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 displacing fluid could be lowered to 10–2 to 10–3 dynes/cm, the oil droplets could be deformed and squeeze through the pore constrictions. A miscible process is one in which the interfacial tension is zero—that is, the displacing fluid and residual oil mix to form one phase. If the interfacial tension is zero, then the capillary number NVC becomes infinite and the microscopic displacement efficiency is maximized.
Figure 11.5 is a schematic of a miscible process. Fluid A is injected into the formation and mixes with the crude oil, forming an oil bank. A mixing zone develops between fluid A and the oil bank and will grow due to dispersion. Fluid A is followed by fluid B, which is miscible with fluid A but not generally miscible with the oil and which is much cheaper than fluid A. A mixing zone will also be created at the fluid A–fluid B interface. It is important that the amount of fluid A that is injected be large enough that the two mixing zones do not come in contact. If the front of the fluid A–fluid B mixing zone reaches the rear of the fluid A oil mixing zone, viscous fingering of fluid B through the oil could occur. On the other hand, the volume of fluid A must be kept small to avoid large injected-chemical costs.
Figure 11.5 Schematic of an enhanced oil recovery process requiring the injection of two fluids.
Consider a miscible process with n-decane as the residual oil, propane as fluid A, and methane as fluid B. The system pressure and temperature are 2000 psia and 100°F, respectively. At these conditions, both n-decane and propane are liquids and are therefore miscible in all proportions. The system temperature and pressure indicate that any mixture of methane and propane would be in the gas state; therefore, the methane and propane would be miscible in all proportions. However, the methane and n-decane would not be miscible for similar reasons. If the pressure were reduced to 1000 psia and the temperature held constant, the propane and n-decane would again be miscible. However, mixtures of methane and propane could be located in a two-phase region and would not lend themselves to a miscible displacement. Note that, in this example, the propane appears to act as a liquid when in the presence of n-decane and as a gas when in contact with methane. It is this unique capacity of propane and other intermediate range hydrocarbons that leads to the miscible process.
There are, in general, two types of miscible processes. The first type is referred to as the single-contact miscible process and involves such injection fluids as liquefied petroleum gases (LPG) 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 fraction; this fraction of alkanes has the unique ability to behave like a liquid or a gas at many reservoir conditions. The injected fluid and oil are usually not miscible on first contact but rely on a process of chemical exchange of the intermediate hydrocarbons between phases to achieve miscibility. These processes are discussed in great detail in other texts.16–20,23
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