Hydrocarbon-miscible flooding

Mannhardt, K. Novosad, J.J. Adsorption of Foam-Forming Surfactants for Hydrocarbon-Miscible Flooding at High Salinities in Foams, Fundamentals and Applications in the Petroleum Industry, Schramm, L.L. (Ed.), American Chemical Society Washington, 1994, pp. 259-316. 

Pande, K.K. Effect of gravity and viscous cross-flow on hydrocarbon miscible flood performance in heterogeneous reservoirs. 67th Annual Conference of SPE, Washington D.C., Oct 4-7, 1992 SPE 24935. 

Adsorption of Foam-Forming Surfactants for Hydrocarbon-Miscible Flooding at High Salinities... 

Hydrocarbon-miscible flooding refers to an oil recovery process in which a solvent , usually a mixture of low and intermediate molecular-weight hydrocarbons (methane through hexane), is injected into a petroleum reservoir. Several mechanisms contribute to oil recovery in this process displacement of oil by solvent through the generation of miscibility between solvent and oil, oil swelling with a resulting increase in oil saturation and therefore in oil relative permeability, and reduction of oil viscosity. When solvent and oil remain immiscible, a reduction of gas-oil interfadal tension leads to improved oil recovery. 

Foams for Hydrocarbon-Miscible Flooding in Reservoirs Containing High-Salinity Brines... 

The reason for the large number of hydrocarbon-miscible flooding projects in Canada is the preponderance of reasonably priced gas throughout the province of Alberta. Also, a large number of gas plants separates intermediate components, such as ethane, propane, and butane, and allows custom design of individual solvents tailored to specific reservoir conditions. Because of the large number of hydrocarbon-miscible projects in Canada, the application of mobility-control foams seems an attractive means to significantly increase oil production. 

The Gilwood formation and the Beaverhill Lake group provide excellent examples of pools that are candidates for hydrocarbon-miscible floods. There are 19 producing pools in the Gilwood formation and 27 in the Beaverhill Lake group. The range of properties encountered in these pools is listed in Table II. 

Because the surfactants described previously were selected for hydrocarbon-miscible flooding, the effect of hydrocarbon solvent on foam performance should be included in the surfactant screening process. Data on the characteristics of foams generated with light hydrocarbons as the gas phase are not readily available in the literature. Limited data comparing nitrogen foams with hydrocarbon solvent foams are shown in Table... 

The evaluation of surfactant adsorption is particularly important when foams for high salinity reservoirs, such as many Canadian reservoirs subjected to hydrocarbon-miscible flooding, are considered. This puts stringent requirements on the solubility, foaming, and adsorption properties of surfactants that may be considered for foam applications, and severely limits the types of surfactant that may be used. 

Moreover, these surfactants precipitate at moderate salinities, making them unsuitable for foam applications in many Canadian reservoirs currently subjected to hydrocarbon-miscible flooding. Although surfactants that remain soluble and form effective mobility-control foams in near-saturated salt solutions have been identified (2), these surfactants adsorb at moderate to high levels under some conditions. Therefore, consideration of adsorption is extremely important in the selection of foamforming surfactants. 

Dependence of Adsorption on Brine Salinity and Divalent Ion Content. Brine salinity and composition probably constitute the primary criteria for selecting surfactants for foam applications. Many Canadian pools that are being flooded with hydrocarbon solvents contain near-saturated formation brines. Some of these pools have been waterflooded with fresh water, and therefore, salinity gradients exist. In addition, the majority of hydrocarbon-miscible floods in Canada are conducted in carbonate formations that contain formation waters with high levels of hardness. 

Many hydrocarbon-miscible floods are run in reservoirs containing brines of extremely high salinity and hardness. Surfactants that may be used for mobility control foams at such conditions are commercially available. The effectiveness of foams generated with these surfactants was illustrated by way of representative mobility reductions factors measured in oil-free porous media. 

A large percentage (80%) of Canadian enhanced oil production comes from hydrocarbon miscible flooding [1], The low density and viscosity of the injected fluids cause hydrocarbon miscible EOR processes to suffer from poor sweep efficiency, due to viscous fingering and gravity override. Mobility control foams provide a means for improving the sweep efficiency and could significantly increase oil production from Canadian reservoirs. 

