Page 140 - Fundamentals of Gas Shale Reservoirs

120 PETROPHYSICAL EVALUATION OF GAS SHALE RESERVOIRS

50 Carynginia formation Clay
Kockatea Shale 0 100
40 Free gas 20 10 90 80
Gas content (scf/ton) 30 50 40 30 70 60

20

10 Adsorbed gas 60 50 40
70 30
80
0 20
0 200 400 600 800 90
Pressure (psi) 10
100
FIGURE 6.4 Variation of adsorbed gas versus free gas as a 0 102030405060708090 100 0
function of pressure for a potential gas shale sample from the Perth Carbonate Quartz
Basin, WA.
FIGURE 6.5 Variability of mineralogical composition in the
on solids is an exothermic process (Lu et al., 1995). Reservoir potential gas shale layers of Perth Basin.
pressure, meanwhile, has an important impact on the
adsorbed gas capacity. Adsorption is a very efficient mecha­ clay, and those with abundant carbonates are moderate.
nism for storing gas at low pressures, while at high pressures Therefore, brittleness of the shale layers could be defined
the role of free gas is highlighted (Alexander et al., 2011). It upon the basis of their mineralogical composition as follows:
is due to the nature of gas adsorption on the shale layers
which increases rapidly at relatively low pressures; there­ BI mineralogy Quartz 100 (6.1)
after, the adsorption capacity plateaus as the system reaches Quartz CarbonatesClays
gas saturation (Fig. 6.4) (Ross and Bustin, 2007b). Moisture
content competes with the methane molecules for adsorption There are different methods for determining shale composi­
sites (Bustin and Clarkson, 1998); therefore, the gas shale tion: X‐ray powder diffraction (XRD), Fourier transform
with higher moisture content should have lower gas adsorp­ infrared transmission spectroscopy (FTIR), X‐ray fluores­
tion capacity. cence (XRF), energy‐dispersive X‐ray spectroscopy setting
on the scanning electron microscopy (EDS‐SEM), and thin
section analysis (TS). Among these techniques XRF, XRD,
6.2.5 Shale Composition
and FTIR are used more than others for determining shale
Petrophysical evaluation of gas shale reservoirs is complex composition. XRD could determine the bulk mineralogy and
due to the variable mineral composition. Figure 6.5 is a ter­ clay mineralogy, whereas XRF quantifies elemental abun­
nary plot based on quartz, total clay, and total carbonate for dances which are then stoichiometrically apportioned to
the potential gas shale layers in the Perth Basin, WA, which common minerals. FTIR could identify 16 different minerals
shows the variability of mineral composition in the Kockatea based on the absorption of the infrared energy onto the sample.
Shale and Carynginia Formation. Mineralogy plays a
significant role in controlling shale properties. As was men­
tioned before, clays can affect the shale pore structure and 6.2.6 Geomechanical Properties
consequently may provide adsorption sites for methane. The Evaluation of the gas shale mechanical properties is very
nonclay minerals especially quartz content is very important important as screening criteria for determining the potential
for estimating the brittleness index of the rock. There is a intervals for hydraulic fracturing and, as a result, in gas shale
relationship between mineralogical content and the brittle­ sweet spot mapping. The starting point for doing the
ness of the shale layers. Brittleness, a measure of the rock’s hydraulic fracturing is determination of the rock’s mechanical
ability to fracture, is a complex function of lithology, mineral properties. Young’s modulus and Poisson’s ratio are two
composition, TOC, effective stress, reservoir temperature, controlling mechanical properties that dictate the brittleness
diagenesis, thermal maturity, porosity, and type of fluid of the gas shale layers. These parameters can be determined
(Wang and Gale, 2009). Based on the study by Jarvie et al. in the laboratory by testing the rock sample under different
(2007) on the Barnett shale, the most brittle section of conditions (static method) or can be calculated using the
Barnett has abundant quartz, the least brittle has abundant dipole sonic log data (refer to Section 6.2.5).