Page 288 - Fundamentals of Gas Shale Reservoirs

268 A REVIEW OF THE CRITICAL ISSUES SURROUNDING

for free hydrocarbon storage and transport, so parameters S i
such as permeability and free gas porosity will be explicit V R (12.3)
functions of pore pressure, not just effective pressure. i i
S is the surface area corresponding to the ith volume, ρ is the
p
i
12.2 MICROGEOMETRY OF ORGANIC‐RICH pore surface relaxivity, R is the characteristic size parameter, and
i
SHALE RESERVOIRS α is the pore shape parameter. For a sphere, R is its radius and α
i
is 3, for a cylinder R is the radius and α is 2, and for a slit pore R i
i
The microgeometry of organic‐rich shales has been shown to be is its half aperture and α is 1. To get a pore size distribution, an
fairly heterogeneous even on the scale of nanometers (Curtis assumption is required for the value of α and a value for ρ .
p
et al., 2010) and our current understanding of the microstructural The Langmuir adsorption data provide an estimate for S
features of shale is primarily because of the recent advances in the total surface area. From Equations 12.2 and 12.3,
nanoimaging technology done with the help of scanning elec­ V
tron microscopes aided by fixed ion beam milling to smooth and S S i i (12.4)
prepare samples for imaging (Ambrose et al., 2010; Curtis et al., i i p T i
2010, 2011, 2013; Quirein et al., 2012). Although SEM image
interpretation tends to be subjective, there is a growing body of Sigal et al. (2013) show that
work demonstrating the ability of these systems to image 3D S
connected pore networks within shales. A wide variety of pore S r 2 aLmax (12.5)
types can be found in organic‐rich shale reservoir rocks and m maMax
pores of various types exist in both the organic and inorganic In Equation 12.5, r is the radius of a methane molecule,
matrix material (Figures 9–12 in Curtis et al., 2010). Within the ρ is the maximum methane density in the adsorbed
m
maMax
organic material, irregular cross‐sectional pores characterized by layer, and S is the maximum number of moles of methane
aLmax
some very small pore openings in the order of a few nanometers that can be adsorbed. S is obtained from the standard
aLmax
may be observed. In comparison, a methane molecule is approx­ Langmuir adsorption measurement. Based on discussion in
imately 0.37 nm in diameter. The intrinsic porosity of the organic Sigal et al. (2013), a reasonable estimate for the adsorp­
material can be 30% or greater (Sigal, 2013a). For mature tion layer density is 0.0281 mol/cm . The methane radius is
3
organic shales, the pores in the organic material probably pro­ 1.865 × 10 cm.
−8
vide most of the gas storage capacity. Sigal and Odusina (2011) reported the NMR methane
Using image analysis methods, SEM images can supply spectra for several Barnett samples. The NMR samples also had
information about the volume weighted pore size distributions, methane adsorption data on companion plugs. Figure 12.1
for a volume that is on the order of a cubic micron in size. shows the volumetric pore size distribution data for one of these
They typically show pore sizes ranging from a nanometer to samples assuming spherical or cylindrical pores. The pore sizes
hundreds of nanometers, with the majority of the pore volume have a minimum value of about 1 nm. This is because the
associated with pores having characteristic sizes larger than adsorbed methane relaxes too fast to be detected by the NMR
10 nm (Curtis et al., 2013). measurements reported in Sigal and Odusina (2011), so each
Methods such as NMR spectra and gas adsorption measure­ NMR pore size estimate has been enlarged by twice the diam­
ments can provide pore size distributions on larger scale. For the eter of a methane molecule. The maximum pore size for pores
pores that store methane in a core plug, Sigal (2015) has used that store methane is a couple of hundred nanometers. Assuming
NMR measurements combined with standard Langmuir adsorp­ spherical pores, 20% of the pore volume is contained in pores
tion isotherms to provide volumetric pore size distributions. smaller than 10nm, and assuming cylindrical pores, 30% of the
These samples came from the dry gas zone of the Barnett. volume is in the small pores. The pore size distributions obtained
For NMR relaxation curves produced by relaxation from using NMR and adsorption data are consistent with distributions
interaction with wall potentials, the curves are modeled as a obtained from SEM studies.
sum of pore volumes V where the pores in each V have the Studies on shale samples of varying organic maturity indi­
i
i
same relaxation time as T . The V sum to the total pore cate that porosity development in these shales tends to coincide
2i
i
volume V(t = 0). That is
with the initiation of the formation of hydrocarbon liquids
n within the organic material although there are no clear trends
/
Vt () Ve tT i2 (12.1)
i
1 between organic maturity and porosity. However, the organic
material is typically associated with porosities as high as 30%
Equation 12.1 is inverted to get the V associated with each T . or higher (Sigal, 2013a) and typically most of the hydrocarbon
i
2i
The pore sizes are captured in the relaxation times. One has storage is associated with these organic pores (Alfred and
1 S i (12.2) Vernik, 2013; Gouth et al., 2007; Sigal and Odusina, 2011). For
T 2i p V i highly mature shales that are often associated with the gas
window, the organic pore walls are probably dominantly gas
and wetting while the inorganic pores are generally water wetting.