An Experimental Comparison of CO2 Huff-N-Puff Process in Shale and Tight Reservoirs

The shale and tight sandstone reservoirs, the important parts of unconventional oil and gas resources, are extremely difficult to be exploited due to the complex geological features and the diverse modes of occurrence of fluids. CO2 huff-n-puff technology, an important and efficient method of EOR, is expected to be widely applied in unconventional reservoirs. In order to understand and evaluate the application potentiality of this technology in different unconventional reservoirs, a series of CO2 huff-n-puff experiments were carried out in both shale and tight sandstone cores. The effects of core types, dissolved gas, operating pressure, and number of cycles were investigated to evaluate the different development effects of CO2 huff-n-puff technology in different unconventional reservoirs. The cumulative amount of injected CO2 during the soak period was analyzed to understand the differences of CO2 dissolution and diffusion processes in shale and tight sandstone reservoirs. The results show that the oil recovered by CO2 huff-n-puff technology in tight sandstone reservoirs is higher than that in shale reservoirs due to the difficulty of the dissolved oil being extracted in the organic matter. The existence of dissolved gas can further enhance recovery factors in both shale and tight sandstone reservoirs and partly improve the extraction of oil by CO2 in the organic matter of the shale. Moreover, the increased pressure beyond MMP still has effect on the recovery factor in shale reservoirs but has little effect in tight sandstone reservoirs. In addition, more cycles of CO2 huff-and-puff are conducive to enhancing oil recovery in the shale, compared to the tight sandstone. Therefore, different production measures should be adopted in CO2 huff-n-puff technology for different unconventional reservoirs.