Jointing and Fracturing in the Marcellus Shale

PA 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 AT H T H E S U R FA 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
Jointing and Fracturing in the
Marcellus Shale
A discussion of natural fractures, or joints, present in the Marcellus Shale
and the hydraulic fractures that are induced during unconventional gas
drilling to extract natural gas.
Introduction
DID YOU KNOW?
Fractures are commonly
known by geologists as
‘joints.’
•
The difference between
• joints
and faults, like the
San Andreas system of
faults, is that faults have
experienced marked directional movement along the
fracture, whereas joints only
experience separation.
though water is
•Even
used, the “hydro” in hy-
drofracking doesn’t refer to
water. It references the
hydraulic pressures used
to fracture the rock.
The Marcellus Shale is a natural gas-bearing rock found beneath
the surface of the Earth in parts of
Pennsylvania, Ohio, West Virginia,
and New York. It was deposited
in a shallow sea that covered these
states nearly 400 million years ago.
Shale is a type of sedimentary rock
formed from very tiny, flat grains
packed together like decks of cards
strewn across a table. Grains found
in the Marcellus Shale accumulated
together as muds–along with a large
quantity of organic material–in the
shallow sea. Some of the organic
material trapped in the mud has matured (changed chemically with heat
and pressure) into natural gas. The
natural gas now present in the Marcellus Shale remains mostly trapped
between the grains and in natural
fractures already present in the shale.
Economically extracting the natural gas tightly trapped by the shale
grains requires a process called hydraulic fracturing. There are various
types of hydraulic fracturing technology, but all hydraulic fracture jobs
aim to create or extend a network of
joints, or fractures, in the shale that
allows the trapped natural gas to flow
into a natural gas well.
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Marcellus Shale • Issue Number 5 • July 2011
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There are environmental concerns about hydraulic fracturing in
the Marcellus Shale, in part because
the shales surrounding and including
the Marcellus have already experienced natural fracturing, or jointing,
as a part of their geologic history. It
has been suggested that stimulating
the Marcellus with hydraulic fracturing may cause the pre-existing fractures to connect and create a pathway
that leads drilling mud, hydraulic
fracture fluid, formation water, and
methane gas to drinking water aquifers or water sources.
The Role of
Hydraulic Fracturing
At present, hydraulically fracturing a well requires 3-5 million
gallons of water, mixed with sand
grains of different sizes and a number of chemicals, to be pushed into
the rock at very high pressures. The
chemicals perform various functions
in the fracturing process, like killing
bacteria and reducing the viscosity of
the hydraulic fracturing fluid. While
some of the chemicals are considered
benign, others can be considered
toxic: even when diluted with the
large quantities of water used in the
process. The fluid is injected into the
well, and when the pressure of the
fracturing fluid exceeds the pressure
of the rock at depth, the rock breaks
at weak points, usually along planes
of weakness. The direction of these
planes of weakness can be predicted
to some extent based on the physical characteristics of the rock and
the orientation of stresses the rock is
under. During a hydraulic fracture,
the rock breaks, creating fractures,
or joints, which would, without
Figure 1. Taughannock Falls
Aerial view of the creek near Taughannock Falls near Ithaca, NY. Note the joint pattern in the
rocks. The J1 and J2 joint sets intersect at roughly 90˚, creating a pattern of repeating “corners”
in the rock. In these joints, the other side of the rock has fallen away, but beneath Earth’s surface, the joints are planar cracks that exist within the rock mass. This same jointing pattern is
exhibited at depth in the Marcellus Shale.
something to prop them open, naturally close when the water pressure
is subsequently lowered. The sand
or sand-like grains in the fracturing
fluid act as propping agents (prop-
2 / Marcellus Shale • Issue Number 5 • July 2011
pants) remaining in the joints after
the water is removed and the pressure
is lowered to keep the joint networks
open for natural gas extraction.
In sum, hydraulic fracturing
operates to open and maintain pathways for fluid to flow where it otherwise could not, and the process uses
chemicals that would be unsafe if
they entered drinking water aquifers
or streams. If hydraulic fracturing
were conducted near drinking water
aquifers or if the induced fractures
were sufficiently long or connected
to long natural fractures, it is conceivable that the process could also
connect new fracture networks with
pre-existing ones to create a pathway
for the natural gas, and the water and
chemicals used to release the gas, to
flow into drinking water aquifers.
Finally, there are other substances
trapped in the rocks around the Marcellus Shale, like brines and natural
gas, which could contaminate drinking water aquifers if a pathway was
created during the hydraulic fracturing process. For these reasons, it is
important to understand how the
naturally occurring joints and the
fractures stimulated by oil and gas
companies could interact in order
to assess the risk of drinking water
contamination in areas where drilling
is done.
