Page 472 - Enhanced Oil Recovery in Shale and Tight Reservoirs

436 Enhanced Oil Recovery in Shale and Tight Reservoirs

least 5 seconds after ignition by an open flame. The flash point is lower than
the fire point. At the flash point, the vapor burns briefly, but may not sustain.
During LTO, heat may be released. And oxidation reaction rates increase
almost exponentially with temperature, whereas the heat loss rate increases
linearly; it is feasible for the LTO reaction to achieve the spontaneous igni-
tion (Gray, 2016).
If spontaneous ignition can occur in reservoirs, the following advantages
may be realized:
1. The air injection project will be much more economic.
2. The sweep efficiency of air injection will be more uniform because spon-
taneous ignition leads to combustion, consuming oxygen, reducing air
(oxygen) fingering.
Ignition is fundamentally important for air injection, as many failures in
field projects were caused by the failure of ignition (Turta, 2013). Under-
standing of spontaneous ignition and the resulting thermal effect will aid
in designing and optimizing air injection projects so that the advantages of
spontaneous ignition and the thermal effects are fully taken of. Therefore,
it is important to study spontaneous ignition.
In this section, the field and laboratory observations are reviewed, and
simulation results are discussed in terms of the feasibility of spontaneous igni-
tion. Spontaneous ignition is a result of heat accumulation. In other words, it
takes some time for spontaneous ignition to occur. Therefore, to discuss the
feasibility of spontaneous ignition, ignition delay is discussed.

13.5.1 Field observations
Evidences of in-situ combustion from cores and production performance
were observed in the air injection project in the light oil reservoirs in the
North and South Dakota portions of the Williston Basin. These fields had
permeability less than 20 mD, porosity 11%e19%, and oil viscosity less
than 2 cP. The CO 2 content in some of production wells exceeded 12%;
a thin-section photomicrograph showed a high content of halite indicating
a high temperature experienced by the rock; the core showed a very low oil
saturation (4%) and some parts were absent of hydrocarbons (black spots)
(Gutierrez et al., 2008). More than half of the cumulative oil production
could be attributed to thermal effects in the Red River zone in West Buffalo
(Kumar et al., 2007). Production data showed flat gas-oil ratio (GOR)
compared with exponential increase in GOR for the wells without thermal
effect (Gutierrez et al., 2009). As earlier as in 1956, spontaneous ignition was