Page 144 - Fundamentals of Gas Shale Reservoirs

124 PETROPHYSICAL EVALUATION OF GAS SHALE RESERVOIRS

NMR T distribution allows differentiation between free and Staudt, 2005). This method allows for the direct
2
fluid and bound fluid. The ability of NMR to distinguish bet­ measurement of the amount adsorbed, which is not the case
ween bound and free fluid increases the capability of NMR for volumetric method.
to estimate the formation permeability (Rezaee et al., 2012). The adsorption isotherm is the relationship between the
Two common models for estimating permeability are the amount of adsorbed gas and the gas pressure at constant
free‐fluid (or Coates model) and mean T (or SDR model) temperature (Lu et al., 1995). Therefore, by repeating
2
(Coates et al., 1999). T 2cutoff , (the) T value which differentiates adsorbed gas measurement at different pressures, the
2
between free fluid and bound fluid, and T 2gm , the geometrical Langmuir isotherm will be obtained (Fig. 6.8). According to
mean of T distribution, are the effective parameters for the the Langmuir equation, the adsorbed gas capacity (G ) can
s
2
Coates model and mean T model, respectively. Small pore be expressed as follows:
2
sizes in the shales resulted in fast T relaxation time and
2
small T 2cutoff and T 2gm compared to normal values for sand­ G VP (6.2)
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stone or carbonates. Coates et al. (1999) proposed a T 2cutoff of s PP 1
33 ms for sandstone and 92 ms for carbonates, while Sigal
and Odusina (2011) showed that for the gas shale most of the where V and P are the Langmuir volume and pressure
l
l
NMR signals occur below 20 ms. There are many challenges respectively, and P is the reservoir pressure. Due to the
with the interpretation of NMR signals for gas shale samples exothermic nature of adsorption phenomena, it is necessary
that make it difficult to differentiate between free fluid and to do the adsorbed gas measurement at the reservoir temper­
bound fluid and, as a result, to estimate permeability. As Sigal ature to simulate the reservoir conditions exactly. Zhang et
and Odusina (2011) proposed, some of these challenges are al. (2012) and Ross and Bustin (2007a) derived some
as follows: formulas for determining the effect of temperature on the
adsorbed gas capacity. Zhang et al. (2012) proposed the
• Although T mode is on the order of 1 ms for brine following relationships between the Langmuir pressure and
2
response and 10 ms for methane, water and gas NMR temperature based on the organic matter type:
signals are very similar, and it is difficult to separate
between gas and brine response. 1241
• Resolving the effect of adsorbed gas in the determined ln P 1 . 589 T fortype Ikerogen (6.3)
porosity from NMR is an issue.
• The effect of temperature on methane relaxivity should 2628
be investigated. ln P 1 . 975 fortype II kerogen (6.4)
T

3366
6.3.4 Adsorbed Gas Measurement ln P 11 .06 fortype III kerogen (6.5)
1 T
Gas storage evaluation of the gas shale is performed through
two different measurements. Measuring the free gas compo­ where P is the Langmuir pressure in MPa and T is tempera­
l
nent is the same as with the conventional reservoirs and can ture in Kelvin. However, as was mentioned by Zhang et al.
be done using helium pycnometry or other conventional (2012), the aforementioned correlations were obtained from
methods that measure effective porosity. But for measuring
the adsorbed gas capacity, there are two common methods:
volumetric and gravimetric.
The physics of a volumetric gas adsorption experiment is V I Langmuir volume: gas volume at in nite pressure
simple: a given amount of sorptive gas is expanded into a
vessel which includes the degased shale sample, and which
has first been evacuated. Upon expansion, the sorptive gas is
partly adsorbed on the surface (external and internal) of the Gas content (scf/ton) 1/2 V
I
sorbent material, and partly remains as gas phase around the
adsorbent. By measuring mass balance, the amount of gas Langmuir pressure:
being adsorbed can be calculated if the free space volume of pressure where gas storage capacity
the adsorbent is known (Keller and Staudt, 2005). The is half of Langmuir volume
gravimetric method consists of exposing a degased shale
sample to a pure gas at constant temperature. The change in P I Pressure (psi)
the weight of the adsorbent sample as well as pressure and FIGURE 6.8 A typical Langmuir isotherm showing the quantity
temperature is measured when equilibrium is reached (Keller of adsorbed gas at a given pressure.