOF THEEARTHA T T H E P A L E O N T O L OG I C A L R E S E A R C H I N S T I T U T I O N

D I D Y O U K N O W ?

Fractures are commonly

known by geologists asjoints.

e difference betweenjoints and faults, like theSan Andreas system offaults, is that faults haveexperienced marked direc-tional movement along thefracture, whereas joints onlyexperience separation.

Even though water isused, the hydro in hy-drofracking doesnt refer towater. It references thehydraulic pressures usedto fracture the rock.

Introduction

e Marcellus Shale is a natu-ral gas-bearing rock found beneath

the surface of the Earth in parts ofPennsylvania, Ohio, West Virginia,and New York. It was depositedin a shallow sea that covered thesestates nearly 400 million years ago.Shale is a type of sedimentary rockformed from very tiny, flat grainspacked together like decks of cardsstrewn across a table. Grains foundin the Marcellus Shale accumulated

together as mudsalong with a largequantity of organic materialin theshallow sea. Some of the organicmaterial trapped in the mud has ma-tured (changed chemically with heatand pressure) into natural gas. enatural gas now present in the Mar-cellus Shale remains mostly trapped

between the grains and in naturalfractures already present in the shale.Economically extracting the natu-

ral gas tightly trapped by the shalegrains requires a process called hy-draulic fracturing. ere are varioustypes of hydraulic fracturing technol-ogy, but all hydraulic fracture jobsaim to create or extend a network ofjoints, or fractures, in the shale thatallows the trapped natural gas to flowinto a natural gas well.

Marcellus Shale Issue Number 5 July 2011 / 1


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Figure 1. Taughannock Falls

Aerial view of the creek near Taughannock Falls near Ithaca, NY. Note the joint pattern in therocks. e J1 and J2 joint sets intersect at roughly 90 , creating a pattern of repeating cornersin the rock. In these joints, the other side of the rock has fallen away, but beneath Earths sur-face, the joints are planar cracks that exist within the rock mass. is same jointing pattern is

exhibited at depth in the Marcellus Shale.

ere are environmental con-cerns about hydraulic fracturing inthe Marcellus Shale, in part becausethe shales surrounding and includingthe Marcellus have already experi-enced natural fracturing, or jointing,as a part of their geologic history. It

has been suggested that stimulatingthe Marcellus with hydraulic fractur-ing may cause the pre-existing frac-tures to connect and create a pathwaythat leads drilling mud, hydraulicfracture fluid, formation water, andmethane gas to drinking water aqui-fers or water sources.

The Role of

Hydraulic Fracturing

At present, hydraulically frac-turing a well requires 3-5 milliongallons of water, mixed with sandgrains of different sizes and a num-ber of chemicals, to be pushed intothe rock at very high pressures. echemicals perform various functionsin the fracturing process, like killingbacteria and reducing the viscosity of

the hydraulic fracturing fluid. Whilesome of the chemicals are consideredbenign, others can be consideredtoxic: even when diluted with thelarge quantities of water used in theprocess. e fluid is injected into thewell, and when the pressure of thefracturing fluid exceeds the pressureof the rock at depth, the rock breaksat weak points, usually along planes

of weakness. e direction of theseplanes of weakness can be predictedto some extent based on the physi-cal characteristics of the rock andthe orientation of stresses the rock isunder. During a hydraulic fracture,the rock breaks, creating fractures,or joints, which would, without

something to prop them open, natu-rally close when the water pressureis subsequently lowered. e sandor sand-like grains in the fracturingfluid act as propping agents (prop-

pants) remaining in the joints afterthe water is removed and the pressureis lowered to keep the joint networksopen for natural gas extraction.

In sum, hydraulic fracturing

2 / Marcellus Shale Issue Number 5 July 2011


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operates to open and maintain path-ways for fluid to flow where it other-wise could not, and the process useschemicals that would be unsafe ifthey entered drinking water aquifersor streams. If hydraulic fracturingwere conducted near drinking water

aquifers or if the induced fractureswere sufficiently long or connectedto long natural fractures, it is con-ceivable that the process could alsoconnect new fracture networks withpre-existing ones to create a pathwayfor the natural gas, and the water andchemicals used to release the gas, toflow into drinking water aquifers.Finally, there are other substances

trapped in the rocks around the Mar-cellus Shale, like brines and naturalgas, which could contaminate drink-ing water aquifers if a pathway wascreated during the hydraulic fractur-ing process. For these reasons, it isimportant to understand how thenaturally occurring joints and thefractures stimulated by oil and gascompanies could interact in orderto assess the risk of drinking water

contamination in areas where drillingis done.

