Experimental evaluation of enhanced oil recovery in unconventional reservoirs using cyclic hydrocarbon gas injection

Abstract

The significant amount of gas produced from unconventional reservoirs in the US in recent years has reduced market prices for hydrocarbon gas and created an opportunity for using low-cost gas in enhanced oil recovery (EOR) projects. In addition to its availability in large quantities and at low cost, injecting produced gas back into the reservoir prevents flaring the gas, reducing environmental concerns associated with greenhouse gas emissions to the atmosphere. Furthermore, while CO2 has shown tremendous success as an EOR injectant in huff-and-puff operations in tight liquid-rich shales, the lack of sufficient pipeline infrastructure supplying low-cost CO2 as well as operational issues (e.g., corrosion) has forced field operators to consider other gas options, such as produced gas, to substitute for or mix with CO2.While limited, experimental and numerical studies conducted on produced gas huff-and-puff in unconventional resources have shown promising results and highlighted the great potential of this technique for enhanced oil recovery from ultra-tight shale resources. The objective of the current work is to investigate the feasibility of produced gas huff-and-puff operations in one of the major oil-producing shale formations in North America further by performing several flow experiments using various mixtures of hydrocarbon gases. Three reservoir-condition core-flooding experiments are carefully designed and executed on shale core plugs in the absence and presence of proppant-filled fractures. After establishing initial two-phase brine-oil conditions in the core samples, hydrocarbon gas is injected into the medium through a huff-and-puff approach. The cycle of injection-soaking-production is repeated multiple times in each experiment. The results show that the use of hydrocarbon gas in huff-and-puff operations has great potential similar to CO2 for enhanced oil recovery from tight shale rocks and can lead to remarkable oil recovery. The hydrocarbon gas mixture in the fracture penetrates the matrix due to pressure and concentration gradients during the injection and soaking periods and forces the oil to leave the matrix during the depressurization process. It is observed that the rate of oil production is high in the first cycle and decreases in subsequent cycles as the rate of mass transfer and the amount of oil remaining in the matrix decrease. The average oil production due to the cyclic gas injection process in an intact and fractured composite rock sample was more than 48 and 21%, respectively. The results show that even though the ultratight formations could be heavily fractured, a proper cyclic gas injection EOR scheme could potentially improve oil recovery from the shale oil reservoirs, significantly.