Miscible processes

Miscible processes are aimed at recovering oil which would normally be left behind as residual oil, by using a displacing fluid which actually mixes with the oil. Because the miscible drive fluid is usually more mobile than oil, it tends to bypass the oil giving rise to a low macroscopic sweep efficiency. The method is therefore best suited to high dip reservoirs. Typical miscible drive fluids include hydrocarbon solvents, hydrocarbon gases, carbon dioxide and nitrogen. 

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]. 

It is apparent that the addition of the mobility control agent did not affect the miscibility process, as evidenced by the comparable oil recovery to that of the no mobility control case. At the same time the mobility of carbon dioxide was decreased more than twenty-fold. This important flnding demonstrates that the mobility control method produces effective mobility control without adversely affecting the miscible recovery mechanism. 

The displacement tests show that a signiflcant increase in tertiary oil production can be achieved when one of the better additives is utilized in 1-dimensional laboratory immiscible carbon dioxide floods. In the miscible displacement case no adverse effect on the miscibility process was found. In... 

This developed miscibility process results in a miscible fluid, that is capable of displacing all the oil which it contacts in the reservoir... The efficiency of this displacement is controlled by the mobility (ratio of relative permeability to viscosity) of each fluid. If the displacing fluid (i.e. carbon dioxide) is more mobile than that being displaced (i.e. crude oil) then the displacement will be relatively inefficient. Some of the residual oil saturation will never come into contact with carbon dioxide. Both laboratory and field tests have indicated, that even under favourable condition, injection of 0.15-0.6 10 m of carbon dioxide is required for recovery of an additional barrel (0.16 m ) of oil". Here our goal is to obtain a mass ratio of CO2 to incremental oil of 1 to 4, on the basis of the Bonder s data. 

PARA with PAr are miscible processability, mechanical properties, solvent, weather, HDT, impact and stress-crack resistance Bapat et al., 1992... 

In the previous section, it was noted that the microscopic displacement efficiency is largely a function of interfacial forces acting between the oil, rock, and displacing fluid. If the interfacial tension between the trapped oil and the displacing fluid could be lowered to 10 to 10 dyn/cm, the oil droplets could be deformed and could squeeze through the pore constrictions. A miscible process is one in which the interfacial tension is zero that is, the displacing fluid and the residual oil mix to form one phase. If the interfacial tension is zero, then the capillary number Nyc becomes infinite and the microscopic displacement efficiency is maximized. 

Consider a miscible process with -decane as the residual oil, propane as fluid A, and methane as fluid B. The... 

There are, in general, two types of miscible processes. One is referred to as the single-contact miscible process and involves such injection fluids as liquefied petroleum gases (LPGs) and alcohols. The injected fluids are miscible with residual oil immediately on contact. The second type is the multiple-contact, or dynamic, miscible process. The injected fluids in this case are usually methane, inert fluids, or an enriched methane gas supplemented with a C2-C6 fi action. The injected fluid and oil are usually not miscible on first contact but rely on a process of chemical exchange between phases to achieve miscibility. 

The oil-LPG-dry gas system will be used to illustrate the behavior of the first-contact miscible process on a ternary diagram. Figure 5 is a ternary diagram with the points O, Z, and V representing the oil, LPG, and dry gas, respectively. The oil and LPG are miscible in all proportions. A mixing zone at the oil-LPG interface will grow... 

FIGURE 6 Ternary diagram illustrating the multicontact dry gas miscible process. 

Reservoir pressures sufficient to achieve miscibility are required. This limits the application of LPG processes to reservoirs having pressures at least of the order of 1500 psia. Reservoirs with pressures less than this might be amenable to alcohol flooding, another first-contact miscible process, since alcohols tend to be soluble with both oil and water (the drive fluid in this case). The two main problems with alcohols are that they are expensive and they become diluted with connate water during a flooding process, which reduces the miscibility with the oil. Alcohols that have been considered are in the C1-C4 range. 