Natural Fractures

e Marcellus Shale is a naturallyfractured rock because of the com-bination of the quantity of organicmatter trapped in the rock and thehistorical plat tectonic activity that

occurred in the area. e conversionfrom organic material to natural gasin the Marcellus created pressure inthe fluids trapped in the rocks, whichhelped created natural fractures inthe rock, called joints. ese jointswere further exacerbated by the colli-sion of plates during the Alleghanian

Orogeny, a mountain-building eventthat began around 350 million yearsago. is combination of forces cre-ated the majority of the joints in theMarcellus Shale. Because the jointswere formed from the same processes,most of the joints in the Marcellus

Shale run in roughly parallel sets inone of two directions.

ese two sets of joints are calledJ1 and J2, and each has uniquecharacteristics. Joints in the J1 setrun east-northeast throughout theMarcellus Shale. e J2 joint setruns generally north-northwest andcuts across the J1 joint set. e J1joint set happens to coincide with

the modern underground stressesfound throughout the Northeast.Joints in the J1 joint set are spacedmore closely together than those inthe J2 joint set. Joints in the J2 setare more likely to have been healed,which means the joint has been filledin with minerals and is no longer apathway for gas flow. ese healedjoints are stil l relatively weak becausethe mineral cement gluing them

together is weaker than the originalbedding planes of the rock, and aretherefore places where new fracturescould grow.

In addition, the Marcellus Shalewas buried by numerous other rocklayers in the last 400 million years,some of which have been erodedsince the Alleghanian Orogeny(in part by the numerous glacia-

tions that have sculpted the currentlandscape of the Northeast). eseerosional forces have brought theMarcellus Shale closer to the surfaceof the Earth and thereby decreasedthe downward pressure provided byoverlying rocks. As overlying weightthat had compressed the ground was

released, much like a memory foammattress, the decrease in pressureresulted in the J3 joint set. ey, too,run roughly parallel, and are referredto as release joints and unloadingjoints.1

Release and unloading joints cre-

ated by the various unloading of rockand ice are only present at or nearEarths surface. is is why J3 jointsare not found at the depth of Marcel-lus Shale natural gas extraction.2

How Stimulated

Hydraulic Fractures Form

Hydraulic fractures form when

the fluid pressure within the rockunit exceeds the external pressurespushing on the rock unit. To under-stand how hydraulic fractures couldpropagate (be stimulated) in Mar-cellus Shale natural gas drilling, drill-ing engineers must understand thepressures acting upon the rock.

Beneath the surface, there isvertical pressure from the weight ofthe overlying rock. ere are also

horizontal pressures in all directionspressing on the Marcellus Shale.ese pressures are the result ofplate tectonic forces interacting. eEarths outer shell is made of verylarge plates, like puzzle pieces, whichslowly jostle each other. e interac-tions of these plates can build moun-tains and cause earthquakes, but inthe Marcellus Shale region, they are

weaker than the vertical pressurecaused by the weight of the overly-ing rocks. Hydraulic fractures growperpendicular to the direction (theplane) of minimum principle stress.In the Marcellus Shale region, frac-tures made by hydraulic fracturing atdepth will have a vertical orientation



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and will separate against the mini-mum horizontal stress.

Because the vertical stress changeswith depth, the orientation of thehydraulic fractures also changes as thefractures move closer to the surface. Insufficiently shallow rock, the directionfractures propagate can leave the verti-

cal plane and propagate on an angleand could even approach the horizon-tal plane. So as fractures propagateinto shallower rock above, they be-come less likely to propagate verticallyand more likely to extend laterallyinto the rock. e depth at which thisoccurs varies by rock type and thick-ness and by the change in related pres-sures and is difficult to estimate.

While much of the Devonianrock surrounding the Marcellus Shaleis also shale, there are many layersof silt, limestone, and sandstone, aswell. Even the different shales sur-rounding the Marcellus are uniquein their composition. ese differentkinds of rock layers are called litholo-

gies, and each lithology has its ownunique set of physical characteristicsthat react differently to the pressuresthat cause hydraulic fractures. Eachlithology has been deposited underslightly different marine and terres-trial conditions, which can affect itsgrain size and provenance (where the

grains came from and what types ofgrains are present), composition ofthe mineral glue cementing grainstogether, and the quantity of organ-ics present in theshale. For example,some layers containevidence of hurri-cane-force storms,and others record

times of little or nosediment deposi-tion. Some shalesare black due to ahigh quantity oforganic material, while others withless organic material are grey. edepositional conditions determine

the physical characteristics of thelithology and thus how it may reactto hydraulic fracturing. In general,limestones have higher fracturethresholds than shales and can act asbarriers to vertical fracture growth.3

As a fracture grows above or be-low the Marcellus Shale, it encoun-

ters other lithologies, and this canplay an important role in determin-ing the ultimate extent of a hydraulicfracture. Lithologies often change

abruptly, represent-ing events or chang-ing environments inEarth history; suchdistinct boundariesbetween litholo-

gies are often moreweakly cementedtogether than thelayers, or beds, with-in each lithology.

Because of this, these boundariesare considered planes of weakness.Layers, or beds, within a lithology

Africa

Eurasia

Indo-Australia

Antarctica

Nazca

Pacific

SouthAmerica

NorthAmerica

CocosCaribbean

Somalia

ArabiaPhilippines

Scotia

Juan de Fuca10

23

29

79

106

46

32

12

6

8

8

14

54

59

74

9

921

105

102

11

Figure 2. Map of current plateboundaries throughout the world.