Another EOR approach to reducing the viscosity of oil in the reservoir is ntiscible flooding— the injection of fluids that mix with the oil under reservoir conditions. Such fluids include carbon dioxide, light hydrocarbons, and ititrogen. Supply and cost of carbon dioxide are often more favorable than for other injectants. Extensive research and field testing have established the techiucal viability of miscible flooding, and a nnmber of commercial carbon dioxide miscible flooding projects are in operation. 

Miscible Recovery. Oil and water do not mix and they do not flow with equal facility through a porous rock. Over the years, many miscible flood processes have been tested, the most successful of which have been (1) hydrocarbon misdble recovery (2) carbon dioxide miscible flooding and (3) chemically enhanced recovery. 

The chapter by Fulton and Smith (Chapter 5) shows that ionic surfactants can form microemulsions with ethane and water under conditions that might be encountered in miscible floods with light hydrocarbons. These microemulsions correspond to the single-phase regions of the model diagrams in Figure 11. 

Shelton, J.L. and Schneider, F.N. The Effects of Water Injection on Miscible Flooding Methods Using Hydrocarbons and Carbon Dioxide, SPEJ. June 1975, 217-26... 

CO2 and hydrocarbon gas flooding remain popular methods to recover oil. Between 1982 and 1992, oil production from gas floods increased from 72,028 to 296,020 b/d in the United States. " The latest data can be found in the review of EOR projects published by Oil Gas Journal every two years. In 1992, there were 45 active CO2 miscible projects in the USA. About 2.4 BCF/D of CO2 was being injected into these reservoirs. About 40%i of this gas was being produced and reinjected. The incremental production response was about 142,000 b/d of oil. The industry had booked about 1.9 billion barrels of oil because of CO2 flood. Hydrocarbon flooding has been used in Prudhoe Bay field in Alaska, Block 31 field in West Texas, and Hassi-Messaoud field in Algeria, to give a few examples. 

Miscible flooding CO2, nitrogen, flue gas, hydrocarbon, solvent... 

The proportion of enhanced oil production that is caused by hydrocarbon-miscible and, to a lesser extent, immiscible flooding is evident from Table I. Hydrocarbon-flooding dominates in Canada, where it accounts for 81% of Canada s enhanced oil recovery (EOR) production. Although steam-flooding is the most commonly applied EOR process in the United States, oil production from hydrocarbon-flooding is significant and has increased by more than the production from any other EOR process over the past 2 years. In countries other than the United States and Canada, most of the oil recovered by hydrocarbon-flooding comes from Libya, but hydrocarbon injection is also applied in the Soviet countries and in the United Arab Emirates (J). 

Table I shows some statistics on the types of reservoir subjected to enhanced recovery operations. Most EOR projects in Canada, particularly hydrocarbon-miscible projects, are being conducted in carbonate reservoirs, and hydrocarbon injection in the United States is mostly in sandstones. As will be evident from this chapter, the type of reservoir rock is of significance to surfactant propagation during foam-flooding.

Schramm, L.L., Mannhardt, K., and Novosad, J.J. (1993) Selection of oil-tolerant foams for hydrocarbon miscible gas flooding, in Proceedings, 14th International Workshop and Symposium, International Energy Agency Collaborative Project on Enhanced Oil... 

In the eastern and western portion of the Prudhoe Bay field the natural gas is also used for miscible flooding process. The heavier hydrocarbons are stripped from the natural gas and injected into the Prudhoe Bay Reservoir to achieve miscibility and thus enhance the oil recovery. Such an enhanced oil recovery process is certainly cost effective due to availability of natural gas. This is probably the lowest cost option for the North Slope gas utilization. 

Enhanced oil-recovery processes include chemical and gas floods, steam, combustion, and electric heating. Gas floods, including immiscible and miscible processes, are usually defined by injected fluids (carbon dioxide, flue gas, nitrogen, or hydrocarbon). Steam projects involve cyclic steam (huff and puff) or steam drive. Combustion technologies can be subdivided into those that autoignite and those that require a heat source at injectors [521]. 

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