Environmental Engineering Reference
In-Depth Information
2.1 Shale Gas
Shale gas resource plays are distinguished by gas type and ''system'' char-
acteristics,
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typically with either biogenic or thermogenic gas production
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from low porosity and permeability shales, forming a self-contained source -
reservoir - seal system.
Biogenic gas shales are thermally immature (vitrinite reflectance (VR)
o
0.5% Ro) due to only shallow burial, typically with formation temperatures
of 40-80 1C. At temperatures above 80 1C most bacteria are destroyed by
''pasteurisation'' and thermally generated gas becomes predominant. Bio-
genic gas shales include the Antrim Shale Formation in the Michigan Basin,
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and the shallow buried sections of the New Albany Shale, typified by dry gas
adsorbed onto organic matter with, after de-watering, modest initial pro-
duction (IP) rates of 40-500 million cubic feet of gas per day (mmcf d
1
)with
long production of over 30 years. Thermogenic shale gas plays range in ma-
turity from early gas-window such as the New Albany Shale to the very prolific
late gas-window plays such as the Barnett, Marcellus and Haynesville Shales,
with EURs for the Barnett Shale in excess of 2.5-3.5 billion cubic feet (BCF)
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and 5-7 BCF in the high-maturity Marcellus Shale in northern Pennsylvania.
Most shale-gas plays have been affected by tectonic uplift, which effect-
ively shuts off the hydrocarbon generation process by lifting them out of the
gas generation ''window''. Due to the impermeable character of the shales,
most of the gas is retained in the rock unless released by fracturing or
faulting related to the tectonic activity. This has two benefits: the gas is
generally overpressured in uplifted reservoirs and the cost of drilling to
reach the shales is reduced.
2.2 Shale (Tight) Oil
Shale or 'tight' oil is a more recent concept, with large potential in the USA in
formations such as the Devonian Bakken Shale and the Cretaceous Niobrara,
Mowry and Eagle Ford Shales. While the basic principles are similar to that
of shale gas, the much larger oil molecule size relative to pore throat
diameter results in large capillarity effects which greatly restrict the flow
capabilities of these shales. To be effective, shale oil plays require what are
described as hybrid reservoirs which comprise a coarser grained, more-
permeable lithology intimately associated with the shale source rock in
which the production wells can drilled and completed. In the case of the
Bakken Shale, the hybrid reservoir comprises a limestone unit which is
sealed between the Lower and Upper Bakken source rock shales (see Figure
5). Lateral wells are completed within this limestone unit to create a per-
meable reservoir that can effectively flow the oil charge from the adjacent
shales. Thin interbeds of more permeable lithologies, such as siltstones and
limestones within the shale source rock, can also enhance the shale pro-
duction capabilities, examples being limestones in the Eagle Ford Shale (see
Figure 6) and siltstones in the Mowry Shale. In these cases, oil production is
