Page 124 - Fundamentals of Enhanced Oil and Gas Recovery

112 Pouria Behnoudfar et al.

Multiplying this factor by the predicted value of Eq. (4.15),wehave
MMP 5 1:723 3 11:562 5 19:926 MPa

from Eq. (4.15)

4.3.3 CO 2 Flooding Properties and Design
By far, the most common application of solvent methods is in a displacement mode,
but injection and production through the same wells have been reported [39 41].
Carbon dioxide can be injected and dissolved in water in a distinctly immiscible
fashion that recovers oil through swelling and viscosity reduction [12]. If the solvent is
completely miscible with the oil (first contact), the process has a very high ultimate
displacement efficiency since there can be no residual phases. If the solvent is only
partially miscible with the crude, the total composition in the mixing zone between
the solvent and the oil can change to generate or develop in situ miscibility.
Regardless of whether the displacement is developed or first contact miscible (FCM),
the solvent must immiscibly displace any mobile water present with the resident fluids.
The economics of the process usually dictates that the solvent cannot be injected
indefinitely. Therefore, a finite amount or a slug of solvent is usually followed by a
chase fluid whose function is to drive the solvent toward the production wells. This
chase fluid—N 2 , air, water, and dry natural gas seem to be the most common
choices—may not itself be a good solvent. But it is selected to be compatible with the
solvent and because it is available in large quantities [4].
A phase behavior or pressure temperature plot (P T diagram) for different pure
components and air is reported in the work of McCain [4,42]. For each curve, the
line connecting the triple and critical points is the vapor pressure curve; the extension
below the triple point is the sublimation curve. The P T diagram for air is really
similar to an envelope, although its molecular weight (MW) distribution is so narrow
that it appears as a line. The critical pressures for most components fall within a rela-
tively narrow range of 3.4 6.8 MPa (500 1000 psia), although critical temperatures
vary over a much wider range. The critical temperatures of most components increase
with increasing MW. Carbon dioxide (MW 5 44) is an exception to this trend with a

critical temperature of 304K (87.8 F), which is closer to the critical temperature of
ethane (MW 5 30) than to propane (MW 5 44). Most reservoir applications would be
in the temperature range of 294 394K (70 250 F) and at pressures greater than

6.8 MPa (1000 psia); hence, air, N 2 , and dry natural gas will all be supercritical fluids
at reservoir conditions. Solvents such as LPG, in the MW range of butane or heavier,
will be liquids. Carbon dioxide will usually be a supercritical fluid since most reservoir