Page 357 - Fundamentals of Gas Shale Reservoirs

REFERENCES 337
carbon (TOC) of 43.9% at 45°C on a dry and ash‐free because gases are shown to be primarily adsorbed on the
basis, and of 0.43 and 0.46 mmol/g on two other coal minerals in single layers.
samples with undetermined TOC at 35°C.
Coal with 3 wt% moist has the lowest adsorption capacity
as compared to that of 1.2% moist and dry coal; and at ACKNOWLEDGMENTS
pressure of 10 MPa the reduction of adsorption capacity in
reference to dry coal to coal with 1.2 and 3.0% moist are 10 This work is financially funded by the Research Institute
and 30%, respectively. of Petroleum Exploration and Development, PetroChina
through the “Element and process constraint petroleum
system modeling” project (No. 2011A‐0207) under the
15.3 CONCLUSIONS PetroChina Science Innovation program. We are grateful to
many colleagues for their support and encouragement for
MS comprising MD and MC simulations can mimic physical the research work including Dr Junfang Zhang of CSIRO
movements of interacting atoms and molecules in a complex Division of Earth Science and Resource Engineering and
system. In MS, the trajectories of atoms and molecules are Prof. Xiancai Lu of Nanjing University.
determined by numerically solving the Newton’s equations
of motion, where forces between atoms and molecules are
defined by fields of molecular mechanics force. MS can deal NOMENCLATURE
with a vast number of particles and associated properties in
complex systems numerically and make it ideal for simu­ RDF, also called pair correlation function, is defined as the
lating gas adsorption in porous medium. The MS of the ratio of the number of atoms at a distance r from a given
adsorption of natural gas, CH and CO on quartz, montmo­ atom compared with the number of atoms at the same dis­
4
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rillonite (Wyoming‐type), zeolite (FAU), and coal under tance in an ideal gas with the same density.
various reservoir conditions has shown that MS can generate RASPA 1.0 is a computer program for MC molecular
adsorption isotherms for both minerals and coal that comple­ simulations developed by Dubbeldam and coauthors from
ments the laboratory measurements and provide the follow­ the Computational Chemistry Group, the University of
ing insight for understanding gas occurrence and phase Amsterdam. The software is described in Dubbeldam, S.
behavior in shales: Calero, D. Ellis, R. Snurr, “RASPA, 1.0: Molecular Software
Package for Adsorption and Diffusion in Nanoporous
1. Gas (C –C ) adsorbed on minerals (e.g., SiO and Materials,” in Northwestern University, Evanston 2008.
2
1
3
clay) in single layer; GROMACS is a computer software for MD simulation
2. C H and C H have comparatively stronger adsorp­ developed by Eril Lindahl and coauthors (Lindahl et al.,
8
3
6
2
tion capacity than CH at low pressure; 2001) and is described by in the widely cited paper
4 “GROMACS 3.0: a package for MS and trajectory analysis,”
3. CH adsorption increases with pore/channel sizes bet­
4
ween 1 and 2 nm; in Molecular Modelling Annual 7 (8) 306–317.
PRODRG is a server can take a description of a small
4. At high pressure (>30 MPa), temperature has little molecule (as PDB coordinates/MDL Molfile/SYBYL Mol2
effect on the CH adsorption capacity;
4 file/text drawing) and from it generate a variety of topologies
5. Both CH and CO have similar adsorption capacity at for use with GROMACS, WHAT IF, Autodock, HEX, CNS,
4
2
high pressure; REFMAC5, SHELX, O and other programs, as well as
6. Water has an adverse effect on gas adsorption in min­ energy‐minimized coordinates in a variety of formats.
erals and coal. (http://davapc1.bioch.dundee.ac.uk/cgi‐bin/prodrg)
MS: Molecular simulation
In relation to the shale gas resource assessment (i.e., adsorp­ MD: Molecular dynamics
tion capacity), the simulation indicates that pressure has MC: Monte Carlo
strong effect on the gas adsorption capacity as compared GCMC: Grand canonical statistical ensemble
with temperature. In fact at high pressure, temperature does
not appear to have any effect on the gas adsorption capacity.
At high pressure, the contribution of the free gas in the pore REFERENCES
space becomes significant due to the increased bulk density.
Natural gas can enter into nanochannels of 1 nm in width. Allen MP. Computer Simulation of Liquids. Oxford: Clarendon
Water‐wet channels are conducive to gas migration and Press; 1987.
transport due to the adverse effect of water on gas adsorp­ Allen MP, Tildesley DJ. Computer Simulation of Liquids. Oxford:
tion. Gas can move freely even in nanometer channels Clarendon Press; 1989.