Orogeny, a mountain-building event
that began around 350 million years
ago. This combination of forces created the majority of the joints in the
Marcellus Shale. Because the joints
were formed from the same processes,
most of the joints in the Marcellus
Shale run in roughly parallel “sets” in
one of two directions.
These two sets of joints are called
J1 and J2, and each has unique
characteristics. Joints in the J1 set
run east-northeast throughout the
Marcellus Shale. The J2 joint set
runs generally north-northwest and
cuts across the J1 joint set. The J1
joint set happens to coincide with
the modern underground stresses
found throughout the Northeast.
Joints in the J1 joint set are spaced
more closely together than those in
the J2 joint set. Joints in the J2 set
are more likely to have been healed,
which means the joint has been filled
in with minerals and is no longer a
pathway for gas flow. These healed
joints are still relatively weak because
the mineral cement gluing them
together is weaker than the original
bedding planes of the rock, and are
therefore places where new fractures
could grow.
Natural Fractures
In addition, the Marcellus Shale
The Marcellus Shale is a naturally was buried by numerous other rock
fractured rock because of the comlayers in the last 400 million years,
bination of the quantity of organic
some of which have been eroded
matter trapped in the rock and the
since the Alleghanian Orogeny
historical plat tectonic activity that
(in part by the numerous glaciaoccurred in the area. The conversion tions that have sculpted the current
from organic material to natural gas
landscape of the Northeast). These
in the Marcellus created pressure in
erosional forces have brought the
the fluids trapped in the rocks, which Marcellus Shale closer to the surface
helped created natural fractures in
of the Earth and thereby decreased
the rock, called joints. These joints
the downward pressure provided by
were further exacerbated by the colli- overlying rocks. As overlying weight
sion of plates during the Alleghanian that had compressed the ground was
released, much like a memory foam
mattress, the decrease in pressure
resulted in the J3 joint set. They, too,
run roughly parallel, and are referred
to as release joints and unloading
joints.1
Release and unloading joints created by the various unloading of rock
and ice are only present at or near
Earth’s surface. This is why J3 joints
are not found at the depth of Marcellus Shale natural gas extraction.2
How Stimulated
Hydraulic Fractures Form
Hydraulic fractures form when
the fluid pressure within the rock
unit exceeds the external pressures
pushing on the rock unit. To understand how hydraulic fractures could
propagate (be “stimulated”) in Marcellus Shale natural gas drilling, drilling engineers must understand the
pressures acting upon the rock.
Beneath the surface, there is
vertical pressure from the weight of
the overlying rock. There are also
horizontal pressures in all directions
pressing on the Marcellus Shale.
These pressures are the result of
plate tectonic forces interacting. The
Earth’s outer shell is made of very
large plates, like puzzle pieces, which
slowly jostle each other. The interactions of these plates can build mountains and cause earthquakes, but in
the Marcellus Shale region, they are
weaker than the vertical pressure
caused by the weight of the overlying rocks. Hydraulic fractures grow
perpendicular to the direction (the
plane) of minimum principle stress.
In the Marcellus Shale region, fractures made by hydraulic fracturing at
depth will have a vertical orientation
Marcellus Shale • Issue Number 5 • July 2011
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North
America
Juan de Fuca
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21
9
Eurasia
10
23
29
79
5
Nazca
46
10
2
11
Africa
South
America
Arabia
Philippines
54
Somalia
14
59
8
32
74
10
6
106
Pacific
Caribbean
Cocos
Indo-Australia
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Antarctica
Scotia
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and will separate against the minimum horizontal stress.
Because the vertical stress changes
with depth, the orientation of the
hydraulic fractures also changes as the
fractures move closer to the surface. In
sufficiently shallow rock, the direction
fractures propagate can leave the vertical plane and propagate on an angle
and could even approach the horizontal plane. So as fractures propagate
into shallower rock above, they become less likely to propagate vertically
and more likely to extend laterally
into the rock. The depth at which this
occurs varies by rock type and thickness and by the change in related pressures and is difficult to estimate.
While much of the Devonian
rock surrounding the Marcellus Shale
is also shale, there are many layers
of silt, limestone, and sandstone, as
well. Even the different shales surrounding the Marcellus are unique
in their composition. These different
kinds of rock layers are called litholo-
Figure 2. Map of current plate
boundaries throughout the world.
