Improved Cyclic Stimulation Using Gas Mixtures
Authors: Shayegi Sara , Jin Zhengguo , Schenewerk Philip , Wolcott Joanne 
Published: December 2007

Abstract
Cyclic gas injection processes have been primarily restricted to the use of pure CO2 or CO2 that has been slightly contaminated with reservoir gases. However, it is difficult to employ CO2 in certain circumstances due to transport, economic, or corrosion problems. This paper presents the results of laboratory investigations of the cyclic gas injection process using methane, nitrogen and mixtures of these gases with CO2. Cyclic gas injections were performed at immiscible conditions in consolidated sandstone cores that contained waterflood residual oil. Cyclic CO2 injections were performed under analogous conditions for comparative purposes.
Pure methane recovered approximately the same amount of waterflood residual oil as CO2, whereas pure nitrogen recovered about half that amount. Pure CO2 was more mobile in porous media than pure CH4 or N2. Certain CO2/N2 and CO2/CH4 mixtures yielded outstanding results, recovering 2-3 times the waterflood residual oil produced by pure CO2. Maximum recovery was obtained with mixtures containing 10-25% CO2.

Introduction
Cyclic gas injection is a single-well enhanced oil recovery (EOR) process which involves the injection of a slug of gas into a well. After gas injection, the well is shut-in for a "soak" period to allow time for equilibration. Then, the well is reopened and production is resumed.
Cyclic CO2 injection, which is also known as CO2 huff 'n' puff, is the most common cyclic gas injection process. Although the process was originally proposed as an alternative to cyclic steam injection for the recovery of heavy crude, it has primarily been used for the recovery of light oils. Numerous field tests have been performed and the results revealed that the process is economically feasible in diverse reservoir environments.
Although field results indicate that cyclic CO2 injection may be successfully implemented under a variety of conditions, application of CO2 processes offshore, and in certain isolated locations onshore, are limited due to CO2 transportation costs. Offshore application of CO2 is additionally restricted due to difficulties in isolating CO2 contamination, and the resulting corrosion.
Gases other than CO2 have been employed to a wide extent in full-scale EOR processes that involve well-to-well flooding. The gases that have been used include methane, rich gas, nitrogen, and flue gas. Methane and rich gas are non-corrosive, and are frequently available from production streams. Nitrogen is also non-corrosive, and there are inexpensive procedures available for extraction of nitrogen from air. Flue (or engine exhaust) gas is primarily nitrogen mixed with 10-20% CO2. It is a product of combustion and is generated by combustion facilities such as power plants.
Despite the widespread use of these gases in well-to-well processes, only a limited number of reports concerning the use of gases other than CO2 have appeared in the cyclic injection literature. Haines and Monge examined the feasibility of cyclic natural gas injection for the recovery of light oils. The natural gas employed in the study was primarily methane contaminated with less than 2% nitrogen, CO2, and ethane. Coreflood and numerical simulation results indicated that the natural gas injection process was a technically and economically feasible EOR option, and that repressurization and gas relative permeability hysteresis were the most important recovery mechanisms.
Shelton and Morris utilized cyclic rich gas injection to improve production rates in viscous oil reservoirs. The rich gas employed in the field tests consisted of methane enriched with propane.
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