Page 14 - Enhanced Oil Recovery in Shale and Tight Reservoirs

4 Enhanced Oil Recovery in Shale and Tight Reservoirs

Table 1.2 Preliminary definitions of shale and tight formations.
Tight formation Shale formation

Types of Converted (oil and gas) Unconverted organic materials
hydrocarbon migrated from nearby and converted (oil and gas)
source rocks and/or migrated from nearby
sources, and
Rocks Storage reservoirs Interbedded source rocks and
storage reservoirs
Matrix <0.1 mD <1 mD
permeability

Because the same unconventional technology (horizontal well drilling
and fracturing) has to be used to produce shale and tight reservoirs, it is
convenient to combine the discussion of these two. Therefore, we do not
differentiate the terms of shale oil and tight oil in this paper, except for
some places where a differentiation is necessary. Note that sometimes shale
oil includes oil from oil shale and shale formation (NPC, 2011; Jia et al.,
2012). Such definition gradually loses its use because the technologies to
produce oil from oil shale and shale formation are very different. Producing
oil from oil shale generally uses high-temperature pyrolysis.
1.2.2 Shale oil versus oil shale

There is a huge difference between oil shale and shale oil. Oil shale is a rock
that contains a solid organic compound known as kerogenea precursor to
oil. Oil shale is a misnomer because kerogen is not really a crude oil, and
the rock holding the kerogen is not necessarily shale. Shale oil refers to
hydrocarbons that are trapped in so tight formations that the oil and gas
cannot easily flow into production wells.
To generate (before production) oil and gas synthetically from oil shale,
the kerogen-rich rock is heated to a high temperature (about 950 For

500 C) in a low oxygen environment, a process called retorting. There are
two methods to heat the rock. One is to mine the rock and heat the rock
at the ground surface. The other one is to heat the rock underground. To
heat the rock underground, ExxonMobil has developed a process to create
underground fractures in oil shale, to lay electrically conductive materials in
the fracture, and pass electric currents through the shale to gradually convert
the kerogen into liquid oil. The oil company Shell buries electric heaters un-
derground to heat the oil shale. Compared to the technologies to produce
hydrocarbon from oil shale, the current technology to produce shale oil is