Arrows indicate the speed and direction of
plate movement, represented in millimeters
per year. Dots indicate the presence of earthquakes, which are concentrated along plate
boundaries. Notice how the dots concentrate
when two plates are moving toward each
other, and are less common where plates are
moving away from each other. This image
is courtesy of Incorporated Research Institutions for Seismology (IRIS), and is part of
their educational series of pamphlets about
earthquakes that can be accessed online at
http://www.iris.edu/hq/publications/brochures_and_onepagers/edu.
the physical characteristics of the
gies, and each lithology has its own
lithology and thus how it may react
unique set of physical characteristics
to hydraulic fracturing. In general,
that react differently to the pressures
limestones have higher fracture
that cause hydraulic fractures. Each
thresholds than shales and can act as
lithology has been deposited under
barriers to vertical fracture growth.3
slightly different marine and terresAs a fracture grows above or betrial conditions, which can affect its
grain size and provenance (where the low the Marcellus Shale, it encounters other lithologies, and this can
grains came from and what types of
play an important role in determingrains are present), composition of
ing the ultimate extent of a hydraulic
the mineral glue cementing grains
fracture. Lithologies often change
together, and the quantity of organabruptly, representics present in the
ing events or changshale. For example,
The stiffness of
ing environments in
some layers contain
the
Marcellus
Shale
is
Earth history; such
evidence of hurricane-force storms,
not uniform, so some distinct boundaries
between litholoand others record
areas
are
more
prone
gies are often more
times of little or no
weakly cemented
sediment deposito longer fractures
together than the
tion. Some shales
layers,
or beds, withthan
others.
are black due to a
in each lithology.
high quantity of
Because of this, these boundaries
organic material, while others with
are considered planes of weakness.
less organic material are grey. The
Layers, or beds, within a lithology
depositional conditions determine
4 / Marcellus Shale • Issue Number 5 • July 2011
can also be planes of weakness, but
these are usually more cohesive than
boundaries.
Boundaries play an important
role in the growth of hydraulic fractures, because individual fractures
tend to have greater difficulty propagating straight through them. Often
the boundary between two lithologies acts as a buffer, and the some
of the pressure causing a hydraulic
fracture is diverted to planes of weakness, radiating horizontally along the
boundary (because it is weaker than
the surrounding rock). As a result,
there is less pressure available to create a fracture in the overlying lithology. When the pressure creating the
fracture succeeds in extending into
the overlying lithology, the energy is
distributed in this new rock layer differently based on the characteristics
of the rock, and the direction and
pattern of the fracture can change.
Field geologists commonly note that
a set of fractures end at a boundary
between two lithologies, and sometimes use this characteristic to orient
themselves at a new outcrop.
Hydraulic fracture characteristics
are dramatically impacted by the measure of the modulus (ratio of stress to
strain) of the rock. If the modulus is
large, the rock is considered stiff. In
stiff rock units, fractures grow long
and narrow away from the source of
additional pressure, and in less stiff
materials, fractures are wider and
vertically shorter because the pressures causing the fractures can penetrate and dissipate further laterally
(into planes of weakness like bedding
planes and existing fractures) within
the rock unit. Shales are usually very
stiff, but highly fractured black shales,
like the Marcellus, are generally much
less stiff than unfractured grey shales.
The stiffness of the Marcellus Shale is
not uniform, so some areas are more
prone to longer fractures than others.
However, when a hydraulic fracture
hits the less stiff, already fractured
materials in the Marcellus Shale, the
energy can move laterally within the
shale and, as a result, cause shorter,
wider fractures.4,5,6
those fractures is expected to grow
significantly as additional data are
gathered. In some respects, the only
way to gather additional data is to
drill more wells and examine rock
carefully removed from the well (a
process called core sampling), so
some things could remain unknown
until after additional drilling has taken place. While this pamphlet highlights much of the base level understanding of Marcellus Shale jointing,
Understanding Current
some generalizations will be refined
Fracture Networks within the
only after engineers have collected
Marcellus Shale
and analyzed data from numerous
The Marcellus Shale is a natuwells and core samples throughout
rally-fractured gas shale, and the body the Marcellus.
of literature on the characteristics of
There are a number of physical
Figure 3. Fracture Propagation
Scenarios 1, 2, and 3 represent theoretically likely directions of fracture propagation resulting from increasingly higher levels of fracture-inducing force. As force increases in relatively
unfractured rock, fractures propagate perpendicular to the direction of maximum principle
stress. When they encounter a rock layer boundary (which is naturally weaker), the energy
forcing the fracture dissipates laterally, making it harder for a fracture to continue across
boundaries. 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. Because 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 rock
was relatively unfractured.
