vxEPA
United States
Environmental Protection
Agency
Office of Environmental Engineering
and Technology
Washington DC 20460
Research and Development
EPA-600/S7-82-053 Jan. 1983
Project Summary
Unconventional Natural Gas
Resources: An Overview
Covering the Resources and
Environmental Aspects of
Production
L. Hoffman
As part of our Nation's study of
alternative means to achieve energy
independence, especially over the
critical mid-term period from 1985 to
approximately the year 2000, an
overview of unconventional sources
of natural gas, with emphasis on
associated environmental aspects,
was produced.
Natural gas is the cleanest fuel at the
point of utilization and is currently the
major fuel for residential heating, a
very dispersed application that en-
vironmentally is relatively unregu-
lated. It is therefore in the interest of
those having the major responsibility
for insuring a clean environment to
encourage the use of this clean
burning fuel. In order to accomplish
this goal we must know the extent of
natural gas resources, including the
unconventional sources that may
supply much of this fuel by the year
2000. and any significant environ-
mental consequences of producing
gas from unconventional sources.
This Project Summary was devel-
oped by EPA's Office of Environ-
mental Engineering and Technology,
Washington. DC. to announce key
findings of the research project that is
fully documented in a separate report
of the same title (see Project Report
ordering information at back).
Introduction
Natural gas is a mixture of the low-
molecular weight paraffin-series hydro-
carbons consisting mainly of methane,
with lesser quantities of ethane, pro-
pane and butane and small amounts of
higher hydrocarbons. It also frequently
contains small proportions of nitrogen,
carbon dioxide, hydrogen sulfide and
occasionally helium. Sour natural gas
contains objectionable amounts of
hydrogen sulfide and other sulfur
compounds. Natural gas accompanying
petroleum always contains appreciable
quantities of ethane, propane, and
butane as well as some pentane and
hexane vapors and is known as wet gas.
Dry gas contains less of these higher
hydrocarbons. The exact composition of
natural gas varies with locality. The
heating value of natural gas is usually
over 1,000 Btu/cu ft unless nitrogen or
carbon dioxide are important compo-
nents of the gas. Natural gas is used
directly as a fuel and as a chemical
feedstock. When the gas comes from an
oil well, its higher hydrocarbon content
is often extracted and used for blending
in motor fuel and as a liquefied gas To
be acceptable for injection into a
pipeline, gas must meet standards
acceptable to the pipeline carrier.
Natural gas reserves are located in
underground formations that can be
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economically and legally tapped at the
time of discovery. Proved natural gas
reserves may be divided into drilled, i.e.
producible by means of existing opera-
ting practices, and undrilled, i.e. under-
lying areas which are so related to
developed acreage and to the known
geology and structure that their produc-
tive ability is considered assured. The
term "reserves" connotes gas deposits
of near-term economic interest, where-
as the term "resources" embraces
marginal, sub-marginal, and latent
deposits as well as reserves. As the
price level increases, a portion of the
resource base is transferred to the
reserve base. Even though the natural
gas reserve level is well defined, there is
considerable uncertainty about the
extent of remaining recoverable natural
gas resources.
Table 1 indicates our natural gas
reserves as of year-end 1979. The bulk
of these are found in high permeability
sandstone and carbonate reservoirs
that are generally located in the Gulf
Coast and Southwest regions of the
United States.
In addition to conventional natural-
gas resources, a number of unconven-
tional gas resources may augment our
natural gas supply. The report addresses
the potential of unconventional gas
sources for providing significant levels
of industrial and pipeline quality gas.
The following sources are addressed:
• The gas-bearing Devonian shales
of the eastern United States (eastern
gas shales),
• The low-permeability (tight) gas
sandstones of the Rocky Mountain
region (western gas sands),
• The free methane (natural gas)
present within coal seams (methane
from coal),
Table 1. Year-end 1979 Natural Gas Reserves
Proved Reserves
Reserve Additions in 1979
Production in 1979
Reserve to Production Ratio
Lower-48 States
163.0 Tcf
13.7Tcf
19.7 Tcf
8.3 years
Total U.S.
