Page 137 - Fundamentals of Gas Shale Reservoirs

6

PETROPHYSICAL EVALUATION OF GAS SHALE
RESERVOIRS

Mehdi Labani and Reza Rezaee
Department of Petroleum Engineering, Curtin University, Perth, WA, Australia

6.1 INTRODUCTION for a shale formation to be considered as a potential gas
shale layer are discussed.
For many years, shale formations were viewed as a hydro­
carbon source rock or cap rock. Due to this traditional point 6.2.1 Pore System Characteristics
of view, only geochemical analysis has been routinely
performed on the shale layers. However, for sweet spot Pore system characteristics are very important for evaluation
mapping of the gas shale layers, it is necessary to know of gas shale reservoirs. Pores within the matrix of the gas
about petrophysical and geomechanical properties as well as shale reservoirs are smaller than pores in the conventional
geochemical ones. The main focus of this chapter is the reservoirs (Nelson, 2009). The main types of pore spaces in
petrophysical evaluation methods of shale formations. In the productive gas shale systems are matrix porosity, either
first section, the key properties for evaluation of potential associated with mineral particles or organic matter, and frac­
gas shale intervals are defined, and then the available ture porosity, either natural or induced fractures (Wang and
techniques for measuring these parameters will be discussed. Reed, 2009). In gas shale systems, matrix pores along with
The chapter will be wrapped up with the common well log natural networks of fractures provide the flow of gas from
signatures of the gas shales and ways to interpret them for matrix to induced fractures during production. Generally, in
finding petrophysical properties of shale intervals. describing the matrix pore size in shales the pores are all
considered to fall within the nanopore range (Javadpour,
2009; Javadpour et al., 2007; Loucks et al., 2009). There is a
6.2 KEY PROPERTIES FOR GAS SHALE pore classification system for materials that contain nanometer‐
EVALUATION scale porosity developed by Rouquerol et al. (1994).
According to this pore classification, micropores are <2 nm
Shale is a fine‐grained detrital sedimentary rock, formed by diameter, mesopores are between 2 and 50 nm, and
the consolidation of clay (less than 4 µm) and silt (between macropores are >50 nm. Mesopores and micropores are
4 and 62.5 µm) sized particles into rock layers of ultralow economically important to gas shale production because
permeability. In general, shales are characterized by finely of their large contribution to porosity and storage sites
laminated rocks and/or fissility approximately parallel to the for methane (Keller et al., 2011). Recently, Loucks et al.
bedding (Serra, 1988). This definition gives the lowest (2012) defined a new pore size classification for matrix‐
opportunity for shale as a reservoir. However, the right related mudrock pores; however, herein the Rouquerol’s
combinations of geological, geochemical, petrophysical, classification has been used.
and geomechanical properties would result in a productive To find out more about the pore system of organic‐rich
gas shale interval. In the following sections, the key properties shale layers, it is necessary to know about the accessible

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