Page 56 - Fundamentals of Enhanced Oil and Gas Recovery

44 Ehsan Mahdavi and Fatemeh Sadat Zebarjad

based on some factors such as reservoir geology, reservoir depth, MMP, API of reser-
voir oil, and oil viscosity. As discussed before, efficiency of miscible CO 2 EOR
method is much higher than that of immiscible injection. Accordingly, screening of
oil reservoirs plays a crucial role in identification of the most likely candidate reservoir
for miscible CO 2 which directly affects efficiency of CO 2 EOR process.

Aforementioned, MMP is the determinative factor in miscibility condition of a CO 2
EOR process. As a rule of thumb, oil reservoirs with a minimum mid-point reservoir
depth of 3000 ft or deeper can be selected for CO 2 EOR due to appropriate reservoir
pressure and temperature which facilitate the achievement of miscibility in CO 2 oil
systems. Generally, MMP of CO 2 oil system increases as the viscosity of oil increases.
Regarding CO 2 EOR, there is another rule of thumb stating that oils with bubble-
point viscosities less than 10 cp and API of 25 or greater can become miscible with
CO 2 at reservoir pressure greater than 1000 psia [3]. It is noted that reservoirs not
having these criteria is not rejected for CO 2 EOR, because the above criteria are not
strict and depends on reservoir size and potential of oil recovery.
The role of temperature is significant due to its effect on miscibility of CO 2 oil
system; MMP for reservoirs having lower temperature are lower. A good example for
this condition is the Permian Basin reservoirs having low geothermal gradient, leading
to a lower required pressure to reach miscibility condition. Generally, if MMP and
sufficient residual oil saturation conditions be satisfying, CO 2 EOR is not adversely
affected by reservoir geological complexity; therefore, carbonate or sandstone reser-
voirs can be selected for CO 2 EOR. Typically, reservoirs with successful implementa-
tion of waterflooding can be suitable candidates for CO 2 EOR. Regarding fluid
properties, most of CO 2 EOR projects have been performed on reservoirs with
medium to light oils. Among all 123 CO 2 projects in the United States until 2012,
114 projects [6,7] were reported as miscible flooding in reservoirs with light to ultra-
light oils and viscosity of less than 3 cp except for two reservoirs. Other nine immisci-

ble projects were performed on reservoirs with heavy to light oils (11 35 API).
Table 2.1 presents screening criteria of CO 2 EOR.
In addition to the above-mentioned projects in the United States, Joffre and
Pembina fields in Canada, Buracica and Rio Pojuca fields in Brazil, Budafa and
Lovvaszi in Hungary are some sandstone reservoirs in which CO 2 EOR projects have
been carried out [16,17]. In the case of carbonate reservoirs, Permian Basin in the
United States and Weyburn in Canada are two large projects that have made a major
contribution to the total world oil production by CO 2 EOR. Judy Creek and Swan
Hills in Canada, Bati Raman in Turkey, and Ghawar in Saudi Arabia are other exam-
ples of CO 2 EOR projects in Carbonate formations [17].
Currently, there are some ongoing CO 2 EOR projects worldwide which contri-
butes in oil production of around 300,000 bbl/day. Most of the projects have been
implemented in North America (United States and Canada). Actually, large part of