Abstract
It has been proven that polymer injection at early times is beneficial to offshore heavy oil recovery. It is of significant importance to optimize the polymer injection timing and decide the residual oil distribution after polymer flooding. Aiming at a specific offshore heavy oil reservoir in Bohai, China, the optimum polymer injection timing is investigated through laboratory experiments. The influence of polymer injection timing on oil displacement and remaining oil distribution is analyzed by combining macroscopic and microscopic flooding experiments. The results reveal that the optimum polymer injection timing should be close to the water breakthrough, i.e., just before the waterflooding front reaches the outlet of the core. In addition, the waterflooding front position is analytically solved by using the Buckley–Leverett method and verified by experimental results, which supply an approach to predict the polymer injection timing. When polymer is injected before the waterflood front reaches the outlet of the core, the mobility control ability of polymer solution can reduce the fraction of bypassed volume of the reservoir by waterflooding. The early injected polymer mainly enters the high permeability zone, which works positively in two ways. Firstly, it improves the oil displacement efficiency of the high permeability zone. Secondly, the polymer establishes a flow resistance in the high permeable zones, thus improving the sweep efficiency in the low and medium permeability zones. However, our residual oil distribution experiments illustrate that there is still a large amount of oil remaining in the low and medium permeability zones. Therefore, it is necessary to explore additional EOR methods to recover the abundant residual oil.
Highlights
In the early stage of heavy oil reservoir in Bohai Sea, water injection is used to supplement formation energy, and the unfavorable mobility ratio leads to the rapid rise of water cut
In our previous work [17], it was concluded that the optimum injection timing for a 70 mPa·s heavy oil was when the water saturation was in the range of 0.204–0.256
Our macroscopic and microscopic experimental studies revealed that the optimum polymer injection timing is close to the water breakthrough
Summary
In the early stage of heavy oil reservoir in Bohai Sea, water injection is used to supplement formation energy, and the unfavorable mobility ratio leads to the rapid rise of water cut. In 2002, field tests on early polymer injection (when water cut was 60%) were carried out in China Offshore Heavy Oil S Oilfield. Processes 2020, 8, 93 that early polymer injection could improve both the production rate and the final oil recovery [4,5,6]. The success of oilfield tests has proven that the conclusions of early polymer flooding obtained through laboratory experiments and numerical simulations were valid, which motivated further investigation into establishing an optimum injection timing range for polymer flooding and shedding light on more efficient offshore oilfield development. In this paper, the optimum injection timing of polymer flooding was investigated by conducting both macro and micro laboratory experiments based on our previous study [17]. Characteristics of the remaining oil were decided by means of saturation monitoring [20,21], which directed ways to further enhance oil recovery after early polymer injection
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