Page 471 - Enhanced Oil Recovery in Shale and Tight Reservoirs

Air injection 435

Figure 13.21 Activation energy of 25 crude oil samples with additives.
13.4.2.4 Light oil versus heavy oil
A heavy oil has more heavy components. A light oil has larger amount of
light aliphatic hydrocarbons which vaporize into injected air. Thus, a light
oil has less oxidation inhibitors like aromatics; and oxidation can more
rapidly proceed in the gas phase for a light oil. As a result, a lower temper-
ature peek for a light oil usually occurs earlier than a heavy oil. Because a
light oil does not have significant coke to form by pyrolysis, the temperature
cannot reach high in a light oil reservoir. Thus, generally LTO occurs in
light oil reservoirs.
A heavy oil has more heavy components that are aromatic in nature.
Aromatic-based oxidation inhibitors become ineffective near or above
180 C. Thus, as LTO increases the reservoir temperature above 220 C,

the aromatic, resin, and asphaltene fractions in a heavy oil enter a new
kinetically controlled regime (Freitag and Verkoczy, 2005). Above this
temperature, the peroxides that form from aromatic compounds (ROOR)
begin to participate in branching and cease to inhibit oxidation. As the
temperature is further increased, more coke is formed by pyrolysis. More
fuel leads to high combustion temperature (HTO) (Freitag, 2016).

13.5 Spontaneous ignition

Spontaneous ignition (autoignition) occurs when the temperature is
raised to a fire point by self-heating (due to exothermic internal reactions).
At the fire point, the vapor produced by a given fuel continue to burn for at