Summary A pragmatic method has been developed to efficiently design the production-injection parameters to optimize the water-alternating-gas (WAG) performance in a field-scale CO 2 -miscible flooding project. The net present value (NPV) is selected as the objective function, while the controlling variables are chosen to be the injection rates, ratios of gas slug size to water slug size (WAG ratio) and cycle time (i.e., the injection time for each gas or water slug) for the injectors and bottomhole pressures (BHPs) for the producers. A hybrid technique, which integrates the orthogonal array (OA) and Tabu technique into the genetic algorithm (GA), is then developed and employed to determine the optimum WAG production-injection parameters. Sensitivity analysis of the WAG parameters on oil recovery is conducted and a field case is finally presented to demonstrate the successful application of the newly developed technique.

The recovery of oil by secondary waterfloods and tertiary miscible floods is significantly affected by the native wettability state of the reservoir. In gas injection EOR processes, the initial wettability state of the reservoir could be significantly altered due to mass transfer and phase behaviour interactions between the injected gas and the reservoir crude oil. This, in turn, would influence the water-oil and water-gas flow behaviour by altering their relative permeabilities. Should the reservoir engineer include these dynamic interactions and flow behaviour modifications in designing gas injection projects for optimum economic benefits? We seek to address this question by extending our understanding of the influence of the native wettability state on watergas relative permeability by experimental measurements and by quantifying the effects of induced wettability and relative permeability changes on gas-flood design parameters. A sequence of coreflood steps was designed to mimic the field practices used in water-alternating-gas (WAG) injection projects. The sequence has been applied to two reservoir rockfluids systems. Each sequence consists of four cycles of immiscible nitrogen floods in the core assembly at four different stages starting with clean core (unexposed to oil), core containing liveoil at waterflood-residual saturation, core subjected to a miscible flood, and the core after a repeat of the miscible flood. Both the rock-fluids systems studied indicate that the watergas flow behaviour is modified due to the miscible displacement. These results, combined with the knowledge of our previous work on oil-water flow behaviour modifications, imply the need for a dynamic mode of miscible flood design that would incorporate the in situ rock-fluids interactions. The results of this experimental study and their impact on WAG ratio calculations are presented and discussed in the paper. Introduction A summary of Canadian, US and worldwide EOR production data for the past decade has been recently provided (1) . These data indicate that nearly all of the gas-based EOR production in Canada has been due to hydrocarbon gas injection. However, the production share of hydrocarbon gas injection projects (as a per cent of total EOR production) has steadily declined from 49% in 1992 to about 18% in 2000, while steam-based thermal EOR of heavy oil has steadily grown during this period. The historical predominance of hydrocarbon gases over CO 2 in Canadian miscible projects appears to be changing. With PanCanadian's Weyburn CO 2 miscible project coming on stream this year, the share of EOR production by CO 2 injection will increase significantly. The EOR data for the US indicate that, while the production (and number) of CO 2 miscible projects has increased steadily over the last two decades, all other gas injection projects (CO 2 immiscible, N 2 and flue gas) have declined or become extinct except for the hydrocarbon miscible projects. The production from miscible hydrocarbon gas injection projects in the US has steadily increased, in spite of their decreasing numbers. The overall effect is that the share of production from gas injection EOR in the US has almost doubled from 23% in 1990 to 42% in 2000. The worldwide EOR data indicate the increasing dominance of hydrocarbon miscible projects over CO 2 , and a reversal, in 2000, of the decade-long decreasing trend in the production share of gas injection projects. This discussion points out the significant role that gas injection processes will continue to play in the world to enhance oil recovery.