Using huff n' puff with a recycled hydrocarbon gas as a means for enhancing oil recovery in a liquid-rich shale reservoir

In recent years, production in unconventional reservoirs has increased exponentially due to technological breakthroughs in horizontal well completions. However, even with new technology, ultimate recovery after primary production in these reservoirs is extremely low (5-10% of original oil in place). The huff n’ puff process is considered to be a strong candidate for enhancing these notoriously-low recovery factors in unconventional reservoirs, especially those that are liquid-rich, in a cost-effective manner. Huff n’ puff is an enhanced oil recovery method in which one well alternates between injection, soaking, and production. Gas injection is often used in this scenario because of its high injectivity compared to water and its ability to develop miscibility with the reservoir oil. In this work, a recycled hydrocarbon gas was used due to its ease of accessibility within the target reservoir. In this work, the application of huff n’ puff to a liquid-rich shale reservoir with nanodarcy-range permeability was investigated both experimentally and numerically. A completely unique experimental setup was fabricated in order to execute oil recovery experiments on preserved core plugs taken from the target reservoir. In these experiments, it was shown that significant amounts of oil could be recovered after two huff n’ puff cycles lasting approximately one day each. Using propane as the injection gas resulted in higher recoveries when compared to the recycled gas due to enhanced miscibility with the oil. It was also shown that the ratio between soaking pressure and production pressure is a significant factor in recovering oil via huff n’ puff. An additional cycle was run with a longer soaking time, but no additional oil was recovered. A set of numerical reservoir models was also created to further investigate the recovery mechanisms in the huff n’ puff process. Lab-scale models were created in an attempt to replicate the experimental findings. The results showed that the recoveries seen in the experiments and simulations were very similar. Also, as long as injection took place above MMP, it was shown that the gas and oil mixed similarly in all cases regardless of pressure. Furthermore, lower production pressures allowed for more gas expansion and therefore better recovery, proving that production pressure alone may be an important parameter rather than the ratio between production and injection pressures. Field-scale models were also created. These models also showed that gas expansion plays a significant role in recovering oil. However, there were several key differences associated with sweep efficiency and the use of live oil versus dead oil.