Arrows indicate the speed and direction ofplate movement, represented in millimetersper year. Dots indicate the presence of earth-quakes, which are concentrated along plate

boundaries. Notice how the dots concentratewhen two plates are moving toward eachother, and are less common where plates aremoving away from each other. is imageis courtesy of Incorporated Research Institu-tions for Seismology (IRIS), and is part oftheir educational series of pamphlets aboutearthquakes that can be accessed online athttp://www.iris.edu/hq/publications/bro-chures_and_onepagers/edu.

The stiffness of

the Marcellus Shale is

not uniform, so some

areas are more prone

to longer fractures

than others.



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Relatively highly fractured rock

Minimumprincipalstress

Minimumprincipalstress

Relatively unfractured rock

1 2 3 3a

can also be planes of weakness, butthese are usually more cohesive thanboundaries.

Boundaries play an importantrole in the growth of hydraulic frac-tures, because individual fracturestend to have greater difficulty propa-

gating straight through them. Oftenthe boundary between two litholo-gies acts as a buffer, and the someof the pressure causing a hydraulicfracture is diverted to planes of weak-ness, radiating horizontally along theboundary (because it is weaker thanthe surrounding rock). As a result,there is less pressure available to cre-ate a fracture in the overlying lithol-

ogy. When the pressure creating thefracture succeeds in extending intothe overlying lithology, the energy isdistributed in this new rock layer dif-ferently based on the characteristicsof the rock, and the direction andpattern of the fracture can change.Field geologists commonly note thata set of fractures end at a boundarybetween two lithologies, and some-times use this characteristic to orient

themselves at a new outcrop.Hydraulic fracture characteristics

are dramatically impacted by the mea-sure of the modulus (ratio of stress tostrain) of the rock. If the modulus islarge, the rock is considered stiff. Instiff rock units, fractures grow longand narrow away from the source ofadditional pressure, and in less stiffmaterials, fractures are wider and

vertically shorter because the pres-sures causing the fractures can pen-etrate and dissipate further laterally(into planes of weakness like beddingplanes and existing fractures) withinthe rock unit. Shales are usually verystiff, but highly fractured black shales,like the Marcellus, are generally much

less stiff than unfractured grey shales.e stiffness of the Marcellus Shale isnot uniform, so some areas are moreprone to longer fractures than others.However, when a hydraulic fracturehits the less stiff, already fracturedmaterials in the Marcellus Shale, the

energy can move laterally within theshale and, as a result, cause shorter,wider fractures.4,5,6

Understanding Current

Fracture Networks within the

Marcellus Shale

e Marcellus Shale is a natu-rally-fractured gas shale, and the body

of literature on the characteristics of

those fractures is expected to growsignificantly as additional data aregathered. In some respects, the onlyway to gather additional data is todrill more wells and examine rockcarefully removed from the well (aprocess called core sampling), so

some things could remain unknownuntil after additional drilling has tak-en place. While this pamphlet high-lights much of the base level under-standing of Marcellus Shale jointing,some generalizations will be refinedonly after engineers have collectedand analyzed data from numerouswells and core samples throughoutthe Marcellus.

ere are a number of physical

Figure 3. Fracture Propagation

Scenarios 1, 2, and 3 represent theoretically likely directions of fracture propagation result-ing from increasingly higher levels of fracture-inducing force. As force increases in relativelyunfractured rock, fractures propagate perpendicular to the direction of maximum principlestress. When they encounter a rock layer boundary (which is naturally weaker), the energyforcing the fracture dissipates laterally, making it harder for a fracture to continue acrossboundaries. In more fractured rock (scenario 3a), even with the same pressures as seen in 3,

pre-existing fractures can cause energy forcing a fracture to dissipate in other directions. Be-cause of the many boundaries between rock layers and pre-existing fractures in scenario 3a,the fractures propagated are wider and shorter than they would normally be if the same rockwas relatively unfractured.



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characteristics of the Marcellus Shalethat are not homogeneous. Forexample, some geologists have hy-pothesized that the lower part of theMarcellus Shale, sometimes calledthe Union Springs shale, which hasa higher organic

content andlarger quantitiesof pyrite (foolsgold) than therest of the Mar-cellus, may havemore jointing(as a result ofthe maturingorganics in

concert withthe Alleghanianorogeny). Itwould thus havea higher naturalinterconnec-tion of fracturesthrough whichgas can moveand a highernatural gas yield. Because the eastern

part of the Marcellus Shale has beensubjected to greater plate tectonicforces, jointing may be more abun-dant in the east. But thinner unitsare generally more fractured thanthicker units, and the Marcellus isthinner in the west than it is in theeast, so jointing could be hypoth-esized to be more prevalent in thewestern portion of the Marcellus

