Page 12 - Enhanced Oil Recovery in Shale and Tight Reservoirs

2 Enhanced Oil Recovery in Shale and Tight Reservoirs

In this chapter, shale and tight reservoirs are defined first. Then current
production technologies are described. Detailed EOR methods are discussed
in the subsequent chapters.

1.2 Definitions of shale and tight reservoirs

In this section, shale and tight reservoirs are defined. The terminol-
ogies of shale oil and oil shale are also differentiated. Different injection
modes are defined.

1.2.1 Shale tight reservoir
Shale is a laminated or fissile claystone or siltstone. If claystones (or siltstones,
not listed in Pettijohn, 1957) are neither fissile nor laminated but they are
blocky or massive, they are termed mudstone. Claystone is indurated clay.
A clay is a sediment with grains less than 0.002 mm (in radius or
1/256 mm in diameter (Pettijohn, 1957). A tight formation is a reservoir.
One common and important characteristic about shale and tight formations
is very low permeability. Tight formation oil permeability is less than 0.1
milliDarcy (mD) (air permeability is less than 1 mD) (Jia et al., 2012); and
matrix shale formation permeability is in the order of nanoDarcies (nD).
Zou et al. (2015) divided conventional and unconventional oil and gas res-
ervoirs using 1 mD air permeability. Song et al. (2015) grouped shale forma-
tion, tight formations, coal-bed methane formations, and oil shale in
unconventional reservoirs. Another related term is ultralow permeability
formation whose permeability is 1 nD to 1 mD (Speight, 2017). In other
words, ultralow permeability formations cover tight formations and shale
formations. But ultralow permeability is defined 0.3e1 mD (air perme-
ability) in China (Yang et al., 2013). Some shale formations, if not all,
have small natural fractures, which can make the effective permeability
higher than the order of nanoDarcies. Some key parameters about tight
oil reservoirs are the porosity less than 10%, total organic carbon (TOC)
higher than 1%, thermal maturity 0.6%e1.3%, and the API gravity higher
than 40 (Jia et al., 2012). Based on the shale pore size distribution, the micro-
pore is for the pore diameter d 2 nm, mesopore 2 nm d 50 nm, and
macropore d 50 nm (Fakcharoenphol et al., 2014). According to a DOE
report, shale originated from mud deposition in low-energy environment
and it primarily consists of consolidated clay-sized particles (Ground Water
Protection Council and All Consulting, 2009).