Page 211 - Fundamentals of Gas Shale Reservoirs

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ROCK PHYSICS ANALYSIS OF SHALE RESERVOIRS

Marina Pervikhina , Boris Gurevich , Dave N. Dewhurst , Pavel Golodoniuc
2,3
1
1,2
1
and Maxim Lebedev 2
1 CSIRO Energy Flagship, Perth, WA, Australia
2 Department of Exploration Geophysics, Curtin University, Perth, WA, Australia
3 CSIRO Mineral Resources Flagship, Perth, WA, Australia

9.1 INTRODUCTION the microstructure of organic‐rich source shales using high‐
resolution petrographic techniques to identify the micro­
Oil, gas condensate, and dry gas from organic‐rich shales structural changes caused by HC maturation processes.
(ORSs) are transforming the energy outlook of the world These studies aimed to observe possible paths of the HC
economy. The US Energy Information Administration transportation from the source to reservoir rocks. Some of
assessment from 2013 (EIA, 2013) estimates the technically these studies reported the existence of subvertical micro­
recoverable resources of shale gas at 186,000 MTOE distrib­ cracks caused by the organic matter decomposition and
uted between 41 countries with China having 15% of the related pore pressure generation (e.g., Meissner, 1978;
total amount, Argentina 11%, Algeria 10%, United States Momper, 1978). Other researchers detected development
9%, Canada 8%, Mexico 7%, Australia 6%, South Africa of subhorizontal cracks parallel to bedding (e.g., Lewan,
5%, and Russia 4%. The volumes of extractable shale oil 1987; Price et al., 1984).
accounted for 87 billion tons (Aguilera and Ragetzki, 2013; Such microstructural and petrographic studies of ORSs
DERA, 2012). New technological advancements have made were complemented with laboratory measurements of
previously irrecoverable resources extractable. This fact in acoustic wave velocities at ultrasonic frequencies (Vernik,
addition to more informed estimations of ORS resources 1993, 1994; Vernik and Landis, 1996; Vernik and Liu, 1997;
may further drastically increase the assessments of recover­ Vernik and Nur, 1992). These earliest rock physics experi­
able shale oil and gas. A striking example is the US Bakken ments on ORSs encountered a number of difficulties such as
Shale formation: between 2008 and 2013, the estimate of insufficient core material or its poor quality. Additional dif­
shale oil in this formation increased more than fivefold to ficulties were caused by the necessity to establish completely
2.5 billion tons (IEA, 2013). new practices of sample storage and experimental proce­
Organic‐rich shales were investigated for decades as dures. These new practices of sample preservation are
source rocks. The focus of this research was to improve necessary as dewatering of clay minerals in clay‐bearing
understanding of the processes that lead to chemical trans­ cores can alter elastic properties of shales substantially.
formations of solid and immobile organic matter to mobile Experimental procedures need to be able to evaluate the
hydrocarbons (HCs). Another practically important goal extremely anisotropic elastic properties of organic‐rich
was to get an insight into the microstructural changes that shales. Polar anisotropy (also called vertical transverse isot­
result from these maturation processes and allow versatile ropy, VTI), which is intrinsic for shales without vertical
HC migration from the low‐permeability source rocks into fractures, requires measurements of ultrasonic velocities on
high‐quality reservoirs. A number of studies investigated at least three different samples cut normal, parallel, and at

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