Shale.6

e Marcellus Shale is the baserock unit in a group of shales andlimestones called the HamiltonGroup. e Hamilton Group is wellknown in New York State for theabundant fossils present in some ofthe rock units. While the Marcellus

is not the only shale in the Hamil-ton Group, it is the only uniformlythick, gas-bearing, black shale. Blackshales have some unique distinctionsover grey shales, including a higherquantity of organic material and a

higher amount of

naturally-occur-ring radioactivematerial. Becauseof the organiccontent of blackshales, the num-ber of joints ina square unit ofrock (joint den-sity) is usually

much higher thanin grey, lowerorganic contentshales, with spac-ing usually lessthan one meterapart in blackshales.7Becauseof this and othershale character-

istics, many times the fractures in

black shales consistently tend not toextend beyond lithologic boundariesinto overlying (or underlying) greyshales.

e Marcellus Shale representsa dynamic system that has beenchanging for almost 400 millionyears. While there is consensus onbasic fracture patterns likely to occurin the Marcellus Shale as a result of

stimulation, the shale is not homoge-neous, and different regions will reactdifferently to the same hydraulic frac-turing treatment.ere will be areaswith concentrated fractures and areasalmost devoid of them, and whilewe can predict where those might bebased on our understanding of how

fractures form, there is not yet muchobservational data to support thosepredictions.

Tracking the Results of

Hydraulic Fracturing

Because hydraulic fracturing isoccurring beneath Earths surface,the only way to track fractures result-ing from a hydraulic fracture is touse proxy evidence in the form ofmodels, field tests, and monitoringproduction of wells, and incorporat-ing that information into a body ofknowledge that grows as more wellsare drilled in the Marcellus region.

M O D E L I N G

Models are used to predict theeffectiveness of hydraulically frac-turing wells in shale gas layers likethe Marcellus, and to inform futurehydraulic fracturing in nearby wells.ese models are based on data gath-ered in the field which are importedinto software programs to predict thecharacteristics of the rock at depth.

Field data usually incorporated intohydraulic fracture models includeseismic and microseismic data, welllog data, expected pressures through-out the subsurface, and other rockcharacteristics. Models predict howfracture fluid (and the associatedpressures and fractures already pres-ent in the rock) will act during hy-draulic fracturing. When measured

values differ significantly from themodeled values, the fluids are notbehaving in the way the model pre-dicted they would and one or morecharacteristics of the rock have notbeen adequately modeled. is mayoccur if the model itself needs to berevised or if estimates of rock proper-

While there is

consensus on basic

fracture patterns

likely to occur in the

Marcellus Shale as a

result of stimulation,

the shale is not homo-

geneous, and differ-

ent regions will react

differently to the same

hydraulic fracturing

treatment.



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ties where the hydraulic fracturingis done were not accurate. us, tounderstand what is happening in therock below the surface, modeling isused in concert with field testing toassess most effective hydraulic frac-turing treatment for different regions

in the Marcellus Shale.8,9,10

F I E L D T E S T S

Common field tests used initiallyto inform and eventually to truthfracture propagation models includeobserving well logs, seismic testing,sonic logs, gamma ray logs, resistivitylogs, and monitoring fluid injectionand downhole pressures of the well

during hydraulic fracturing, amongother techniques.11Seismic testing isa technique that can be run on thesurface of the Earth, but most tech-niques require an observational wellto be drilled for monitoring purposes.A few field techniques will be dis-cussed below.

Seismic testing uses seismicwaves, created by thumping theground with heavy weights, to read

the subsurface. e waves travelthrough the ground, hit differentlayers of rock, fractured zones, andother variations in density beneaththe surface of the Earth, and returnto the surface at different times. ewaves are recorded and analyzedto create an image of the rock lay-ers beneath the surface. ese testsare usually run along roads or other

straight paths, and interpreted withother seismic tests to search for exist-ing fractures and other structures inthe subsurface that could potentiallyinteract with a hydraulic fracturingtreatment. Because seismic testingcan be done from Earths surface,it is sometimes used after hydraulic

fracturing to explore the accuracy ofthe model of fracturing in predictingactual fracture patterns.

Microseismic testing works inmuch the same way as seismic testing,but small seismic monitors are placedat the surface, or shallowly buried,

above the area undergoing hydraulicfracturing. ese monitors recordsmall seismic changes and relaythem, frequently in real time, to theengineers conducting the hydraulicfracturing treatment. is is the mosteffective way to visualize the fracturesthat are created during a hydraulicfracture and evaluate model effective-ness.12

Well logs are descriptions of therock layers as observed by the drillersthat document the rock type, joint-ing, and other visual characteristicsof the rock. Sonic logs are similar inprinciple to seismic tests, but use son-ic waves instead of sound waves. Also,sonic logs are run down a well sothat, instead of from the surface, thesonic logs record information nearthe well bore. Gamma ray logs also

run down the well bore, but theselook specifically for natural radioac-tivity, which is much higher in blackshales than in other rocks. Gammaray logs help drillers determine thethickness and depth of the reservoirrock in the region. Resistivity logs usevarious methods to extract the poros-ity and fluid content of different rocklayers.

