Gas Injection EOR- A New Meaning in the New Millennium
Authors: Rao Dandina 
Published: March 2010

Abstract
Although the idea of injecting gases to improve oil recovery has been known for over three-quarters of the past century, it appears to be taking on a new meaning as we step into the new millennium. The oil price scenario remains consistently loyal to its rollercoaster past; the long awaited jump in the natural gas price has become a reality; the oil industry has made new strides in horizontal drilling with significant cost reductions; and the world's attention is focused on the industrialized nations to lead the way to control global warming by reducing greenhouse gas emissions. These evolving global issues combined with the ever-declining reserves of conventional crude oils appear to bring a new optimism to enhanced oil recovery (EOR) by gas injection. Added to this optimism are the well-accepted facts that waterflood recoveries from conventional oil reservoirs rarely exceed 40﹪ of the original oil in place (OOIP); that most waterfloods are maturing or close to their economic limits; and that chemically enhanced waterfloods appear to have become practically extinct in spite of their conceptual soundness. Therefore, gas-based EOR processes using horizontal wells appear to be the solitary hope for continuing profitable production from the large remaining reserves of conventional oil in Canada, the United States and the rest of the world. In what follows, we will briefly discuss-but not review-the conceptual basis for gas injection EOR processes and what we have conveniently overlooked in it, the historical developments, process design considerations, problem areas, and the future direction and potential of this time-tested technology.

The Background
The concept of injecting gases into reservoirs to improve oil recovery is an old one, dating back to the 1920s when the early research papers appeared. However, much of the progress in our knowledge of the physics and chemistry of multiphase flow through porous media seems to have occurred after the early 1950s. This progress encompassed the development of three main processes of enhanced oil recovery (EOR): thermal, chemical, and miscible. While the thermal EOR process and its variations were aimed mainly at recovering heavy oils by lowering their viscosity to enable their flow, the chemical and miscible gas processes targeted the light and medium gravity crude oils by lowering the interfacial tension between the injected fluid and the crude oil to minimize the trapping of oil in the rock pores by capillary or surface forces.
Before discussing the background of gas injection EOR processes, we need to distinguish the term EOR from the more recent term of improved oil recovery (IOR). The EOR processes involve the injection of fluids to supplement the natural energy in the reservoir to displace oil to the producing wells. More importantly, the injected fluids interact with the reservoir rock-fluids system to create conditions favourable for enhancing oil recovery. These favourable interactions include oil swelling, interfacial tension reduction, oil viscosity reduction, rock wettability modification, and phase behaviour effects(1). Thus, simple waterflooding and dry gas injection for pressure maintenance are not EOR processes, as they do not involve any of these interactions.