Page 67 - Fundamentals of Gas Shale Reservoirs

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GEOCHEMICAL ASSESSMENT OF UNCONVENTIONAL
SHALE GAS RESOURCE SYSTEMS

Daniel M. Jarvie
Worldwide Geochemistry, LLC, Humble, TX, USA

3.1 INTRODUCTION whereas unconventional systems are considered to be less
than 0.10 mD. Shale gas systems often have permeabilities
The global search for shale gas resource systems is based on less than 100 nD such as found in the Barnett Shale of the
the astounding success in North America as up to 750 trillion Fort Worth Basin, Texas (Loucks et al., 2009; Reed and
cubic feet (tcf) is estimated to be technically recoverable Loucks, 2007).
according to the US Energy Information Administration Mudstone and shale nomenclature are used interchange­
(EIA, 2011). The current surplus of gas due primarily to shale ably in this text and are not necessarily referring to principal
gas production has kept natural gas prices modest, aiding mineralogical components or even particle size. Most of the
American industries and consumers as well as aiding the US ongoing plays are mudstones based on particle size, but may
quest for lowered dependence on overseas sources of energy. have quite variable mineralogical contents.
In addition, the United States reported the lowest level of What shale resource systems lack in porosity and perme­
carbon dioxide (CO ) emissions in 20 years (EIA, 2012). ability, they counter with massive areal and volumetric
2
Shale gas resource systems are typically characterized by extents. One of their defining characteristics of an uncon­
organic‐rich, gas‐window mature, very low porosity, and ventional resource is being a continuous accumulation over
ultralow permeability mudstones. Typical storage capacities extensive distances that generally do not have obvious struc­
range from about 4–14% porosity and is a combination of tural components as targets (Zou, 2012). They may extend
porosity created by organic matter decomposition (organopo­ over thousands of square miles with thicknesses ranging
rosity) as well as classical storage (e.g., matrix porosity). from tens to hundreds of feet. This volume of continuous
It is also possible that some storage is in and among atomic mudstone can store trillions of cubic feet of gas, despite
pores within the organic matter itself (Locke and Winans, their overall low porosity when reservoir PVT properties are
2013; Orendt et al., 2011) due to sorption of generated petro­ considered (Jarvie et al., 2007). However, due to this storage
leum. Additional storage is often found along expulsion con­ and ultra‐low permeability, it is necessary to shatter the rock
duits and matrix pores. Fractures are present but are commonly matrix, enabling gas flow to reach the well bore. This is
calcite‐filled and do not contribute significant storage volume. achieved by high‐energy stimulation, that is, pumping high
Nanodarcy permeability is predominant in these plays rates of slick water (freshwater with surfactants) with high
with values ranging from tens to hundreds of nanodarcies. amounts of proppant to open and prop open fractures as
Such low permeability restricts the flow of petroleum par­ well as diverting energy, optimally creating a dendritic
ticularly compared to conventional reservoir rocks that fracture network with extensive reservoir volume contact.
have five to nine orders magnitude higher permeabilities. In Nonetheless, flow from shale gas reservoirs is typically very
this chapter, conventional reservoir systems are considered low when compared to conventional gas reservoir rocks.
to be equal to or greater than 0.10 mD (Williams, 2012), In order to be able to develop these systems commercially,

Fundamentals of Gas Shale Reservoirs, First Edition. Edited by Reza Rezaee.
© 2015 John Wiley & Sons, Inc. Published 2015 by John Wiley & Sons, Inc.