Relatively unfractured rock
Relatively highly fractured rock
Minimum
principal
stress
Minimum
principal
stress
1
2
3
3a
Marcellus Shale • Issue Number 5 • July 2011
/ 5
characteristics of the Marcellus Shale is not the only shale in the Hamilthat are not homogeneous. For
ton Group, it is the only uniformly
example, some geologists have hythick, gas-bearing, black shale. Black
pothesized that the lower part of the shales have some unique distinctions
Marcellus Shale, sometimes called
over grey shales, including a higher
the Union Springs shale, which has
quantity of organic material and a
a higher organic
higher amount of
content and
naturally-occurWhile there is
larger quantities
ring radioactive
consensus on basic
of pyrite (fool’s
material. Because
gold) than the
of the organic
fracture patterns
rest of the Marcontent of black
likely to occur in the
cellus, may have
shales, the nummore jointing
ber of joints in
Marcellus Shale as a
(as a result of
a square unit of
result of stimulation,
the maturing
rock (joint denorganics in
sity) is usually
the
shale
is
not
homoconcert with
much higher than
the Alleghanian
in grey, lower
geneous, and differorogeny). It
organic content
ent
regions
will
react
would thus have
shales, with spaca higher natural
ing usually less
differently to the same
interconnecthan one meter
hydraulic
fracturing
tion of fractures
apart in black
through which
shales.7 Because
treatment.
gas can move
of this and other
and a higher
shale characternatural gas yield. Because the eastern istics, many times the fractures in
part of the Marcellus Shale has been black shales consistently tend not to
subjected to greater plate tectonic
extend beyond lithologic boundaries
forces, jointing may be more abuninto overlying (or underlying) grey
dant in the east. But thinner units
shales.
are generally more fractured than
The Marcellus Shale represents
thicker units, and the Marcellus is
a dynamic system that has been
thinner in the west than it is in the
changing for almost 400 million
east, so jointing could be hypothyears. While there is consensus on
esized to be more prevalent in the
basic fracture patterns likely to occur
western portion of the Marcellus
in the Marcellus Shale as a result of
6
Shale.
stimulation, the shale is not homogeThe Marcellus Shale is the base
neous, and different regions will react
rock unit in a group of shales and
differently to the same hydraulic fraclimestones called the Hamilton
turing treatment. There will be areas
Group. The Hamilton Group is well with concentrated fractures and areas
known in New York State for the
almost devoid of them, and while
abundant fossils present in some of
we can predict where those might be
the rock units. While the Marcellus
based on our understanding of how
6 / Marcellus Shale • Issue Number 5 • July 2011
fractures form, there is not yet much
observational data to support those
predictions.
Tracking the Results of
Hydraulic Fracturing
Because hydraulic fracturing is
occurring beneath Earth’s surface,
the only way to track fractures resulting from a hydraulic fracture is to
use proxy evidence in the form of
models, field tests, and monitoring
production of wells, and incorporating that information into a body of
knowledge that grows as more wells
are drilled in the Marcellus region.
MODELING
Models are used to predict the
effectiveness of hydraulically fracturing wells in shale gas layers like
the Marcellus, and to inform future
hydraulic fracturing in nearby wells.
These models are based on data gathered in the field which are imported
into software programs to predict the
characteristics of the rock at depth.
Field data usually incorporated into
hydraulic fracture models include
seismic and microseismic data, well
log data, expected pressures throughout the subsurface, and other rock
characteristics. Models predict how
fracture fluid (and the associated
pressures and fractures already present in the rock) will act during hydraulic fracturing. When measured
values differ significantly from the
modeled values, the fluids are not
behaving in the way the model predicted they would and one or more
characteristics of the rock have not
been adequately modeled. This may
occur if the model itself needs to be
revised or if estimates of rock proper-
ties where the hydraulic fracturing
is done were not accurate. Thus, to
understand what is happening in the
rock below the surface, modeling is
used in concert with field testing to
assess most effective hydraulic fracturing treatment for different regions
in the Marcellus Shale.8,9,10
fracturing to explore the accuracy of
the model of fracturing in predicting
actual fracture patterns.
Microseismic testing works in
much the same way as seismic testing,
but small seismic monitors are placed
at the surface, or shallowly buried,
above the area undergoing hydraulic
fracturing. These monitors record
FIELD TESTS
small seismic changes and relay
Common field tests used initially them, frequently in real time, to the
to inform and eventually to truth
engineers conducting the hydraulic
fracture propagation models include fracturing treatment. This is the most
observing well logs, seismic testing,
effective way to visualize the fractures
sonic logs, gamma ray logs, resistivity that are created during a hydraulic
logs, and monitoring fluid injection
fracture and evaluate model effectiveand downhole pressures of the well
ness.12
during hydraulic fracturing, among
Well logs are descriptions of the
11
other techniques. Seismic testing is rock layers as observed by the drillers
a technique that can be run on the
that document the rock type, jointsurface of the Earth, but most teching, and other visual characteristics
niques require an observational well
of the rock. Sonic logs are similar in
to be drilled for monitoring purposes. principle to seismic tests, but use sonA few field techniques will be disic waves instead of sound waves. Also,
cussed below.
