Page 120 - Hybrid Enhanced Oil Recovery Using Smart Waterflooding

112 Hybrid Enhanced Oil Recovery using Smart Waterflooding

Because the first-contact miscibility between the DETERMINATION OF SOLVENT PHASE
displacing and displaced fluids results in a completely BEHAVIOR
single phase, the displacing fluid is able to recover all There are several experiments to characterize the solvent
displaced fluid contacted. The miscible displacement phase behavior such as single contact, multiple contact,
achieving the first-contact miscibility indicates that the and slim tube tests (Lake, 1989). The single contact
injecting gas is miscible in all proportions with experiment is useful to construct the P-z diagram, which
the reservoir fluid under the prevailing reservoir temper- only shows the number and types of phases and liquid
ature and pressure conditions. It is termed as first- volume, since the pressure can be changed. The multiple
contact miscible process. contact experiment can formulate the ternary diagram
Another immiscible displacement can be classified and imparts the compositional information, which is
with two processes (Slobod & Koch, 1953), and it not described from P-z diagram. The experiment of
corresponds to the CO 2 injection in the real field. In slim tube displacement can bridge the static experiment
the first immiscible process of type I, the equilibrium and coreflood. Although it does not exactly mimic the
phases at the gas-oil front area are essentially immiscible. displacement process, it still offers the dynamic proper-
In other words, the CO 2 -rich phase displaces oil-rich ties of phase behavior. Although the slim tube test
phase with the interface between the two phases. It intro- measures an effluent concentration, its main objective
duces the advantages of more viscous displacing gas, oil is to identify the MMP. The pressure, in which certain
swelling, and oil viscosity reduction. In the second amount of oil recovery is produced, is the MMP or
immiscible flood of type M, the injected gas is suffi- minimum dynamic miscibility pressure. Although there
ciently enriched at the front as to be completely miscible are a number of definitions to indicate MMP, they
with the reservoir fluid. It is the miscible process show the similar trends in correlations. Corresponding
achieving multiple-contact miscibility. In the CO 2 to MMP, the critical tie line passes through the crude
injection, CO 2 - and oil-rich phases are miscible on not composition. The MMP is conventionally less than the
the first contact but multiple contacts. During the multi- pressure of first-contact miscibility. The determination
ple contacts between oil- and CO 2 -rich phases, the mass of MMP using slim tube test requires a high cost. There-
transfer between oil and CO 2 proceeds until the oil-rich fore, alternative approaches to determine MMP are (1)
phase cannot be distinguished from the CO 2 -rich phase. mathematical models and (2) thermodynamic MMP
The transfer process between oil- and CO 2 -rich phases is correlations (Jarrell et al., 2002). The mathematical
described with condensing/vaporizing mechanisms model uses phase equilibrium data and EOS to estimate
(Zick, 1986). Following the condensing mechanism, the thermodynamic MMP. Once the appropriate data are
the CO 2 firstly condenses into the hydrocarbon liquid, given, an excellent determination of MMP is possible at
i.e., oil-rich phase. The condensation makes the oil-rich low cost. When the acquisition of phase equilibrium
phase lighter and, often, drives some lightest methane data is unavailable, MMP correlations can be applied
out ahead of oil bank. Because of the succeeding vapor- for the specific condition.
izing mechanism, the lighter oil components vaporize
into the CO 2 -rich phase. The vaporization makes the
CO 2 -rich phase denser to be soluble in the oil phase. SOLUBILITY OF CO 2 IN WATER
When the multiple-contact miscibility between the The solubility of most hydrocarbon components in water
CO 2 - and oil-rich phases is achieved, the two phases is negligible over the range of temperature and pressure
cannot be distinguishable in terms of properties. During conditions. However, the solubility of CO 2 in water is
the multiple-contact miscible process of CO 2 injection, much higher than that of hydrocarbon components in
there are some advantages of viscosity ratio, oil swelling, water. The solubility of CO 2 is a function of pressure,
and condensing/vaporization to recover oil. The misci- temperature, and salinity. Søreide and Whitson (1992)
bility between CO 2 and oil is sensitive to the reservoir developed the numerical model of PR EOS considering
fluid composition, pressure, and temperature. Because the temperature-, pressure-, and salinity-dependent
the real production commonly undergoes little change aqueous solubility of CO 2 based on the experimental
in the temperature except for the thermal EOR process, database. Chang, Coats, and Nolen (1998) also
the main interesting factor on the miscibility is the developed the EOS model using CO 2 solubility, as well
pressure. The condensing CO 2 in oil-rich phase and as the properties of CO 2 -saturated water including
vaporizing oil in CO 2 -rich phase increase with an formation volume factor, compressibility, and viscosity
increase in pressure. based on the experimental database.