Introduction

The initial production of hydrocarbons from an oil-bearing formation is accomplished by the use of natural reservoir energy. As discussed in this type of production is termed primary production. Sources of natural reservoir energy that lead to primary production include the swelling of reservoir fluids, the release of solution gas as the reservoir pressure declines, nearby communicating aquifers, and gravity drainage. When the natural reservoir energy has been depleted, it becomes necessary to augment the natural energy from an external source. The Society of Petroleum Engineers has defined the term enhanced oil recovery (EOR) as the following: “one or more of a variety of processes that seek to improve recovery of hydrocarbon from a reservoir after the primary production phase.”¹

These EOR techniques have been lumped into two categories—secondary and tertiary recovery processes. It is these processes that provide the additional energy to produce oil from reservoirs in which the primary energy has been depleted.

Typically, the first attempt to supply energy from an external source is accomplished by the injection of an immiscible fluid—either water, referred to as waterflooding, or a natural gas, referred to as gasflooding. The use of this injection scheme is called a secondary recovery operation. Frequently, the main purpose of either a water or a gas injection process is to repressurize the reservoir and then maintain the reservoir at a high pressure. Hence the term pressure maintenance is sometimes used to describe most secondary recovery processes.

Tertiary recovery processes were developed for application in situations where secondary processes had become ineffective. However, the same tertiary processes were also considered for reservoir applications where secondary recovery techniques were not used because of low recovery potential. In the latter case, the name tertiary is a misnomer. For most reservoirs, it is advantageous to begin a secondary or a tertiary process concurrent with primary production. For these applications, the term EOR was introduced.

A process that is not discussed in this text is the use of pumpjacks at the end of primary production. A pumpjack is basically a device used with a downhole pump to help lift oil from the reservoir when the reservoir pressure has been depleted to a point where the oil cannot travel to the surface. Most producing companies will employ this technology, but it is not considered an enhanced oil recovery process.

On the average, primary production methods will produce from a reservoir about 25% to 30% of the initial oil in place. The remaining oil, 70% to 75% of the initial resource, is a large and attractive target for enhanced oil recovery techniques. We will provide an introduction to the main types of EOR techniques that have been used in the industry. Much of the information in this has been taken from an article written by one of the authors and published in the Encyclopedia of Physical Science and Technology (third edition).² The information is used with permission from Elsevier.