Observation wells are sometimesdrilled near a well undergoing hy-draulic fracturing while a region isbeing established to collect data onthe effectiveness of hydraulic fractur-ing. A similar suite of field tests andobservations can be conducted on anobservation well to more objectively

test the effectiveness of a hydraulicfracturing treatment, the model usedto predict the fracture behavior, andother parameters.13,14

Existing Fracture Networks

Natural gas has been rising to-ward areas of low pressure since itformed in the Marcellus Shale, whichis part of the reason for the existingfractures in the unit. Natural gas hasreached near-surface layers alongfracture networks where the Marcel-lus Shale (and other shales and lime-stones) comes within several hundredfeet of the surface. Some of these

seeps were sources for natural gas fortowns during the 19thcentury. is iswhy methane can also be found ris-ing from some abandoned, uncappedwells.

e NYS DEC has recom-mended that stimulation more than2,000 feet beneath the surface is deepenough to avoid impacts on potablewater sources.15Fluid and gas migra-tion through fracture networks from

below 2,000 feet to the surface is nottheoretically impossible. A recentstudy documented the presence ofthermogenic gas in water wells withinclose proximity of active gas wells.16ermogenic gas, the kind of gasfound in the Marcellus Shale, is theresult of temperature and pressurechanges deep beneath Earths sur-face that change carbon matter into

methane. It has a different chemicalsignature than biogenic natural gas,which is produced by the metabolicdecay of organisms close to Earthssurface. is study correlates a highpercentage of thermogenic gas (as op-posed to a mix of thermogenic andbiogenic gas) contamination with ac-



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tive gas drilling, but does not explainhow the migration from deep rockshas occurred. Migration pathways aredifficult to trace, and no conductedstudies have been able to confirm ordeny a fracture connection betweendeep gas and drinking water aquifers.

is study did not record thepresence of fracturing fluid contami-nation or brine contamination in anywater well in close proximity to anactive gas well, which suggests thatfaulty well casings are a more likelypathway than deep fracture connec-tions.17e presence of natural gasand other fluids at the depth of theMarcellus Shale and in surrounding

layers suggests that existing fracturenetworks have not allowed these sub-stances to escape.

e study also noted that somewater wells that were not locatednear active gas wells had methanecontamination. However, the amountof methane in these wells was farless than those water wells near ac-tive gas drilling, and the chemicalmakeup of the methane was a mix

of thermogenic and biogenic naturalgas. e reason gas and other fluidsmay migrate from fractures near thesurfaceleaving a small amount ofmixed methane contaminationbutnot from a mile beneath the surfaceis likely related to the number of rockunits acting as barriers to migrationand the degree of discontinuous frac-tures passing from the Marcellus to

the surface.In considering existing fracturenetworks, it is also important to notethat Marcellus Shale drilling is not theonly type of drilling that has occurredin NY and throughout the northeast.Conventional oil and gas drilling hascommonly occurred throughout NY

for decades, and some of these wellswere improperly abandoned. At least70,000 wells have been drilled inNYS and only 30,000 are accountedfor by the DEC.18

When a well is no longer pro-ducing commercial quantities of oil

or natural gas, a company abandonsthe well. Abandoning a well properlyinvolves removing some of the casingand other surface equipment, plug-ging the well with cement so thatit is no longer a conduit for fluidsmigrating from beneath the surface,and then reclaiming the surface. Ifwells are improperly abandoned, theycan act as a connection between the

surface and the depth of the well. Inmost cases, improperly abandonedwells are older and relatively shallow,but when the Marcellus Shale is be-ing exploited at shallower depths, likein central NY, improperly abandonedwells could potentially connect afracture network to a groundwatersource. Improperly abandoned wellswere considered when the DEC sug-gested that stimulation of Marcellus

Shale should be at least 3,000 feetbeneath the surface to sufficientlydecrease risk of contamination of po-tential groundwater sources.

Fracture networks are not theonly consideration of how fluids cantravel through rock layers to the sur-face. Because of the greater pressuresbeneath the surface, fluid naturallyflows toward areas of lower pressure

if a pathway (through fractures or be-tween connected pore spaces) exists.Permeability (amount of connectedpore space) is measured in mil-liDarcies (after Darcys Law, whichdescribes how fluids flow through po-rous media). e rate at which a fluidcan flow through a rock, based on

the amount of fluid present, pressure,fracture presence, and rock perme-ability, is called conductivity.

e conductivity of the Marcel-lus Shale is relatively low, but variesbetween 0.000011 and 0.00059 feetper day (just over 7/1000thof an inch

of movement per day at its fastest),and the surrounding units also havevariable but low conductivities.17It ismore common for fluid to flow hori-zontally than vertically in the Marcel-lus Shale because of the pressures atdepth and the existence of horizontalbedding planes. However, it is notimpossible for pressures at depth tocreate a gradient angled to some de-

gree toward Earths surface in someregions of the Marcellus or surround-ing units. As the rock unit changes,the direction of flow and conductiv-ity can change.

