This paper examines the feasibility of cyclic natural gas injection for the enhanced recovery of light oil from waterflooded fields. Approximately 40 percent of waterflood residual oil was recovered in corefloods using two huff 'n' puff cycles at immiscible conditions. Gross gas utilizations were as low as 3 MSCF/STB. Response to cyclic injection contrasted favorably with immiscible WAG displacement. Coreflood results and numerical simulation indicated that incremental oil was not highly sensitive to remaining oil saturation. Predicted field recovery could be improved by managing offset wells. Response to cyclic gas injection was mostly dependent upon the amount of gas injected, suggesting that there will be a maximum economic slug size in field applications. Results indicated that repressurization and gas relative permeability hysteresis are the major oil recovery mechanisms.
Single-well, cyclic injection/production processes are characterized by low initial capital outlay and rapid payout, offering a timely alternative to full scale tertiary flooding. The literature explores cyclic gas injection using rich gas, exhaust gas, and CO2. These processes were introduced for the stimulation of viscous-oil reservoirs, 1,2 While cyclic CO2 injection was originally proposed as an alternative to cyclic steam for heavy oil recovery, it is now finding wide application in light-oil reservoirs. 3,4
This study was undertaken to encourage exploitation of natural gas huff 'n' puff. By analogy to other cyclic gas injection processes, especially cyclic CO2, an immiscible process using natural gas seemed feasible for enhanced recovery of light oil from waterflooded fields. Environments like offshore which are often too risky for conventional enhanced oil recovery operations, might be economic targets for natural gas huff 'n' puff, because natural gas is on location, gas consumption is low, and test design is simple. Nonetheless, documentation on natural gas huff 'n' puff could not be found. The purpose of this work was therefore to generate laboratory data so that field potential could be evaluated.
Literature available on other cyclic gas injection processes aided this study, and some interrelationships between natural gas huff 'n' puff and the other cyclic gas processes were found. Coreflood and numerical simulation results indicate that natural gas huff 'n' puff does provide improved oil recovery opportunity.
A schematic of the high pressure, high temperature coreflooding apparatus is shown in Fig. 1. Positive displacement pumps were used to inject fluids at constant volumetric rate, and a back pressure regulator controlled production. Four rodded-piston transfer vessels were used to inject separator oil, recombined oil, gas, or brine. The core represented the production wellbore vicinity. A brine transfer vessel was used not only to establish waterflood residual oil (Sorw), but also to model the remainder to the reservoir and to provide pressure support. Injection and differential pressures were monitored throughout each experiment. Produced oil, brine, and gas were measured volumetrically.
Table 1 lists fluid characteristics for the separator oil, the recombined oil, and the lean natural gas used for these experiments. Brine was three weight percent sodium chloride.