Page 346 - Fundamentals of Gas Shale Reservoirs

326 MOLECULAR SIMULATION OF GAS ADSORPTION IN MINERALS AND COAL

extremely difficult despite the huge reserves predicted by
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EIA (2013) in 137 shale formations of over 40 countries.
Shale gas is known to store primarily in pores of nanometer
Coordinate initialization scale (1–100 nm; e.g., Curtis et al., 2012; Loucks et al., 2012;
Slatt and O’Brien, 2011; Zou et al., 2010) and in adsorbed
phase on organics and minerals. This poses enormous
challenges for the traditional macroscopic characterization
Velocity initialization
and conventional laboratory experiments regarding the
occurrence and phase behavior under reservoir conditions.

Calculate forces
15.1.3 MS of Gas Adsorption

Update coordinates and MS has been used as a powerful tool for solving scientific
velocity and engineering problems and has found extensive applica­
tions in the studies of equilibrium thermodynamics of fluids
Unsatis ed involving phase transition of bulk fluids (Harris and Yung,
Check equilibrium 1995; Wei and Sadus, 2000), nanofluid in confined geometry
(Todd and Daivis, 2000; Zhang and Todd, 2004; Zhang et al.,
Satis ed 2007), gas adsorption on porous materials (Aukett et al., 1992;
Compute properties Sweatman and Quirke, 2001a, b; Zhang et al., 2012), fluid
of interest interfaces with clays (Chang et al., 1998; Skipper et al.,
1995a, b), gas adsorption in coal (Brochard et al., 2012;
Hu et al., 2010; Tambach et al., 2009), hydrate formation
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(Duffy et al., 2004; Moon et al., 2003; Rodger et al., 1996;
FIGURE 15.1 Workflow and procedure of performing molecular Zhang et al., 2008), multicomponent gas separation (Ghoufi
dynamic simulations. et al., 2009; Hamon et al., 2009), and so on.
To adequately evaluate shale gas storage in shale and
but computationally very intensive tool for studies of organic matter, equilibrium adsorption methods can be
chemical and biomolecular systems and has been widely used to characterize those nanoporous materials and quan­
applied to various fields including geochemistry, chemical tify the storage capacity of gas (Gensterblum et al., 2014). In
engineering, materials science, medical science, petroleum laboratory experiments, volumetric methods and gravimetric
science, and so on, to understand and characterize the prop­ methods are most commonly used for high pressure adsorp­
erties of molecular systems. tion. The amount of adsorbed gas is the difference between
that admitted to the sample container and that remaining
in the dead space (or isolated pores). The dead space refers
15.1.2 Major Challenges in Shale Gas Research
to the void space and the volume of the connecting tubing
Shale gas is presently a research focus in the world largely (pore throat). In analyzing a shale reservoir rock, a distinc­
sparked from the recently shale gas revolution in the tion is made between the stored free gas, which satisfies as
United States. Shale gas was first discovered in the Devonian a function of pressure using an equation of state such as
Ohio Shale of the Appalachian Basin, USA, in 1821. the compressibility equation of state, and the adsorbed gas,
However, it was not produced commercially on a large which satisfies as a function of pressure using an adsorption
scale until a decade ago owing to technology breakthrough equation such as the Langmuir equation. The dead space is
in horizontal drilling and hydraulic fracturing, as well as the volume that is occupied by the free gas. It is a combination
the demand of cleaner fossil fuels. So far, over 20 shale of volumes contained in the equipment such as pipes and the
gas fields have been discovered in 11 basins in the United part of the sample pore volume not occupied by adsorbed
States (Energy Information Administration (EIA), 2013). gas. Isolated pores by their definition are not part of the dead
Many countries have now embarked on shale gas R&D and space as no gas can enter them.
exploration attempting to replicate the successful story in the To accurately determine the amount of the adsorbed
United States. gas, the dead space needs to be experimentally obtained as
Because of the complexity of geological conditions of a slight change in the dead space value might shift the high
shale gas reservoirs, the distribution of the shale gas and pressure part of the adsorption isotherms upward or down­
its abundance vary greatly from basin to basin and between ward. The void volume is usually estimated by the helium
different shale formations. This makes the search for expansion method prior to the introduction of gas into the
commercially viable shale gas plays and their development sample cell. During the adsorption isotherm measurement,