Page 102 - Fundamentals of Enhanced Oil and Gas Recovery

90 Ramin Moghadasi et al.

are obtained through a PVT test can directly provide information about oil CO 2
mixtures. For instance, a ternary diagram could be provided, which helps under-
standing the occurrence of miscibility. In a typical PVT test, CO 2 and oil are
injected into a high-pressure cell. The volume of the mixture is then changed.
Such a change will also change the pressure of the system. This test resembles a
standard constant composition expansion. At each pressure, the volume of any
phases present are measured. In the case of one phase, the density of the mixture
represents swollen oil density. Bubble point pressures for the CO 2 oil mixtures
can also be measured accurately by plotting cell pressure versus cell volume [17].
• Core flood experiments
Core flood experiments are useful for estimation of displacement efficiency at
microscopic level. Normally, small cores, which are used for experimentation, are
difficult to obtain from long cores. Without ignoring the usefulness of core flood
experiments, their data are difficult to interpret because even in linear cores, dis-
placement efficiency can be affected by viscous fingering, gravity segregation,
channeling, or bypassing of oil due to core heterogeneities, and trapping or shield-
ing of oil in contact with CO 2 by high mobility of water saturations, as well as by
the complexities of CO 2 oil phase behavior. Core flood experiments could also
be used to investigate whether unexpected problems can occur due to interactions
of CO 2 with reservoir oil, brine, clay, and cementing materials. Any of these phe-
nomena can result in increased or decreased permeability. For instance, asphaltene
deposition due to CO 2 oil interactions leads to decreased permeability. On the
other hand, rock dissolution due to CO 2 rock interactions in carbonates may lead
to increased permeability. It should be mentioned that core flooding data cannot
be readily extrapolated to field dimensions [9,67,68].
• Slim-tube displacement
This type of test is mainly used for determination of MMP. Slim tube consists
of a very slim coiled tube, which is filled with crushed core, sand, or glass bead
materials. This tube is typically very long in order to allow development of
dynamic miscibility. Displacements in slim tubes approach nearby the ideal displa-
cements. Viscous fingering growth is inhibited by the walls of the tube. It is
assumed that fluids are mixed well due to very small diameter of the tube and also
nearly homogeneous porous media within the tube. For MMP measurements, the
tube is first saturated with the oil while keeping the temperature at reservoir con-
dition. The gas is then injected into the tube, and recovery is calculated as the
amount of oil produced divided by the initial oil volume. Fixing the time, the
pore volume recovered are plotted against pressure. Normally, the time is fixed at
1.2 hydrocarbon pore volumes injected. The same procedure is then carried out
for higher pressures. The pressure at which a break or a sharp change occurs in the
oil recovery at 1.2 pore volumes of injection, or the lowest pressure at which the
recovery is about 90% 95%, is often used to define the minimum pressure [69].