Page 109 - Fundamentals of Gas Shale Reservoirs

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POrE GEOmETry IN GaS SHaLE rESErVOIrS

Adnan Al Hinai and Reza Rezaee
Department of Petroleum Engineering, Curtin University, Perth, WA, Australia

Summary and the isolated pores. The pore body to pore‐throat ratio is
a characteristic that controls fluid flow. The connectivity in
Assessing shale formations is a major challenge in the oil and the pore system can be represented by the pore body to pore‐
gas industry. The complexities are mainly due to the ultralow throat size ratio: the lower the ratio, the lower the connec­
permeability, the presence of a high percentage of clay, and the tivity; hence the lower will be the permeability/fluid flow.
heterogeneity of the formation. Knowledge and understanding The results demonstrated a complex geometry of the pore
of rock properties, including pore geometry, permeability, network from clay‐rich rocks.
and fluid distribution are essential for determining shale’s The siliceous and organic‐rich gas shales studied are
hydrocarbon storage and recovery. This chapter discusses the marked by a strong component of clay minerals, mostly made
microstructural characterization of gas shale samples through of kaolinite and illite/smectite (I/S) mixed layers. Three types
mercury injection capillary pressure (MICP), low‐field nuclear of shales can be classified according to their clay content: (i)
magnetic resonance (NMR), and nitrogen adsorption (N ). low I/S but high kaolinite, (ii) high I/S but low kaolinite, and
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High resolution focused ion beam–scanning electron micros­ (iii) high I/S and high kaolinite. It is understood that I/S acts
copy (FIB–SEM) image analysis is used to further support the as a fluid trapping mineral by increasing the pore geometry
experimental pore structure interpretations at submicron level. complexity (surface to volume ratio increase) but generates
The chapter focuses on three key areas: (i) comparisons of low porosity made up of microporosity. Kaolinite acts as fluid
pore size distribution (PSD), (ii) recognizing the relationship storage by clogging pores and helps to keep high porosity
between pore geometry and permeability, and (iii) effects of made up of relatively larger pores.
clay occurrence on fluid transport properties. The combination of MICP, N , and NMR forms an ideal
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MICP and N are destructive techniques used as PSD approach to overcome each of their individual limits in terms
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measurements. MICP is capable of characterizing the PSD of pore size resolution and the external influences (dehydra­
in the range of mesopores (5 nm < pore diameter > 50 nm: tion/hydration state or sample preparation).
intra‐ and inter‐clays) to macropores (pore diameter > 50nm:
inter‐grains and discontinuities) while N can be applied to
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pores less than 2 nm. NMR is a nondestructive technique 5.1 INTrODuCTION
that is performed under room conditions. It supposes that the
sample is fully or partially water saturated. 5.1.1 Gas Shales and Their Challenges
In contrast with MICP PSD that provides only “connected”
pore throats as tube shapes and no pore body sensu stricto, Gas shale systems comprise fine‐grained sedimentary rocks
NMR PSD provides full experimental characterization of that are mainly consolidated from clay‐sized mineral grains.
pore geometry, the size of the pore body behind the throats, It is known as “mudstone” in the category of sedimentary

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