194.9 Tcf
14.3 Tcf
19.9 Tcf
9.8 years
Table 2. Resource Base of Unconventional Sources of Natural Gas
Base (Tcf}
Source
In-place
Recoverable
Western Tight Sands
Devonian Shale
Coal Seam Methane
Geopressured Methane
Total
50-600
75-700
70-860
3.OOO-5O.OOO
3.200-52,000
25-310 (170)
10-500 (30-285)
15-490 (300)
150-2.OOO (15O)
200-3,000 (650-905)
Table 3. Potential Gas Supply from Unconventional Sources — 7355 and 200O
Supply (Tcf)
Source
Western Tight Sands
Devonian Shale
Coal Seam Methane
Geopressured Methane
Total
1985
0.35-0.78
0.12-0.26
0.3 -0.8
O
0.75-2.0
2000
3.0 - 6.9
1.0 - 2.3
2.5 - 5.6
0.22- 0.5
7.0 -15.0
Table 4, Recent Production from Unconventional Resources
Recent Production
Resource
Western Tight Sands
Devonian Shale
Coal Seam Methane
Geopressurized Aquifers
Recovered Landfill Gas
Level (Tcf/yr)
0.10
0.15
(none identified)
(none identified)
Less than 0.004
Comment
In 1980
In 1979 from
9600 wells
In 1980
• The high-pressure, methane-sat-
urated saltwater aquifers of the
Gulf Coast region (geopressured
aquifers), and
• Landfills where residential, com-
mercial, and industrial waste have
been disposed.
Gas from landfills is not generally
categorized as unconventional. How-
ever, since this gas is currently being
commercially extracted, it is included.
Discussion
Natural gas currently supplies ap-
proximately 27 percent of the energy
consumed in the United States. Since
1970, the proved U.S. natural gas
reserves have been declining, with
production exceeding additions. Cur-
rently the proved reserve to annual
consumption ratio is less than 10. The
Energy Information Administration
indicates that over the last 15 years, the
return-to-drilling value (reserve addi-
tions per foot drilled) has rapidly
declined. This implies that unconven-
tional gas resources will needtobeused
to support our natural gas requirement.
The potential contribution of uncon-
ventional gas to our eventual supply is
unknown. Unconventional sources
have not been well exploited. They
include gas-bearing geologic forma-
tions that are hard to reach and/or low-
permeability formations that must await
technical and economical feasibility.
Large uncertainties include: (1) the
magnitude of the gas resource con-
tained in these sources, (2J the capabil-
ity to extract the gas, and (3) the
economics of extraction. Estimates of
the resource base, the reserves, and the
potential gas supply in the foreseeable
future (i.e. to year 2000) are widely
varied. The low- and high-estimates
clearly indicate that knowledge of the
geology and feasibility of unconven-
tionals is tenuous. It should be noted
that relatively small amounts of gas
have been and are currently being
produced from unconventional resources.
Tables 2,3, and 4, based on references
and considerations contained in the full
report, indicate the uncertainty in both
the resource base and the potential gas
supply from these sources.
In general, gas obtained from the
above unconventional sources is either
of pipeline quality or can be upgradedto
pipeline quality through the use of
existing, commercially available
technology.
Pipeline and near-pipeline-quality
gas is currently being obtained from
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Devonian shales and western (tight) gas
sands. The well-completion techniques
(e.g. explosive shooting and hydraulic
fracturing) have been used for some
time and environmental considerations
have not inhibited well development. In
addition, small amounts of gas have
been obtained from landfills and coal
deposits. To date, no gas has been
obtained from geopressured aquifers.
Of the considered unconventional gas
resources, only methane-from-coal and
gas from geopressured aquifers appear
to pose any associated environmental
concerns of significance.
Many factors could impede the
recovery of methane from coalbeds.
These include legal considerations (e.g.
ownership of gas) and the effects of gas
recovery on the mineability of the
inplace coal.
