Page 489 - Enhanced Oil Recovery in Shale and Tight Reservoirs

Air injection 453

13.7 Minimum oil content for combustion
If spontaneous ignition can occur or artificial ignition is executed,
in situ combustion will occur. To maintain this combustion, minimum oil
content W omin is required. According to the energy balance over a unit vol-
ume near the combustion front (Kharrat and Vossoughi, 1985):
r
bC b DT f
o
W omin ¼ S o fr ¼ (13.16)
DH
where

r C b ¼ð1 fÞr C s þ fr C g
s
g
b
DT f is the front temperature minus room temperature from a combus-
tion tube test, DH is the heat value of crude from a DSC test, f is the
porosity, r is the density, C is the specific heat, S is the saturation, and
the subscripts b, o, g, and s are bulk, oil, gas, and solid, respectively. r b C b
may be obtained from Perry et al. (1963). Kharrat and Vossoughi (1985)
calculated W omin for three reservoir cases, 0.0465, 0.0511, and 0.604 g/
3
3
cm , the average being 0.0527 g/cm . For a shale reservoir, if we assume
other parameters are the same as those examples, and if the oil density is
0.85 g/cm3, and the porosity is 0.08, then S o must be higher than 0.77 to
meet this minimum oil requirement. This example calculation indicates
that it is difficult to meet the oil requirement in shale reservoirs. As a result,
combustion cannot be maintained, even if it is initiated. However, Fassihi
and Kovscek (2017) reported a lower range for fuel concentration
3
(0.016e0.04 g/cm ).
It was found from experiments using heavy oils and field observations
that fuel consumed in an in-situ combustion process usually falls within
200e300 barrels per acre-ft of formation burned (Nelson and McNeil,
1961). This number is translated to 2.6%e3.9% porosity, if the oil is
3
completely burned. This number is also converted to 1.6e2.4 lb/ft which
is in the high side of the data presented by Showalter (1963). Probably 5%
e10% the oil in place is burned in an in-situ combustion project
(Hughes and Sarma, 2006).

13.8 Air requirement in combustion
A combustion front can advance only when both air and fuel are
available. Air requirement is closely related to available fuel. And a minimum