Page 141 - Fundamentals of Gas Shale Reservoirs

PETROPHYSICAL MEASUREMENTS OF GAS SHALE RESERVOIRS 121
6.3 PETROPHYSICAL MEASUREMENTS OF GAS The final size of the crushed samples should be higher than
SHALE RESERVOIRS the size of the grains. According to the classification of sed­
imentary rocks based on the grain size, maximum grain size
Petrophysical measurement of gas shale formations is of of the shales is about 62.5 µm in diameter; therefore, 250 µm
critical importance for finding out about potential gas shale would be a proper lower range measure for the crushed
intervals for economic gas production. In conventional res­ sample size. The upper range could be selected as 2 mm
ervoir rocks, characterizing petrophysical parameters like which is the maximum grain size of the sandstones.
porosity, fluid saturation, and permeability are very well Before starting the helium pycnometry, the crushed sam­
documented, and API methodologies are widely adopted ples should be heated to remove the moisture content of the
(API, 1998). However, there are not any well‐established shale samples. The main concern during heating of the shale
laboratory methods specific to gas shale reservoirs, and samples is preserving organic materials and the clay‐bound
sometimes there is not any consistency among the results water. Analysing the evaporated components of the shale
(that are) reported from different commercial laboratories samples shows that by heating up to around 120°C, only free
(Sondergeld et al., 2010). Also, using (the) conventional water evaporates from the matrix of the shale samples
methods for gas shale reservoirs has some limitations. For (Easley et al., 2007; Handwerger et al., 2011).
example, the Dean Stark method, which is a routine Low pressure CO and N isotherms (<18.4 psia) can give
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procedure for water saturation determination of conven­ useful information about pore volume, pore surface area,
tional reservoirs, is unable to separate free water from bound PSD, and pore shape of the shale samples (Quantachrome,
water in gas shale; therefore, it is not possible to calculate 2008). Between 1 and 2 g of ground sample (<250 µm) is
effective saturation, effective porosity, and clay‐bound water degased in the evacuated oven prior to analysis. In different
volume (Handwerger et al., 2011). In the following section, reviews, there are different values for time and temperature
the available laboratory methods used specifically for petro­ that are required for degasing the samples (Clarkson et al.,
physical measurements of gas shale samples are explained. 2012a, b, 2013; Ross and Bustin, 2009). Low pressure
adsorption of CO is useful for characterizing microporosity,
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while nitrogen adsorption is useful for characterizing meso‐
6.3.1 Pore Structure Evaluation Techniques
and part of macroporosity. Low‐pressure gas adsorption
In order to clarify the complex pore structure of the shales, analysis cannot determine pores greater than 300 nm in
researchers have utilized different fluid invasion techniques diameter (Clarkson et al., 2011).
including low‐pressure gas adsorption analysis using Mercury porosimetry provides PSD, total pore volume or
nitrogen and carbon dioxide, helium pycnometry, and mer­ porosity, the skeletal and apparent density, and specific sur­
cury porosimetry. Effective porosity of the shale matrix is face area (Giesche, 2006). Similar to low‐pressure adsorp­
determined by mercury immersion of the sample (for bulk tion measurement, the samples should be evacuated before
volume determination) coupled with helium pycnometry the test to remove moisture and possible gas content.
(for grain volume determination) (Chalmers et al., 2012). Mercury intrusion is useful for characterizing meso‐ and
Measuring the shale matrix porosity is usually performed on macroporosity. Therefore, combining mercury data with
the crushed samples, due to two reasons: (i) Luffel and low‐pressure gas adsorption data allows for the determina­
Guidry (1992) proposed that low porosity of the shale sam­ tion of full pore size spectrum of gas shale reservoirs.
ples might be the result of incomplete penetration of the pore Figure 6.6 shows the representative PSD using mercury
network by helium under the helium porosimetry method. porosimetry and gas adsorption data for a gas shale sample
Crushing the shale samples increases the area accessible by in the Perth Basin, WA. As can be seen in this figure,
gas and thereby increases the accuracy of the measurement, combining these two techniques yields a full pore size spec­
and (ii) the presence of microfractures due to the laminated trum of the shale samples from micro‐ to macropore. By
structure of the organic‐rich shales might affect the comparing the PSD in the overlapped area (mesopore), it is
measurement of matrix porosity. Crushing eliminates both clear that the position of the peaks does not match precisely.
these microfractures and also core‐induced artefacts. For all analyzed samples, mercury porosimetry suggests a
Since the pore sizes of the gas shale vary between lower mode pore diameter compared to that obtained from
nanometer and micrometer scale, crushing the samples in nitrogen adsorption. The possible explanations for this
millimeter size range does not affect the pore structure, and observed shift are (i) mercury intrusion measures the pore
the crushed sample porosity can be considered as represen­ throats and not the actual pore size, and therefore the
tative of the matrix porosity. Any contention arises from measured pore size using mercury would be smaller than
different protocols for crushing and sieving the shale samples. that obtained from the nitrogen adsorption and (ii) in mercury
In different reviews (Chalmers et al., 2012; Karastathis, porosimetry for accessing the smaller pore diameters, mercury
2007; Luffel and Guidry, 1992; Ross and Bustin, 2009), there injection pressure should increase. The experimental results
are different numbers for the size of the crushed samples. show that the mercury injection pressure for accessing pore