Page 29 - Fundamentals of Enhanced Oil and Gas Recovery

An Introduction to Enhanced Oil Recovery 17

1.11.2 Different EOR Processes
EOR methods are mainly categorized into four major techniques: gas injection,
thermal injection, chemical injection, and other methods like microbial EOR. Gas
injection is the most popular method in the world, then thermal injection is second.

1.11.3 Gas Injection
This technique includes miscible injection of different gases such as carbon dioxide,
nitrogen, flue, and natural gas. The objective of miscible injection is to improve oil
displacement and reservoir pressure maintenance by forming a single phase between
the injected gas and oil. The reservoir conditions including temperature, pressure, and
composition of the oil significantly influence oil displacement during miscible gas
injection. Based on reservoir condition and phase behavior of the crude oil, the misci-
ble processes are divided into two majors: first-contact miscible and multiple-contact
miscible gas injection. In the first-contact miscible process, the injected gas is solved
immediately in reservoir crude oil and a single-phase fluid is formed under reservoir
condition. When the fluid is injected to the reservoir, the interference between the
slug of injected fluid and reservoir oil is dropped because of the miscibility of two
fluids. This process improves oil mobility toward the production well.
In the multiple-contact miscible process the injected fluid cannot be solved in res-
ervoir oil at first contact. In this process the composition of the injected fluid strongly
influences the final efficiency. A modified composition of injected fluid can lead to
better mass transfer between injected fluid and reservoir oil through multiple contacts
between them. The miscibility between the injected fluid and reservoir oil is formed,
and subsequently, oil displacement is improved.

1.11.4 Thermal Injection
The thermal methods imply on the processes in which the oil displacement is
improved by heat transfer through the reservoir. Thermal process can be categorized
into two majors: steam drive and in fire flooding.
Steam drive is a means for heat transfer to reservoir oil by injecting steam from the
surface into the reservoir. This process subdivides into two methods: cyclic steam
injection and continuous steam injection. Cyclic steam injection, or the huff and puff
process, is a method in which three stages of injection, soaking, and production are
followed in a single well. Steam is injected for a determined amount of time, then the
well is closed for a certain period, which is called the soak time. In this stage the well
is allowed to be closed for days to allow heat transfer from the steam to viscose/heavy
oil. Then the well is opened for a while and hot oil is produced. Again this process is
repeated. Different mechanisms are active in this process, including viscosity reduc-
tion, oil swelling, and steam stripping. The second process in this classification is