sonic logs are run down a well so
Seismic testing uses seismic
that, instead of from the surface, the
waves, created by ‘thumping’ the
sonic logs record information near
ground with heavy weights, to read
the well bore. Gamma ray logs also
the subsurface. The waves travel
run down the well bore, but these
through the ground, hit different
look specifically for natural radioaclayers of rock, fractured zones, and
tivity, which is much higher in black
other variations in density beneath
shales than in other rocks. Gamma
the surface of the Earth, and return
ray logs help drillers determine the
to the surface at different times. The
thickness and depth of the reservoir
waves are recorded and analyzed
rock in the region. Resistivity logs use
to create an image of the rock layvarious methods to extract the porosers beneath the surface. These tests
ity and fluid content of different rock
are usually run along roads or other
layers.
straight paths, and interpreted with
Observation wells are sometimes
other seismic tests to search for exist- drilled near a well undergoing hying fractures and other structures in
draulic fracturing while a region is
the subsurface that could potentially being established to collect data on
interact with a hydraulic fracturing
the effectiveness of hydraulic fracturtreatment. Because seismic testing
ing. A similar suite of field tests and
can be done from Earth’s surface,
observations can be conducted on an
it is sometimes used after hydraulic
observation well to more objectively
test the effectiveness of a hydraulic
fracturing treatment, the model used
to predict the fracture behavior, and
other parameters.13,14
Existing Fracture Networks
Natural gas has been rising toward areas of low pressure since it
formed in the Marcellus Shale, which
is part of the reason for the existing
fractures in the unit. Natural gas has
reached near-surface layers along
fracture networks where the Marcellus Shale (and other shales and limestones) comes within several hundred
feet of the surface. Some of these
seeps were sources for natural gas for
towns during the 19th century. This is
why methane can also be found rising from some abandoned, uncapped
wells.
The NYS DEC has recommended that stimulation more than
2,000 feet beneath the surface is deep
enough to avoid impacts on potable
water sources.15 Fluid and gas migration through fracture networks from
below 2,000 feet to the surface is not
theoretically impossible. A recent
study documented the presence of
thermogenic gas in water wells within
close proximity of active gas wells.16
Thermogenic gas, the kind of gas
found in the Marcellus Shale, is the
result of temperature and pressure
changes deep beneath Earth’s surface that change carbon matter into
methane. It has a different chemical
signature than biogenic natural gas,
which is produced by the metabolic
decay of organisms close to Earth’s
surface. This study correlates a high
percentage of thermogenic gas (as opposed to a mix of thermogenic and
biogenic gas) contamination with ac-
Marcellus Shale • Issue Number 5 • July 2011
/ 7
tive gas drilling, but does not explain
how the migration from deep rocks
has occurred. Migration pathways are
difficult to trace, and no conducted
studies have been able to confirm or
deny a fracture connection between
deep gas and drinking water aquifers.
This study did not record the
presence of fracturing fluid contamination or brine contamination in any
water well in close proximity to an
active gas well, which suggests that
faulty well casings are a more likely
pathway than deep fracture connections.17 The presence of natural gas
and other fluids at the depth of the
Marcellus Shale and in surrounding
layers suggests that existing fracture
networks have not allowed these substances to escape.
The study also noted that some
water wells that were not located
near active gas wells had methane
contamination. However, the amount
of methane in these wells was far
less than those water wells near active gas drilling, and the chemical
makeup of the methane was a mix
of thermogenic and biogenic natural
gas. The reason gas and other fluids
may migrate from fractures near the
surface–leaving a small amount of
mixed methane contamination–but
not from a mile beneath the surface
is likely related to the number of rock
units acting as barriers to migration
and the degree of discontinuous fractures passing from the Marcellus to
the surface.
In considering existing fracture
networks, it is also important to note
that Marcellus Shale drilling is not the
only type of drilling that has occurred
in NY and throughout the northeast.
Conventional oil and gas drilling has
commonly occurred throughout NY
for decades, and some of these wells
were improperly abandoned. At least
70,000 wells have been drilled in
NYS and only 30,000 are accounted
for by the DEC.18
When a well is no longer producing commercial quantities of oil
or natural gas, a company abandons
the well. Abandoning a well properly
involves removing some of the casing
and other surface equipment, plugging the well with cement so that
it is no longer a conduit for fluids
migrating from beneath the surface,
and then reclaiming the surface. If
wells are improperly abandoned, they
can act as a connection between the
surface and the depth of the well. In
most cases, improperly abandoned
wells are older and relatively shallow,
but when the Marcellus Shale is being exploited at shallower depths, like
in central NY, improperly abandoned
wells could potentially connect a
fracture network to a groundwater
source. Improperly abandoned wells
were considered when the DEC suggested that stimulation of Marcellus
Shale should be at least 3,000 feet
beneath the surface to sufficiently
decrease risk of contamination of potential groundwater sources.
