The global production of tight oil has increased dramatically over the past few years. In order to maintain and extend the production growth from such resources, enhanced oil recovery methods must be applied to shale oil reservoirs. Some of earlier studies have confirmed the potential of cyclic gas injection to improve the oil recovery from shale reservoirs. More studies are needed to explore the mechanisms and to optimize the process.
In this study, the nitrogen huff-n-puff technique has been applied to oil-saturated shale core samples to improve oil recovery. This paper presents the new experimental work to investigate the effect of pressure decline rate on oil recovery. The effect of soaking time is also studied. Eagle Ford shale core plugs were used in the experiment. For the purpose of comparison and validation, two groups of tests were conducted. One group of tests had one-day soaking time, and the other had three-day soaking time. Both of them were under 1,000 psi soaking pressure. Within each group, three pressure depletion rates were controlled, with the initial soaking pressure depleted in 0.05 hours, 4 hours, and 40 hours, respectively. During the puff periods of experiments, the amount of oil recovered with time (history of oil recovery) was closely monitored to understand the mechanisms.
The experimental results show that oil was recovered rapidly during the first two hours during each puff cycle. The oil recovered in each cycle should be measured when no further oil comes out of the core, and we chose six hours as the production time. The results also showed that oil recovery increased as the pressure depletion time was increased, with the same cyclic time and the same number of cycles. For example, for the case of one-day soaking time, the oil recovery factor under the pressure blowout condition was the highest (nearly 50.5% after 10 cycles), while the oil recovery factor was about 43.4% when the soaking pressure was depleted in 40 hours. The oil recovery also increased with longer soaking time only within a certain period, because a further longer period did not present a better performance. The results show that the pressure depletion rate, soaking time and the production time played important roles during the huff-n-puff gas injection process.