Page 265 - Fundamentals of Gas Shale Reservoirs

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GAS TRANSPORT PROCESSES IN SHALE

Farzam Javadpour and Amin Ettehadtavakkol 2
1
1 Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin, Austin, TX, USA
2 Bob L. Herd Department of Petroleum Engineering, Texas Tech University, Lubbock, TX, USA

11.1 INTRODUCTION materials (kerogen) and clays (Javadpour et al., 2007).
In shale gas reservoirs, gas flows through a network of
Nanoscience is science at tiny scales. For many years, high‐tech pores with different diameters, ranging from nanometers
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industries such as microelectronics and bioengineering (1 nm = 10 m) to micrometers (1 µm = 10 m) (Javadpour
systems have benefitted and continue to benefit from new et al., 2012; Loucks et al., 2012; Milliken et al., 2012).
advances in nanoscience. The governing physics at the Figure 11.1 shows exemplary pore images of a sandstone
nanoscale are different from those observed at the large or sample (left) and a shale sample (right) for comparison of
continuum scale. The differences bring new characteristics pore distribution in conventional and unconventional (shale)
and, hence, new applications. Recently, nanoscale character- reservoirs. The average size of the pores is much smaller and
istics of natural systems—in particular, shale gas—have the number of pores is much higher in the shale sample. In
ushered in a new era of nanoscience fossil energy from shale gas systems, nanopores play two important roles. First,
geological systems. Pores in shale gas strata are at the for the same pore volume, the exposed surface area in nano-
nanometer scale, and the physics of fluid transport in these pores is larger than in micropores. This disparity results
pores are different from those described by well‐known because exposed surface area is proportional to 4/d (the
formulations such as the Darcy equation. The petroleum inner surface of a tube divided by the tube volume, where d
industry has extensive experience in characterizing pores is the pore diameter). This large exposed area permits the
and modeling fluid flow in the pore networks of hydrocarbon‐ desorption of large volumes of gas from the surface of the
bearing reservoirs. However, for pores at the nanoscale in nanopores in kerogen. Diffusion from the kerogen bulk to
shale systems, the characterizing methods need modification the nanopores’ inner surfaces may also take place.
and, in many cases, entirely new methods of characteriza- Consequently, high mass transfer of gas molecules occurs
tions are needed. Characterizing pore networks in these sys- inside the bulk kerogen. Second, the applying physics of
tems and developing new formulations for fluid flow in such gas flow in nanopores does not conform to Darcy flow
systems are of great importance and interest. (Akkutlu and Fathi, 2012; Civan, 2010; Darabi et al., 2012;
In addition to having much smaller pore size than conven- Javadpour, 2009 Singh et al., 2014).
tional gas reservoirs, shale gas also differs in that the source Figure 11.2a–e illustrates the sequence of gas production
of the initial gas in place (IGIP) in shale is more complicated at different length scales. Gas production from a new hydrau-
than the gas source in conventional gas reservoirs. The lically fractured wellbore (i) takes place through first the
controlling mechanisms of gas storage and flow in shale gas larger pores, induced microfractures, and larger fracture
sediments are a combination of different processes. Gas is conduits (ii); and then through the smaller pores (iii). During
stored as compressed gas in pores (free gas), as gas adsorbed reservoir depletion, the thermodynamic equilibrium between
to the pore walls, and as dissolved gas in amorphous organic kerogen/clays and the gas phase in the pore spaces changes.

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