COz core floods and related high-resolution, mathematical simulations in which viscous fingering is represented explicitly indicate a synergistic interaction between multicontact COz/crude phase behavior and macroscopic bypassing that causes ultimate oil recovery (when the system has been swept) to be lower in unstable displacements than in stable ones. If this effect is present in field applications, core floods in which fingering is absent should not be used as direct indicators of field-scale CO 2 flood displacement efficiency because they will yield optimistic predictions, all other factors being equal in the laboratory and the field.

EOR applications will not be presented here.

A two-dimensional (2D) analytical model is presented for gas/oil gravity drainage in a homogeneous, dipping reservoir. The sensitivity of gas/oil gravity drainage to key variables such as injection rate, oil relative permeability, and permeability anisotropy can be determined quickly with this model.Example calculations show that miscible-like recovery efficiencies are possible with immiscible gas injection into high-permeability dipping reservoirs with light oil.A procedure based on the analytical model has been developed to simulate immiscible gas injection into highly stratified reservoirs accurately. This simulation procedure allows a great deal of geological detail to be incorporated into reservoir models, because it permits relatively coarse grids.Application of the simulation procedure to a reservoir containing many discontinuous shales reveals that the presence of shales may favorably affect the recovery efficiency of an immiscible gas-injection process.

In this paper, the role of compositional effects during secondary, gravity-stable nitrogen displacements in (dipping) light-oil reservoirs, such as the Statfjord reservoir in the Brent field, is discussed. Displacements of both a gas condensate from the top of the Statfjord hydrocarbon column and a volatile oil from a more downdip location in this column are discussed. The main compositional effects in such vaporizing-gas-drive-type displacements are the following.1. The displacing vapor at the displacement front is not nitrogen, but a rich hydrocarbon gas liberated from the reservoir fluid upon contact with nitrogen. This hydrocarbon vapor is miscible with the reservoir fluid in the case of the gas condensate, but just fails to develop miscibility in the case of the oil-like reservoir fluid.2. The remaining oil is a degassed, more viscous oil than the original reservoir fluid.In addition to equation-of-state (EOS) programs, both a compositional simulator and an analytic, dispersion-free model for vaporizing-gas drives have been used to quantify the effect of phase behavior on recovery efficiency. These compositional calculations indicated that next to the phase behavior, the longitudinal dispersion level and, in the case of the oil-like fluid, the degree of gravity segregation at the displacement front are key factors determining displacement efficiency. Results are presented of a systematic investigation into the relative contributions of phase behavior, physical dispersion, and gravity drainage to the recovery process. A comparison of predicted results and slim-tube and coreflood experiments is made. The extent to which these laboratory experiments might be representative for reservoir-scale displacements is also briefly discussed.