Abstract

Recovering additional oil from partially or totally water flooded reservoirs is a difficult task. Gas injection is one of the most promising EOR method to recover tertiary oil under prevailing economic conditions.

A variety of gases can be used: hydrocarbon (natural or enriched), carbon dioxide, nitrogen, air, flue gas. Gas can be injected in secondary or tertiary stages, in conditions varying from immiscibility to full miscibility.

EOR by gas injection has been used worldwide for decades. This paper presents the technical state of the art of this process, and presents the different mechanisms (taking place in mobilising incremental of), the methodology and the approaches which have to be conducted. In particular, the importance of a good knowledge of the reservoir status and the geological model before initiating any EOR project is emphasised.

The three domains where decisive improvements have been achieved and which appear promising to increase oil recovery are highlighted:

1. lean gas injection where the gravity drainage is the main mechanism,

2. miscible or near miscible gas injection where process optimisation can be studied to make it more cost effective.

3. Air injection into light oil reservoirs

Despite the traditional handicaps (gas availability, high mobility, high compression cost), the historical worldwide EOR surveys show that the number of gas injection projects has increased and will certainly continue to do so. The process is more and more mastered and appears, in many cases, as the only method to be proposed to produce additional oil under economic conditions.