Behind the miscible front, the vapor phase composition continually changes along the dew point curve. This leads to partial condensing of the vapor phase with the resulting condensate being immobile, but the amount of liquid formed will be quite small. The liquid phase, behind the miscible front, continually changes in composition along the bubble point. When all the extractable components have been removed from the liquid, a small amount of liquid will be left, which will also remain immobile. There will be these two quantities of liquid that will remain immobile and will not be recovered by the miscible process. In practice, operators have reported that the vapor front travels anywhere from 20 to 40 ft from the well bore before miscibility is achieved. 

Operational problems involving miscible processes include transportation of the miscible flooding agent, corrosion of equipment and tubing, and separation and recycling of the miscible flooding agent. 

The miscible process requirements are characterized by a low-viscosity crade oil and a thin reservoir. A low-viscosity oil will usually corrtain enough of the intermediate-range components for the multicontact miscible process to be established. The requiremerrt of a thin reservoir reduces the possibility that gravity override will occitr and yields a more even sweep efficiency. 

In general, the chemical processes require reservoir temperatirres of less than 200°F, a sandstone reservoir, and enough permeability to allow sufficient injectivity. The chemical processes will work on oils that are more viscous than what the miscible processes require, but the oils carmot be so viscous that adverse mobility ratios are encountered. Limitations are set on temperature and rock type so that cherrrical corrsumption can be controlled to reasonable values. High terrrperatures will degrade most of the chemicals that are crmerrtly being used in the industry. 

THE ROLE OF ASPHALTENE AGGREGATION IN VISCOSITY VARIATION OF RESERVOIR HYDROCARBONS AND IN MISCIBLE PROCESSES... 

In two of the precipitation tests, the asphalts were first precipitated by addition of n-butane and n-pentane to the tank oil. After separation of asphalts from the crude, the deasphalted crude was analyzed by gas-liquid chromatography. Table 2 shows that the original oil composition is considerably altered by asphalt precipitation. The amount of heavier ends in the deasphalted crude is less than in the original crude. Also, n-butane removed greater amount of heavier fraction iC y + ) than n-pentane. The deasphalted crude has lower density than original crude. This Indicates that in a miscible process, asphaltene precipitation can alter the composition of crude oil which needs to be accounted for in prediction of solvent-oil phase behavior, compositional path and miscibility conditions for solvent-oil systems. 

Asphaltene precipitation results in removal of heavier fractions of the crude leaving behind lighter deasphalted crude. This indicates that in a miscible process, the alteration of crude composition needs to be accounted for in prediction of phase behavior, compositional path and miscibility conditions for solvent-oil systems. 

The common miscible processes can be generalized very loosely by the three-component phase diagram in Figure 5. The fluid which is miscible with both oil and water (or oil and gas) at the prescribed conditions is shown at the apex of the ternary diagram. 

Figure 5 illustrates the conventional diagram normally used for oil-water-alcohol (or surfactant) processes. For CO2 or hydrocarbon-miscible processes, authors conventionally rotate the ternary diagram 120° clockwise, but the illustrated principle is the same. The binodal curve is normally higher for alcohol and lower for surfactants than the curve shown in Figure 5. Although no real reservoir process can be completely miscible in the exact...

All of these hydrocarbon-miscible processes can recover oil, but there are many difficulties. Mobility control is a very serious problem, although techniques such as alternate water and gas injection (WAG) can be used to reduce the mobility of the lighter, driving fluids (113). In the United States, the intermediate hydrocarbons as well as methane are in short supply and are valuable as fuel or chemical feedstocks. Therefore, much attention in the United States is now focused on materials such as CO2 or other inert gases which have no fuel value but which can provide some miscible displacement under certain conditions. In parts of the world where light hydrocarbons and methane are available, the hydrocarbon-miscible process and various modes of gas injection should increase oil recovery for many years. 

Carbon dioxide (CO2) processes are usually listed with miscible processes even though much of the research which is now underway is directed toward finding an answer to the question ... 

Society of Petroleum Engineers (SPE) (1965) Miscible Processes I. SPE Reprint Series, No. 8, Richardson, TX. 

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