Very little data is available on therange of conductivities in the rockssurrounding and including the Mar-cellus Shale in locations being exam-ined for natural gas drilling, althoughmore is gathered by gas companies

as additional wells are drilled.18eimpacts of the short-term increase inpressure caused by hydraulic fractur-ing conductivity from the depth ofthe Marcellus Shale to the surface isnot well understood, and could pos-sibly play a role in the migration ofgas and fracture fluids to adjacentrock units19although are not likely toprovide sufficient pressure change to

migrate gas and fracture fluids all theway into drinking water aquifers.e conductivity of the Mar-

cellus Shale and its overlying unitsdoes not currently allow significantmethane migration from depths beingconsidered for natural gas extractionto the surface. If that were the case,



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the quantity of natural gas storedthroughout the Marcellus Shalewould have been greatly diminishedby now. We might assume, therefore,that the conductivity provided byexisting fracture networks connectingthe Marcellus to much shallower units

is very low. Based on this reasoning, itappears unlikely, though not theoreti-cally impossible, that gas and fracture

fluid could migrate to potential sourc-es of drinking water in this way.

Although it is expected that hy-draulic fractures would propogateupward little further than the top ofthe Marcellus, one ought to considerthe potential impact if there were

sufficient pressure from hydrofractur-ing to connect to and open existingfractures up to very shallow depths,

or to connect them with shallow, im-properly abandoned wells. Fracturesremain open during pressurizationfrom hydrofracturing, for a few hoursat a time for up to a few days, thenremain open if proppant sand hasentered the fractures. If fractures did

connect to existing fracture networks,they would not have sufficient prop-pant to keep them open after hydrau-

Table 1. Geological Characteristics of Coal and Marcellus Shale

depth below surface

Aquifer location

Surrounding rock

lithologies

Thickness of

desired unit

Original deposition

of rock unit

Depth of targeted

formations for

hydraulic fracturing

Pre-existing fractures

in desired rock unit

Rock CharacteristicsCoal Bed

Methane Extraction

06,000 ft; most between 4004,000 ft

mostly within 450 ft

variable; other coal seams, shale, sandstone

inches to 250 ft thick lenses

in terrestrial settings, among braided streams,

wetlands, and deltas

06,000 ft; dependent upon target depth

cleating present; degree of cleating

primarily based on coal type and depthbelow surface

3,00010,000 ft

01,000 ft (most less than 700 ft)

mostly shale, some silt and limestone lenses;varies regionally

one contiguous layer between 50250 ftthick; regionally variable

bottom of shallow, continental sea

3,00010,000 ft; dependent upondepth of unit

fractures present; degree of fracturing

based on organic content, rock unitthickness, and proximity to tectonicaction; varies regionally

Comparison of the geological characteristics of coal bed methane extraction and Marcellus Shale methane extraction.Coal bed information compiled from data from Powder River, San Juan, Raton, Piceance, and Uinta Basins. 21Marcellus Shale information from Hill, et. al., 2002.12

Marcellus ShaleMethane Extraction



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lic fracturing was complete. To datemigration through such fractures offracturing fluids and produced waterhas not been documented.20is maybe because such vertical migrationpaths do not or rarely exist, or be-cause the amount of fracturing fluid

and proppant that can migrate verti-cally over such a distance is small.

Understanding

Fracture Networks in

Different Rock Units

Different rock types, like sand-stones, shales, and coal seams, havedifferent physical characteristics that

impact the likelihood of the ability offractures to grow upward toward thesurface from various depths. Whenthey are sought after for fossil fuelextraction, the depth of the rock unitbelow the surface, the stiffness of therock unit being targeted and of thesurrounding lithologies, the regionalstresses on the rocks, the depth of po-table water, and many other consider-ations are used to predict the impact

hydraulic fracturing will have on aparticular lithology.

Recently, water well contamina-tion in coal bed methane extractionusing hydraulic fracturing has beenused to compare the potential forwater well contamination in theMarcellus Shale. It is important tolearn from the environmental prob-lems experienced in other active gas

plays; comparisons among differentreservoir rocks, however, must alsoconsider the important geologicsimilarities and differences betweenthem.

While the organics in the Mar-cellus Shale were deposited at thebottom of a shallow sea basin and

were comprised primarily of marinealgae, coal beds are deposited in ter-restrial environments and are usuallycomprised of woody and leafy plantmatter. Coals generally form aroundbraided streams, wetlands, or deltaenvironments, and, as a result, an in-

dividual coal layer usually cannot betraced laterally for a long distance. In-stead, the environment conducive tocoal formation moves and migrates,like sand bars in a stream. e resultis that a coal bed can sometimes be aseries of horizontal lenses interspersedin a vertical unit of rock. Coal canalso form in one distinct layer, buteven then the thickness of the coal is

highly variable. Coal deposits usuallyhave a much smaller regional extentthan the Marcellus Shale. is is be-cause they are deposited in areas nearstreams, lakes, and rivers, which aregeographically smaller than shallowcontinental seas.