Since coal beds are hard to penetrate,
production of methane from coal seams
at commercial flow rates is impeded.
The pockets containing gas must be
tapped and the flow of gas stimulated
(e.g. by means of hydraulic fracturing).
The acceptable disposal of the waste-
waters that have collected in the gas
pockets is an environmental problem.
Although the amount of wastewater
produced from the operations has not
been quantified, data describing the
water quality from three coalbed
operations indicate that the two largest
constituents are chlorides and dissolved
solids, and calcium and magnesium are
other important components. Tech-
niques normally used for treating
wastewaters are applicable to this
problem.
The environmental impacts of gas
extraction from geopressured aquifers
are generally thermal and mineral
pollution, and subsidence. One esti-
mate suggests that five trillion barrels
of brines would ultimately be produced
from geopressurized aquifers. This is
the equivalent of the total two-year flow
of the Mississippi River. The brines are
known to be saline, extremely hot, and
to contain boron, which, in minute
quantities, is toxic to marine life.
Economic utilization of gas from
geopressured aquifers dictates the
extraction of large volumes of fluids.
The extraction of these fluids will cause
a reduction in reservoir pressure and
an increase in effective stress in the
reservoir framework. In those areas of
the Gulf Coastal Plain where ground-
water extracted from shallow aquifers
has caused stress changes, surface
subsidence often resulted. Consequent-
ly, the major environmental concerns
associated with the development of
geopressured resources relate to the
possibility of subsidence of land surface
in the immediate vicinity of the produc-
ing wells and the disposal of the large
volume of produced brine.
Landfill gas is typically comprised of
about 50% methane and 50% carbon
dioxide, and the percentage by volume
of both can vary widely. Nitrogen and
oxygen are normally present in small
amounts as a result of air being trapped
as the waste is deposited or as a result
of a negative internal pressure when
landfill gas is extracted. In addition,
there are trace amounts of numerous
compounds in the landfill gas.
If raw landfill gas is to be used for
space heating or hot water heating, the
only processing required is simple
water and particulate removal. An
elaborate and expensive control tech-
nology is unnecessary. Furthermore,
since such uses do not require large
volumes of gas, many smaller landfills
across the country may be tapped.
Landfill gas, upgraded to pipeline
quality (i.e. to close to 1,000 Btu), can be
used interchangeably with natural gas.
Consequently, the gas from most large
landfills may be applied as a substitute
for natural gas. In addition to moisture
and particulate removal, the processing
of landfill gas upgraded to pipeline
quality would include COz removal.
Although the technology necessary to
achieve this quality is relatively straight-
forward and relatively available, large
capital expenditures are involved.
In general, the degree of upgrading
and/or cleanup required for gases from
the considered unconventional sources
are as follows:
Gas Source Gas Cleanup Required
Western Gas
Sands
Devonian Shale
Coal Seams
Geopressured
Aquifers
Landfills
None to minimal
None to minimal
None to moderate
Possibly moderate
or greater
Minimal to moderate,
depending on use
A mature technology base and nu-
merous processes are available for
upgrading gas quality and removing
undesirable components such as HaO,
CO2, H2S, carbonyl sulfide (COS) from
gases. The removed sulfur compounds
may be converted to marketable ele-
mental sulfur or sulfuric acid or may be
disposed of in accordance with environ-
mental regulations. In this regard, many
proprietary and non-proprietary pro-
cesses continue to be used.
L. Hoffman is with Hoffman-Holt, Inc., Silver Spring, MD 20910.
William N. McCarthy, Jr. and Morris H. Altschuler are the EPA Project Officers
(see below).
The complete report, entitled "Unconventional Natural Gas Resources: An
Overview Covering the Resources and Environmental Aspects of Production,"
(Order No. PB 82-260 886; Cost: $7.50, subject to change) will be available
only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officers can be contacted at:
Office of Environmental Engineering and Technology
U.S. Environmental Protection Agency
Washington. DC 20460
•tt U.S GOVERNMENT PRINTING OFFICE. 1983 659-OI7/O89O
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