Fracture networks are not the
only consideration of how fluids can
travel through rock layers to the surface. Because of the greater pressures
beneath the surface, fluid naturally
flows toward areas of lower pressure
if a pathway (through fractures or between connected pore spaces) exists.
Permeability (amount of connected
pore space) is measured in milliDarcies (after Darcy’s Law, which
describes how fluids flow through porous media). The rate at which a fluid
can flow through a rock, based on
8 / Marcellus Shale • Issue Number 5 • July 2011
the amount of fluid present, pressure,
fracture presence, and rock permeability, is called conductivity.
The conductivity of the Marcellus Shale is relatively low, but varies
between 0.000011 and 0.00059 feet
per day (just over 7/1000th of an inch
of movement per day at its fastest),
and the surrounding units also have
variable but low conductivities.17 It is
more common for fluid to flow horizontally than vertically in the Marcellus Shale because of the pressures at
depth and the existence of horizontal
bedding planes. However, it is not
impossible for pressures at depth to
create a gradient angled to some degree toward Earth’s surface in some
regions of the Marcellus or surrounding units. As the rock unit changes,
the direction of flow and conductivity can change.
Very little data is available on the
range of conductivities in the rocks
surrounding and including the Marcellus Shale in locations being examined for natural gas drilling, although
more is gathered by gas companies
as additional wells are drilled.18 The
impacts of the short-term increase in
pressure caused by hydraulic fracturing conductivity from the depth of
the Marcellus Shale to the surface is
not well understood, and could possibly play a role in the migration of
gas and fracture fluids to adjacent
rock units19 although are not likely to
provide sufficient pressure change to
migrate gas and fracture fluids all the
way into drinking water aquifers.
The conductivity of the Marcellus Shale and its overlying units
does not currently allow significant
methane migration from depths being
considered for natural gas extraction
to the surface. If that were the case,
the quantity of natural gas stored
throughout the Marcellus Shale
would have been greatly diminished
by now. We might assume, therefore,
that the conductivity provided by
existing fracture networks connecting
the Marcellus to much shallower units
is very low. Based on this reasoning, it
appears unlikely, though not theoretically impossible, that gas and fracture
fluid could migrate to potential sources of drinking water in this way.
Although it is expected that hydraulic fractures would propogate
upward little further than the top of
the Marcellus, one ought to consider
the potential impact if there were
sufficient pressure from hydrofracturing to connect to and open existing
fractures up to very shallow depths,
or to connect them with shallow, improperly abandoned wells. Fractures
remain open during pressurization
from hydrofracturing, for a few hours
at a time for up to a few days, then
remain open if proppant sand has
entered the fractures. If fractures did
connect to existing fracture networks,
they would not have sufficient proppant to keep them open after hydrau-
Table 1. Geological Characteristics of Coal and Marcellus Shale
Rock Characteristics
Coal Bed
Methane Extraction
Marcellus Shale
Methane Extraction
depth below surface
0–6,000 ft; most between 400–4,000 ft
3,000–10,000 ft
Aquifer location
mostly within 450 ft
0–1,000 ft (most less than 700 ft)
Surrounding rock
lithologies
variable; other coal seams, shale, sandstone
mostly shale, some silt and limestone lenses;
varies regionally
Thickness of
desired unit
inches to 250 ft thick lenses
one contiguous layer between 50–250 ft
thick; regionally variable
Original deposition
of rock unit
in terrestrial settings, among braided streams,
wetlands, and deltas
bottom of shallow, continental sea
Depth of targeted
formations for
hydraulic fracturing
0–6,000 ft; dependent upon target depth
3,000–10,000 ft; dependent upon
depth of unit
Pre-existing fractures
in desired rock unit
cleating present; degree of cleating
primarily based on coal type and depth
below surface
fractures present; degree of fracturing
based on organic content, rock unit
thickness, and proximity to tectonic
action; 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.21
Marcellus Shale information from Hill, et. al., 2002.12
Marcellus Shale • Issue Number 5 • July 2011
/ 9
lic fracturing was complete. To date
migration through such fractures of
fracturing fluids and produced water
has not been documented.20 This may
be because such vertical migration
paths do not or rarely exist, or because the amount of fracturing fluid
and proppant that can migrate vertically over such a distance is small.
