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In view of the difficult problems encountered in the development of tight oil reservoirs, using natural gas as an energy supplementary agent and adopting Huff-N-Puff (HNP) production methods have shown great application potential in the development of such reservoirs. In this study, we took the Chang-7 tight oil cores as the research object. Then, we choose natural gas (methane or the associated gas) as the injection agent and used an experimental simulation device to perform natural gas Huff-N-Puff experiment in tight cores. Finally, we analyzed the factors that affect the enhanced oil recovery factor through parallel comparative experiments. The results show that the natural gas HNP development method can effectively increase oil production based on primary depletion production, and its effective production periods were concentrated in the 1st and 2nd rounds. The flooding mechanism of natural gas HNP process focuses on interfacial aggregation and interfacial slippage of crude oil, which are conducive to enhance oil production. Moreover, when the gas agent was CH4, it was not necessary to reserve too long soaking time during CH4 HNP process; when the gas agent was associated gas, a longer soaking time could be appropriately reserved. Additionally, the higher pressure CH4 injection would aggravate asphaltene precipitation, which will hinder migration of oil and gas in the pore throat of tight cores. Besides, we established the corresponding numerical model via using CMG software. Generally, the CH4 HNP process with higher gas injection pressure had higher oil recovery. However, the ultimate oil recovery under 30 MPa gas injection pressure was lower than that under 28 MPa gas injection pressure. Therefore, the appropriate gas injection pressure should be selected in accordance with reservoir conditions during CH4 HNP process, making its benefits overweigh its harms. [ABSTRACT FROM AUTHOR]

[Display omitted] • Revealing the effect of CO 2 on oil potential energy in nanopores. • CO 2 solubility and oil swelling factor explained within nanopores. • Influence of CO 2 on the recovery law of different components in oil. • Analysis of different water film thickness on CO 2 enhanced oil recovery. Understanding the mechanisms of CO 2 extraction or flooding are vital for enhancing oil recovery (EOR) in tight reservoirs. In this study, the CO 2 EOR mechanism in the displacement-affected area (DPAA) and diffusion-affected area (DFAA) of quartz nanopores were thoroughly investigated using molecular dynamics simulation techniques. First, the following two contents were mainly simulated, namely CO 2 flooding oil in the single/double nanopores of DPAA and CO 2 extraction oil in dead-end nanopores of the DFAA with and without the water film. Then, tight oil potential energy, threshold capillary pressure, CO 2 solubility, and oil swelling in nanopores were calculated to clarify the effects of CO 2 on oil transport. Moreover, different CO 2 injection/flowback rates and different water film thicknesses on dead-end nanopores on oil recovery were discussed. In the DPAA, the CO 2 solubility and the oil swelling factor gradually decreased with distance from the CO 2 -oil interface (Y = 0 nm), where the higher the injection rate, the more easily the CO 2 dissolved in the oil. However, the injection rate of CO 2 was inversely proportional to oil recovery. In addition, it took longer for the displacement efficiency in the 6 nm pore of double pores to reach the same displacement efficiency as in the single 6 nm pore. In the DFAA, the effect of flowback rate on the displacement efficiency of oil was relatively low. However, the thickness of the water film was a key factor that affected the oil displacement efficiency in the DFAA. [ABSTRACT FROM AUTHOR]

Abstract: Alkaline agents have an appeal for enhanced oil recovery because of their low cost and favorable performance. In this paper, sodium carbonate (Na2CO3) and sodium hydroxide (NaOH) are used as the alkaline chemicals, at the same Na2O content, to investigate the oil/water interfacial reactions between the Daqing crude oil and the alkaline solutions. Moreover, oleic acid or the mixture of ethyl acetate and phthalic acid diethyl ester were added into the crude oil, respectively, to facilitate the direct observation of the interfacial reactions and to compare the functional effectiveness of alkalis. The results showed that: Na2CO3 reacted slowly and partly with the acid components in crude oil, while NaOH did it very fast and completely. Interestingly, Na2CO3 is better than NaOH in lowering the oil/water interfacial tension (IFT), due to its buffer effect. These help the optimum formulation design of flooding alkali, which should also be of great importance for tertiary oil recovery. [Copyright &y& Elsevier]