In the context of a depleted oil reservoir, during primary and secondary recovery, specific volumes of oil remain in the porous medium as trapped oil or nondisplaced oil due to the recovery mechanism itself or the execution of any enhanced recovery projects (EOR).
Now, to select the right EOR process to increase oil production as well as recovery factor, knowledge of reservoir properties; evaluation of heterogeneities and their spatial changes; fluid variations, and understanding of the fundamental principles of EOR processes, such as interfacial tension reduction; sweep efficiency increase, and viscosity reduction are essential. Therefore, reinforcing two forgotten concepts in this new technological era, such as Capillary Number and Mobility Ratio, are fundamental in any enhanced recovery project.
Likewise, any EOR method’s success depends on the efficiency of microscopic displacement to extract oil from pores because capillary and viscous forces in the reservoirs influence displacement efficiency. In other words, these forces, together with gravitational forces, determine the distribution of all phases in the reservoir and also cause entrapment or mobilization of stages in a multiphase system. Understanding and studying these forces is significantly necessary for the design of any EOR process.
WHAT IS THE CAPILLARY NUMBER?
The ratio between viscous and capillary forces is called the Capillary Number; literature figures reflect that when this number is lower than 10 -6, the recovery process is inefficient; conversely, if this number is higher than 10 -3, the process tends to be more efficient. In practical terms, the EOR process aims to increase the orders of magnitude of the capillary number by affecting the viscosity of the fluids or by decreasing the interfacial tension.
DEFINING THE MOBILITY RATIO
On the other hand, the Mobility Ratio is defined as the mobility of the displacing fluid (i.e., water) divided by the mobility of the displaced fluid. The overall displacement efficiency of any oil displacement process can be increased by improving the mobility ratio. For example, a favorable mobility ratio is less than 1, which means that the displaced phase (oil) has higher mobility than the displacing phase (water). An unfavorable mobility ratio will be greater than 1.
In practical terms, a favorable mobility ratio means that the displaced oil phase can move faster through the reservoir than the displaced water phase. Finally, understanding these key aspects can lead to the right decision on the implementation of thermal, chemical, or hybrid processes, so this could increase the probability of success in terms of recovery efficiency.
Finally, the consideration, understanding, and evaluation at the laboratory and numerical simulation level of these concepts before implementing any emerging technology is a fundamental aspect to determine if we can indeed talk about enhanced recovery processes and consequently an increase in the recovery factors of hydrocarbon fields.
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