As a result of these geologic dif-ferences, the target depth for coal bedmethane extraction extends from veryshallow depths that sometimes inter-

sect drinking water aquifers to depthsthat are similar to those for MarcellusShale gas drilling. e target depthfor gas extraction has a narrow range,changing verticallyat mostaround500 ft. in a 25 mile radius. However,in a coal bed over the same 25 mileradius, the target depth for methaneextraction can range from around400 to 4,000 feet, and multiple

depths may be targeted. Shallow coalbeds (less than 450 ft deep) are notonly home to sources of methane, butsometimes are the source of potentialdrinking water.21

Shallowly buried coal is usuallyhighly permeable, because as theorganic material matures fractures

form called cleats, which cause minedcoal to appear blocky. Cleats increasecoal permeability dramatically. isis why fresh water can frequentlybe found in shallow coal beds, andwhy coal bed methane extractionfrequently involves what is called

dewatering the coal. is is alsowhy methane can flow easily withinthe coal, especially at shallow depths.When coal is more deeply buried,however, the pressure of the overlyingrocks can close the cleats and lowerthe permeability of the coal. Naturalgas companies prefer to extract gasfrom shallower coal units becauseof this. Because coal bed methane is

sought from a wide range of depths,and because coals have a higher per-meability, stimulated fractures areusually intended to connect betweenmultiple coal layers.

Some similarities can be drawnbetween coal bed methane extractionand Marcellus Shale methane extrac-tion, but in detail, the similarities arefew. Because of the different physicalcharacteristics that dictate how and

where natural gas is stored in the rocktypes, the permeability of the rockunits, and their proximity to poten-tial drinking water sources, the likeli-hood of fractures propagating beyondtheir target zone(s) in coal bed meth-ane extraction is much higher than inthe Marcellus Shale.

Summary

ere has been significant com-munity concern that high volumehydraulic fracturing of the MarcellusShale may allow fluidsincludingnatural gasto rise from connectedfracture networks several thousandsof feet below aquifers. In addition,



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the Marcellus Shale has been deeplyfractured by several geologic eventssince deposition of the shale. Wherethe Marcellus Shale occurs near thesurface, for example in western NewYork, natural gas has been known toseep naturally into drinking wateraquifers and to the surface. Availableevidence suggests that deep shale(several thousand feet) is not natural-ly connected to drinking water sourc-es through fracture networks, likelybecause of the discontinuity of frac-tures across numerous lithologies.22It is not known to be impossible forfluids to migrate to drinking water

sources from deep hydraulic fractur-ing, but the likelihood of significantmigration appears to be very low,especially when compared to the riskof faulty casings or surface spills as apotential source of contamination.Finally, the relative risk of hydraulicfracturing varies substantially by localgeological context, including the na-ture and depth of source rock, lithol-ogy of overlying rocks, and the natureof existing fractures and fault net-works. Any comparison between theMarcellus Shale and another sourceof natural gas must be well framedand consider these parameters.23

References

1. Hill, David, Tracy E. Lombardi, andJohn P. Martin. 2002. Fractured shalegas potential in New York. TICORAGeosciences, Inc., Arvada, Colorado,U.S.A.

2. Engelder, Terry, Gary G. Lash, andRedescal S. Uzcategui. 2009. Joint setsthat enhance production from Middleand Upper Devonian gas shales of the

Appalachian Basin. AAPG Bulletinv.93, 7, 857-889.

3. Jarvie, D.M., R.J. Hill, T.E. Ruble, andR.M. Pollastro, 2007, Unconventionalshale-gas systems: e MississippianBarnett Shale of north-central Texas asone model for thermogenic shale-gasassessment, AAPG Bulletin, v.91:4, pp.475-499.

Potable H2Olevel

Potable H2Olevel

GAS BEARING COAL

MARCELLUSSHALE

Tully LimestoneMoscow Shale

Ludlowville Shale

Skaneateles Shale

Onondaga Limestone

Generic Coal Bed Methane(Modeled in part after Powder River Basin)

Typical Marcellus in NYS

HAMILTON

GROUP

GENESSEEGROUP

SURFACE

1000'

2000'

3000'

4000'

5000'

6000'

COAL BEDCLEATING(detail)

Figure 4. ComparingCoalbed and Marcellus Shale

Methane Extraction

Comparing a generalized depiction of theenvironment from which coal bed meth-

ane is extracted to a generalized depic-tion of the Marcellus Shale in New YorkState. 1) Gas-bearing coal deposits cancommonly be found at multiple depthsin the same region, including as shallowas within the water table, and all of thedeposits are of interest to gas companies.Coal lenses are cleated, which increasestheir permeability. 2) In NYS, the Mar-

cellus Shale averages around 100ft inthickness, andin areas presently beingconsidered for natural gas drillingislocated between 3,000 and 5,000ft be-neath the surface. Potable water depositsin NYS are not deeper than 850ft, butsources that can be treated and becomepotable extend to 1,000ft beneath Earthssurface. See Table 1 for additional detail.



