Page 115 - Fundamentals of Enhanced Oil and Gas Recovery

Miscible Gas Injection Processes 103

Miscible CO 2 injection improves the oil recovery efficiency by means of the below
mechanisms [17 19]:
• Reduction in oil viscosity,
• Reduction in oil density,
• Improvement in volumetric sweep efficiency.
Fulfillment of a miscible or immiscible flood is imposed by the magnitude of mini-
mum miscibility pressure (MMP) and the injection pressure of the gas into the oil res-
ervoir. The MMP is stated as the lowest operating pressure at which the injected gas
and the crude oil become miscible after their dynamic multicontact process at reser-
voir temperature [20]. In other words, the MMP parameter is defined as the pressure
at which the local displacement efficiency approaches 100%. At pressures lower than
the MMP, miscibility can be achieved through condensation, vaporization, or their
combined process [21].
In a miscible gas injection process, the IFT between the trapped crude oil and the
injected fluid is lowered to zero; consequently, the capillary forces will be decreased to
a minimum value leading to miscibility achievement. As a result of miscibility develop-
ment in a gas injection process, the huge amounts of trapped oil will be remobilized
leading to the increased oil recovery factor. In a miscible gas injection process, by
selecting the required operational conditions, oil recovery can be maximized [22,23].

4.3 MINIMUM MISCIBILITY DETERMINATION

4.3.1 Minimum Miscibility Pressure and Interfacial Tension
Measurement

Analytical and experimental methods have been introduced to approximate and measure
the MMP value [24]. The most widely known experimental procedures to determine
the gas oil miscibility pressure under reservoir conditions are shown as follows [23]:
• Slim-tube displacement
• Rising bubble apparatus (RBA)
• Method of constructing pressure composition (P X) diagrams
• Vanishing interfacial tension (VIT) technique/axisymmetric drop shape analysis
(ADSA)
The slim-tube test is considered as the most common practice and it has been
widely accepted as a standard in petroleum industry to determine gas oil miscibility.
In spite of this fact, this approach suggests neither a standard design nor a standard
operating procedure and criteria for the evaluation of miscibility conditions [25].
Furthermore, this technique has a time-consuming and costly experimentation, in
which it takes more than a month to complete one miscibility measurement.