Page 474 - Enhanced Oil Recovery in Shale and Tight Reservoirs

438 Enhanced Oil Recovery in Shale and Tight Reservoirs

temperature was able to increase from 134 to 400 C spontaneously, it could

not increase from 92 to 134 C through oxidation; artificial heating was

needed, indicating spontaneous ignition could not occur through LTO
when the initial core temperature was 92 C or lower.

Huang et al. (2016a, 2016b), and Huang and Sheng (2017c) studied the
exothermic behavior of Wolfcamp shale oil using DSC. They found that the
oil exhibited exothermic behavior only after the temperature was heated
above 300 C. But this high temperature may be caused by the thermal hys-

teresis owing to the high heating rate (5e15 C/min) in the experiments.

The above reviews (both field and laboratory) indicate that there is no
conclusion whether spontaneous ignition can occur in real oil reservoirs.
Whether spontaneous ignition can occur or not depends on the balance be-
tween heat generation and heat release. If the heat generation is faster than
the heat release, a local temperature can reach an ignition point, and spon-
taneous ignition may occur. In a reservoir, it is believed that heat release is
pressed (adiabatic condition), spontaneous ignition may happen. Turta and
Singhal (2001) mentioned that spontaneous ignition could occur in reser-
voirs as low as 30 C. However, spontaneous ignition does not occur in

ventilated asphalt (of high activity component) roads, because heat cannot
be accumulated (A. K. Singhal, personal communication in 2015). A real
reservoir is not adiabatic, but heat load imposed by many of laboratory setups
is much higher than what a reaction zone would experience in the field. Just
looking at the heat capacity of the apparatus, and the small reaction volume
for many of the setups, it is essentially impossible to duplicate the heat loss
environment of an oxidation zone operating in the field. That is why it is
difficult to duplicate ignition temperatures in the laboratory (Gordon
Moore, personal communication on Oct. 20, 2015). Therefore, compared
with laboratory conditions, a reservoir condition is relatively “adiabatic”
(Malcolm Greaves, personal communication on Oct. 27, 2015); and it is
difficult to achieve ignition in the laboratory.

13.5.3 Simulation studies
To study spontaneous ignition experimentally in laboratory, there are two
limitations. (1) Spontaneous ignition is induced by thermal energy accumu-
lation from the LTO reactions; but LTO reactions are slow. Thus, it may
take an unrealistically long time to reach a high temperature for ignition.
(2) A good adiabatic condition may not be able to satisfy. These limitations
may be overcome by a simulation approach. In principle, a simulation model