were comprised primarily of marine
algae, coal beds are deposited in terrestrial environments and are usually
comprised of woody and leafy plant
matter. Coals generally form around
braided streams, wetlands, or delta
environments, and, as a result, an individual coal layer usually cannot be
traced laterally for a long distance. Instead, the environment conducive to
coal formation moves and migrates,
Understanding
like sand bars in a stream. The result
Fracture Networks in
is that a coal bed can sometimes be a
Different Rock Units
series of horizontal lenses interspersed
Different rock types, like sandin a vertical unit of rock. Coal can
stones, shales, and coal seams, have
also form in one distinct layer, but
different physical characteristics that even then the thickness of the coal is
impact the likelihood of the ability of highly variable. Coal deposits usually
fractures to grow upward toward the have a much smaller regional extent
surface from various depths. When
than the Marcellus Shale. This is bethey are sought after for fossil fuel
cause they are deposited in areas near
extraction, the depth of the rock unit streams, lakes, and rivers, which are
below the surface, the stiffness of the geographically smaller than shallow
rock unit being targeted and of the
continental seas.
surrounding lithologies, the regional
As a result of these geologic difstresses on the rocks, the depth of po- ferences, the target depth for coal bed
table water, and many other consider- methane extraction extends from very
ations are used to predict the impact shallow depths that sometimes interhydraulic fracturing will have on a
sect drinking water aquifers to depths
particular lithology.
that are similar to those for Marcellus
Recently, water well contamina- Shale gas drilling. The target depth
tion in coal bed methane extraction
for gas extraction has a narrow range,
using hydraulic fracturing has been
changing vertically–at most–around
used to compare the potential for
500 ft. in a 25 mile radius. However,
water well contamination in the
in a coal bed over the same 25 mile
Marcellus Shale. It is important to
radius, the target depth for methane
learn from the environmental probextraction can range from around
lems experienced in other active gas
400 to 4,000 feet, and multiple
plays; comparisons among different
depths may be targeted. Shallow coal
reservoir rocks, however, must also
beds (less than 450 ft deep) are not
consider the important geologic
only home to sources of methane, but
similarities and differences between
sometimes are the source of potential
them.
drinking water.21
While the organics in the MarShallowly buried coal is usually
cellus Shale were deposited at the
highly permeable, because as the
bottom of a shallow sea basin and
organic material matures fractures
10 / Marcellus Shale • Issue Number 5 • July 2011
form called cleats, which cause mined
coal to appear blocky. Cleats increase
coal permeability dramatically. This
is why fresh water can frequently
be found in shallow coal beds, and
why coal bed methane extraction
frequently involves what is called
“dewatering the coal.” This is also
why methane can flow easily within
the coal, especially at shallow depths.
When coal is more deeply buried,
however, the pressure of the overlying
rocks can close the cleats and lower
the permeability of the coal. Natural
gas companies prefer to extract gas
from shallower coal units because
of this. Because coal bed methane is
sought from a wide range of depths,
and because coals have a higher permeability, stimulated fractures are
usually intended to connect between
multiple coal layers.
Some similarities can be drawn
between coal bed methane extraction
and Marcellus Shale methane extraction, but in detail, the similarities are
few. Because of the different physical
characteristics that dictate how and
where natural gas is stored in the rock
types, the permeability of the rock
units, and their proximity to potential drinking water sources, the likelihood of fractures propagating beyond
their target zone(s) in coal bed methane extraction is much higher than in
the Marcellus Shale.
Summary
There has been significant community concern that high volume
hydraulic fracturing of the Marcellus
Shale may allow fluids–including
natural gas–to rise from connected
fracture networks several thousands
of feet below aquifers. In addition,
“Generic” Coal Bed Methane
“Typical” Marcellus in NYS
(Modeled in part after Powder River Basin)
SURFACE
Potable H2O level
1000'
2000'
GENESSEE GROUP
GAS BEARING COAL
Potable H2O level
COAL BED
CLEATING
(detail)
4000'
HAMILTON
GROUP
3000'
MARCELLUS
SHALE
Tully Limestone
Moscow Shale
Ludlowville Shale
Skaneateles Shale
Onondaga Limestone
5000'
Figure 4. Comparing
Coalbed and Marcellus Shale
Methane Extraction
Comparing a generalized depiction of the
environment from which coal bed methane is extracted to a generalized depiction of the Marcellus Shale in New York
State. 1) Gas-bearing coal deposits can
commonly be found at multiple depths
in the same region, including as shallow
as within the water table, and all of the
deposits are of interest to gas companies.