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Authored by Trisha A. Smrecak and the PRI Marcellus Shale TeamIllustrations by J. Houghton unless otherwise attributed.

Partnering Organizations include

Cornell Cooperative Extension (naturalgas.cce.cornell edu),New York State Water Resources Institute

(wri.cornell.edu), Cornell University Department of

Earth and Atmospheric Sciences, and

Cornell University Agricultural Experiment Station.

Funded by NSF GEO No. 1016359.

Any opinions, findings, and conclusions orrecommendations expressed in this material

are those of the author(s) and do not neces-

sarily reflect the views of the National

Science Foundation.

4. OConnor, K.M. and J.A. Siekmeier,2009. Influence of rock mass stiffnessvariation on measured and simulatedbehavior. e 37thU.S. Symposeumon Rock Mechanics (USRMS), June7-9, 1999, Vail, CO.

5. National Research Council, 2010,Management and Effects of Coalbed

Methane Development and ProducedWater in the Western United States,Appendix A Hydraulic FracturingWhite Paper, National AcademiesAcademic Press, Washington, D.C.,217 p.


7. Sankaran, S., M. Nikolaou, and M.J.Economides, 2000, Fracture Geometry

and Vertical Migration in MultilayeredFormations from Inclined Wells, SPE63177, 8 pp.

8. National Research Council, 2010,Management and Effects of CoalbedMethane Development and ProducedWater in the Western United States,Appendix A Hydraulic FracturingWhite Paper, National Academies Aca-demic Press, Washington, D.C., 217 p.

9. Carter, B.J., J. Desroches, A.R. In-graffea, and P.A. Wawrzynek, 2000,

Simulating fully 3D hydraulic fractur-ing, inModeling in Geomechanics, Ed.Zaman, Booker, and Gioda, WileyPublishers, 730 pp.

10. Jenkins, C., A. Ouenes, A. Zellou,and J. Wingard, 2009, Qunatifyingand predicting naturally fractured

reservoir behavior with continuousfracture models, AAPG Bulletin, v.93,11, 1597-1608.

11. Chipperfield, S., 2008, HydraulicFracturing: Technology Focus, JPTv.60, 3. 56-71.

12. Digital EnergyJournal, 2011, CGGVeritas real time microseismic moni-

toring, http://www.findingpetroleum.com/n/CGG Veritas real time micro-seismic monitoring/892015e8.aspx(accessed June 10, 2011).


14. Raymond, M.S. and W.L. Leffler,2006,Oil and Gas Production in Non-Technical Language, PennWell Corp.,

Tulsa, OK, 255 pp.15. Hyne, N.J., 2001, Nontechnical

Guide to Petroleum Geology, Explora-tion, Drilling, and Production, 2nded.,PennWell Corp., Tulsa, OK, 598 pp.

16. New York State Department of Envi-ronmental Conservation, 2009, Draftsupplemental generic environmentalimpact statement on the oil, gas, andsolution mining regulatory program.http://www.dec.ny.gov/energy/58440.html (accessed March, 2011).

17. Raymond, M.S. and W.L. Leffler,2006,Oil and Gas Production in Non-Technical Language, PennWell Corp.,Tulsa, OK, 255 pp.

18. Osborn, S.G., A. Vengosh, N.R. War-ner, and R.B. Jackson, in press, Meth-ane contamination of drinking water

accompanying gas-well drilling andhydraulic fracturing, PNAS.

19. New York State Department of Envi-ronmental Conservation, 2009, Draftsupplemental generic environmentalimpact statement on the oil, gas, andsolution mining regulatory program.http://www.dec.ny.gov/energy/58440.

html (accessed March, 2011).20. ICF International, 2009, Technical

Assistance for the Draft SupplementalGeneric EIS: Oil, Gas and SolutionMining Regulatory Program WellPermit Issuance for Horizontal Drill-ing and High-Volume HydraulicFracturing to Develop the MarcellusShale and Other Low Permeability GasReservoirs, Agreement #9679, NY-SERDA, Albany, NY.

21. Myers, 2009, Review and Analysis ofDRAFT Supplemental Generic Envi-ronmental Impact Statement On eOil, Gas and Solution Mining Regula-tory Program Well Permit Issuance forHorizontal Drilling and High-VolumeHydraulic Fracturing to Develop theMarcellus Shale and Other Low-Perme-ability Gas Reservoirs, Natural Resourc-es Defense Council, New York, NY.

22. Osborn, S.G., A. Vengosh, N.R. War-ner, and R.B. Jackson, in press, Meth-ane contamination of drinking wateraccompanying gas-well drilling and

hydraulic fracturing, PNAS.23. National Research Council, 2010,

Management and Effects of CoalbedMethane Development and ProducedWater in the Western United States,National Academies Academic Press,Washington, D.C., 217 pp.


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