Page 144 - Fundamentals of Enhanced Oil and Gas Recovery

132 Pouria Behnoudfar et al.

respectively. At any HCPV injection, the generalized form of Eq. (4.55) can be pro-
posed as follows [122]:
OOIP
5 ρ (4.56)
ð
M CO 2 CO 2 res ½ RF BT 1 0:6 RF %HCPV 2 RF BT ފ
S h
where the CO 2 storage capacity in Mt, density of CO 2 at reservoir condition in
3
kg/m , the recovery factor at breakthrough time, recovery factor at any HCPV injec-
tion, in percent, original oil in-place in percent, and oil shrinkage factor in 1/oil for-
, ρ , RF BT , RF %HCPV ,
mation volume factor are exhibited by the symbols M CO 2 CO 2 res
OOIP, and S h , respectively. The oil shrinkage factor is defined as the inverse of the oil
formation volume factor.
ECL Technology Limited (United Kingdom) used a similar method to determine
the net CO 2 retained in the reservoir for different EOR operations. For WAG injec-
tion, the net CO 2 retained in the reservoir is calculated as follows [123]:

Net CO 2retained 5 WAG IOR efficieny 3 WAG score efficiency 3 OOIP
(4.57)
B 0
3 WAG CO 2 factor alpha 3
B g
where WAG IOR efficiency ,WAG score efficiency , and WAG CO 2 factor alpha are targeted incre-
mental oil recovery factor, a factor between 0 and 1 (it is 1 for an efficiently and fully
factor alpha varies between 1 and 2 and
implemented WAG project). The WAG CO 2
is related to the net CO 2 utilization efficiency when expressed in reservoir volumes,
indicating more gas may be stored in the reservoir than required for WAG operation,
respectively. For gravity stable gas injection (GSGI), the net CO 2 retained in the reser-
voir is calculated as follows [122]:

B 0
Net CO 2retained 5 GSGI CO 2 factor 3 GSGI score CO 2 factor 3 OOIP 3 0:7 (4.58)
B g
where GSGI CO 2 factor , GSGI score CO 2 factor , B o , and B g illustrate targeted incremental oil
recovery by GSGI operation, factor between 0 and 1, and gas volume factor, respec-
tively. The GSGI score CO 2 factor permits the user to reduce the injected CO 2 volume in
comparison with the potential target volume. For a fully implemented project,
GSGI score CO 2 factor is equal to 1. The factor “0.7” is responsible for the fraction of
OOIP left in the formation at the end of gas flood and a small amount of mobile water
which is left in the swept region by the injected gas [124]. The GSGI process differs from
the WAG operation. The amount of CO 2 retained in GSGI is proportional to the pore
volume, rather than the recovery of IOR process. More CO 2 is needed in a GSGI pro-
cess; thereby, this process is more favorable for CO 2 storage. Numerical reservoir simula-
tions may also be used, which may take into account the impact of water invasion, gravity
segregation, reservoir heterogeneity, and CO 2 dissolution in formation water [124].