Page 59 - Fundamentals of Enhanced Oil and Gas Recovery

Screening Criteria of Enhanced Oil Recovery Methods 47

reservoir rock. Although N 2 is known as a low-cost EOR method that can be imple-
mented in miscible conditions for light oil reservoirs, no new N 2 injection project
was reported in sandstone and carbonate reservoirs during the last few years; however,
several N 2 flooding projects have been carried out particularly in the United States
over the past decades [2]. In contrast, recent successes and field projects (in Montana,
North and South Dakota) of high pressure air injection shows high potential of this
method as a new option with lower cost than miscible N 2 injection [27,28].
In addition to all above-mentioned advantages and screening criteria for the gas
EOR methods, the effect of gravity leading to gas override should be considered; dur-
ing the immiscible gas injection, the injected gas can move upward through the reser-
voir due to its lower density compared to oil; this allows gas to bypass the oil phase.
Under these circumstances, it is recommended to perforate the bottom part of the
pay zone.
Table 2.1 presents screening criteria for the gas methods. As it was mentioned ear-
lier, based on concept of miscibility, depth of reservoir and oil composition are the
most significant factors that must be considered in screening process of the gas EOR
methods.

2.3 CHEMICAL METHODS

Chemical EOR methods consist of injecting chemicals such as polymer, alka-
line, surfactant, and their combinations to increase oil recovery by improving macro-
scopic and microscopic sweep efficiencies. Generally, only around 1% of the overall
EOR projects have been allocated to chemical EOR which is directly affected by oil
price. Although, high-performance chemicals were introduced to the industry in the
last decade, number of chemical EOR projects compared to other EOR methods has
decreased significantly due to the oil price crisis since 2014. An introduction of the
chemicals and their fundamental mechanisms along with their screening criteria is
provided in this chapter.

2.3.1 Polymer Flooding
Compared to oil phase, water movement is faster in reservoir; therefore, to avoid vis-
cous fingering, polymers are added to water (displacing phase) in order to increase the
viscosity and reduce the mobility of the water and finally increase the sweep
efficiency.
Xanthan gum and partially hydrolyzed polyacrylamide (PHPA) are the most com-
mon types of polymer used in polymer flooding. Polysaccharide structure of xanthan