Coal lenses are cleated, which increases
their permeability. 2) In NYS, the Marcellus Shale averages around 100ft in
thickness, and–in areas presently being
considered for natural gas drilling–is
located between 3,000 and 5,000ft beneath the surface. Potable water deposits
in NYS are not deeper than 850ft, but
sources that can be treated and become
potable extend to 1,000ft beneath Earth’s
surface. See Table 1 for additional detail.
6000'
the Marcellus Shale has been deeply
fractured by several geologic events
since deposition of the shale. Where
the Marcellus Shale occurs near the
surface, for example in western New
York, natural gas has been known to
seep naturally into drinking water
aquifers and to the surface. Available
evidence suggests that deep shale
(several thousand feet) is not naturally connected to drinking water sources through fracture networks, likely
because of the discontinuity of fractures across numerous lithologies.22
It is not known to be impossible for
fluids to migrate to drinking water
sources from deep hydraulic fracturing, but the likelihood of significant
migration appears to be very low,
especially when compared to the risk
of faulty casings or surface spills as a
potential source of contamination.
Finally, the relative risk of hydraulic
fracturing varies substantially by local
geological context, including the nature and depth of source rock, lithology of overlying rocks, and the nature
of existing fractures and fault networks. Any comparison between the
Marcellus Shale and another source
of natural gas must be well framed
and consider these parameters.23
References
1. Hill, David, Tracy E. Lombardi, and
John P. Martin. 2002. Fractured shale
gas potential in New York. TICORA
Geosciences, Inc., Arvada, Colorado,
U.S.A.
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Redescal S. Uzcategui. 2009. Joint sets
that enhance production from Middle
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R.M. Pollastro, 2007, Unconventional
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Barnett Shale of north-central Texas as
one model for thermogenic shale-gas
assessment, AAPG Bulletin, v.91:4, pp.
475-499.
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4. O’Connor, K.M. and J.A. Siekmeier,
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Management and Effects of Coalbed
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Management and Effects of Coalbed
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Water in the Western United States,
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Zaman, Booker, and Gioda, Wiley
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John P. Martin. 2002. Fractured shale
gas potential in New York. TICORA
Geosciences, Inc., Arvada, Colorado,
U.S.A.
14. Raymond, M.S. and W.L. Leffler,
2006,Oil and Gas Production in NonTechnical Language, PennWell Corp.,
Tulsa, OK, 255 pp.
15. Hyne, N.J., 2001, Nontechnical
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16. New York State Department of Environmental Conservation, 2009, Draft
supplemental generic environmental
impact statement on the oil, gas, and
solution 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 NonTechnical Language, PennWell Corp.,
Tulsa, OK, 255 pp.
18. Osborn, S.G., A. Vengosh, N.R. Warner, and R.B. Jackson, in press, Methane contamination of drinking water
accompanying gas-well drilling and
hydraulic fracturing, PNAS.
19. New York State Department of Environmental Conservation, 2009, Draft
supplemental generic environmental
impact statement on the oil, gas, and
solution mining regulatory program.
http://www.dec.ny.gov/energy/58440.
html (accessed March, 2011).
20. ICF International, 2009, Technical
Assistance for the Draft Supplemental
Generic EIS: Oil, Gas and Solution
Mining Regulatory Program Well
Permit Issuance for Horizontal Drilling and High-Volume Hydraulic
Fracturing to Develop the Marcellus
Shale and Other Low Permeability Gas
Reservoirs, Agreement #9679, NYSERDA, Albany, NY.
21. Myers, 2009, Review and Analysis of
DRAFT Supplemental Generic Environmental Impact Statement On The
Oil, Gas and Solution Mining Regulatory Program Well Permit Issuance for
Horizontal Drilling and High-Volume
Hydraulic Fracturing to Develop the
Marcellus Shale and Other Low-Permeability Gas Reservoirs, Natural Resources Defense Council, New York, NY.
22. Osborn, S.G., A. Vengosh, N.R. Warner, and R.B. Jackson, in press, Methane contamination of drinking water
accompanying gas-well drilling and
hydraulic fracturing, PNAS.
23. National Research Council, 2010,
Management and Effects of Coalbed
Methane Development and Produced
Water in the Western United States,
National Academies Academic Press,
Washington, D.C., 217 pp.
Partnering Organizations include
Funded by NSF GEO No. 1016359.
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.
Any opinions, findings, and conclusions or
recommendations expressed in this material
are those of the author(s) and do not necessarily reflect the views of the National
Science Foundation.
Authored by Trisha A. Smrecak and the PRI Marcellus Shale Team
Illustrations by J. Houghton unless otherwise attributed.
12 / Marcellus Shale • Issue Number 5 • July 2011