Page 140 - gas transport in porous media

CHAPTER 8

GAS INJECTION AND FINGERING IN
POROUS MEDIA

1
MUHAMMAD SAHIMI , M. REZA RASAEI 1,2 AND
MANOUCHEHR HAGHIGHI 1,2
1 Department of Chemical Engineering, University of Southern California, Los Angeles, California,
90089–1211, USA
2 Institute of Petroleum Engineering and Department of Chemical Engineering, Faculty of Engineering,
The University of Tehran, Tehran, 11365–4563, Iran

8.1 INTRODUCTION
On average, two-thirds of the original oil in any reservoir remains unrecovered, even
after water injection into the reservoir. The same is true if a low-pressure gas is
injected into a reservoir which is largely immiscible with the oil. The oil is trapped
in the reservoir due to the capillary forces and the interfacial tension, and remains
entrapped regardless of how much water or low-pressure gas is injected into the
reservoir. It forms either a discontinuous phase in the swept zone, or a continuous
phase in the unswept zone of the reservoir.
One way of enhancing the recovery of oil is by injecting into a reservoir a fluid
whichismisciblewiththeoilinplace. Inprinciple, theinjectedfluidshoulddrastically
reduce the capillary and interfacial forces. If that happens, then one may recover a
large fraction of the trapped oil. For example, if one injects a gas (or a gas mixture)
into an oil reservoir and if, at the temperature and pressure of the reservoir, the gas is
in a critical or near critical state, it will mix in large proportions (if not completely)
with the oil in place. Under these conditions, one will have an efficient miscible
displacement process. For example, the critical temperature of CO 2 is only about
◦
31 C and, therefore, it can be an ideal agent for miscible displacement of oil. In fact,
field-scale CO 2 injection has been successfully carried out in the United States. It is
such gas injection processes and the associated phenomena in oil reservoirs that are
of prime interest to us in the present chapter.
If we inject a gas into a reservoir which is completely or partially saturated with oil,
and if the gas and the oil-in-place mix in all proportions, the gas and the oil are said

133
C. Ho and S. Webb (eds.), Gas Transport in Porous Media, 133–168.
© 2006 Springer.