Page 142 - gas transport in porous media

Chapter 8: Gas Injection and Fingering in Porous Media
135
miscible displacement much less efficient than desired. In addition, in most misci-
ble displacement processes involving a gas the required temperature and pressure
for miscibility of the oil and the displacement agent are often high enough that they
limit the number of prospective reservoirs. For example, medium to heavy hydrocar-
bons become miscible with oil only at high temperatures and/or pressures (see, e.g.,
Sahimi et al., 1985; Sahimi and Taylor, 1991). Another negative aspect of a miscible
displacement process is its cost. It may happen that a miscible displacement is more
efficient in terms of the amount of the recovered oil than an immiscible injection,
but the total cost of the miscible displacement (including the cost of transporting the
displacing gas to the oil field from other locations) is so high that makes it econom-
ically unattractive. Moreover, flue gas and nitrogen have only limited application as
agents of a miscible displacement in deep and high pressure reservoirs. For these
reasons, EOR processes based on gas injection have not been as common as immisci-
ble displacement processes, such as water flooding, although they are used relatively
extensively.

8.2 FACTORS THAT AFFECT THE EFFICIENCY OF
MISCIBLE DISPLACEMENTS

Miscible gas injection as an EOR process is influenced by several factors. Although
these factors are well-known, there is still some disagreement on the extent of the
influence of each individual factor. For example, while laboratory experiments indi-
cate that the viscosity contrast between the oil and the displacing gas has a strong
effect on the efficiency of the miscible displacements, the same may not be true at
the field scale which is dominated by large-scale heterogeneities, such as the spatial
distributions of the permeability and porosity and the presence of fractures and/or
faults. In what follows, we describe the effect of some of the most important factors
on the efficiency of a miscible displacement by a gas (or any other fluid, for that
matter).

8.2.1 Mobility and Mobility Ratio

The mobility λ i of a fluid i is defined as the ratio of the effective permeability K i
of the porous medium, experienced by fluid i, and the fluid’s viscosity µ i , λ i =
K i /µ i . When one fluid displaces another, the mobility ratio M, defined as the ratio
of the mobilities of the displacing and displaced fluids, is one of the most important
influencing factors in any displacement process. Normally, M is not constant because
mixing of the displacing gas and the oil changes the effective viscosities of the two
fluids. In addition, the viscosity of the mixed zone also depends on concentrations
of the displacing and displaced fluids. In many cases the viscosity µ m of the mixed
zone is estimated from the following empirical law due to Koval (1963)

C s 1 − C s
µ m = + (8.1)
1/4 1/4
µ s µ o