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The LD oilfield is one of the representative offshore oilfields. After weak gel flooding, the recovery rate is significantly improved. However, the oilfield is then in a medium- to high-water content stage, presenting a complex distribution of the remaining oil. The measures for further enhanced oil recovery (EOR) are uncertain. As a result, it is necessary to clarify the distribution pattern and development potential of the remaining oil during the high-water content period after weak gel flooding. In this study, an online nuclear magnetic resonance (NMR) oil displacement experiment and microscopic oil displacement experiment were conducted, and the mechanisms of weak gel flooding and the distribution pattern of the remaining oil were clarified in the LD oilfield. Additionally, high-multiple water flooding and numerical simulation experiments were conducted to analyze the development potential after weak gel flooding. The results show that the effect of weak gel flooding was more significant in the core of 1500 mD, with an increase in oil recovery of 9% compared to 500 mD. At a permeability of 500 mD, the degree of crude oil mobilization in micropores and small pores caused by weak gel flooding was improved by 29.64% and 23.48%, respectively, compared with water flooding. At 1500 mD, the degree of crude oil mobilization in small pores caused by weak gel flooding was increased by 37.79% compared to water flooding. After weak gel flooding, the remaining oil was primarily distributed in medium and large pores. Microscopically, the remaining oil was dominated by cluster residual oil, accounting for 16.49%, followed by columnar, membranous, and blind-end residual oil. High multiple water flooding experiments demonstrated that weak gel flooding could significantly reduce development time. The ultimate oil recovery efficiency of 500 mD and 1500 mD reached 71.85% and 80.69%, respectively. Numerical simulation results show that the ultimate oil recovery efficiency increased from 62.04% to 71.3% after weak gel flooding. This indicated that the LD oilfield still had certain development potential after weak gel flooding. The subsequent direction for enhanced oil recovery focuses mainly on mobilizing oil in medium pores or clustered remaining oil. This will play a crucial role in further exploring methods for utilizing the remaining oil and increasing the recovery rate. [ABSTRACT FROM AUTHOR]

[Display omitted] • Amphiphilic molybdenum disulfide nanosheets can self-adsorb onto the oil-water interface to form a dense interface adsorption film. • The oil-water interface adsorption film exhibits excellent elastic properties and higher strength. • The oil-water interface adsorption film can intelligently control displacement front in porous media for enhanced oil recovery. Immiscible liquid-liquid interfaces have widespread applications in drug delivery, oil-water separation, oil recovery, emulsion and foam stability, etc. Here, a dense viscoelastic interface film is constructed via the self-adsorption of amphiphilic molybdenum disulfide nanosheets at the oil-water interface, which has significant implications for enhanced oil recovery. The microscopic dispersion characteristics of amphiphilic nanosheets in water phase are characterized using Cryo-EM, and the adsorption regularities of amphiphilic nanosheets in horizontal and vertical directions of oil-water interface are also revealed. Besides the mechanical response and rheological properties of interface adsorption film are systematically studied. Experimental results demonstrate that the amphiphilic nanosheet can self-curl to form a single amphiphilic nanosheet with a size of 30 nm in the water phase as the amphiphilic nanosheets concentration is lower than 50 mg/L. When the amphiphilic nanosheets concentration reaches 100 mg/L, obvious water-phase aggregation behaviors happen. The amphiphilic nanosheets can reduce the oil-water interfacial tension from 18.47 mN/m to 0.27 mN/m by forming multilayer adsorption at the oil-water interface. The adsorption ratio of amphiphilic nanosheets from bulk phase to oil-water interface can reach 97.50 %. In addition, the oil-water interface adsorption film exhibits excellent elastic properties and higher strength because the amphiphilic nanosheets can form a planar network structure at the oil-water interface which can be also directly observed in microfluidic pores. The storage modulus and interface shear viscosity of interface adsorption film are 1.65 Pa and 1.38 mN·s/m, respectively. In theory, the required desorption energy of a single amphiphilic nanosheet from oil-water interface to bulk phase is deduced to be 587 K B T. This work is expected to provide amphiphilic molybdenum disulfide nanosheet as an effective candidate for enhanced oil recovery. [ABSTRACT FROM AUTHOR]

Nitrogen gas drive and foam drive are commonly adopted for practical production in fractured carbonate reservoirs. A systematic study on nitrogen gas drive and foam assisted nitrogen gas drive in varying-aperture fractures is a practical and essential method to understand the EOR mechanism. In this study, we designed and fabricated similarity models and microscopic visual models. The fluid flow mechanisms were discussed combining the pressure response and dynamic displacement phenomenon. Experimental results show that the effective time of nitrogen gas drive was longer in decreasing-aperture fractures than that in increasing-aperture fractures. After water flooding, 0.8 PV nitrogen gas injection lead to gas channeling in decreasing-aperture fracture of 1 mm–0.2 mm while 0.6 PV nitrogen gas injection by reverse displacement direction. Both production rate and average pressure appeared discrete in narrow fracture compared to wide fracture. Enhanced oil recovery efficiency by foam drive was average 5% higher than that by nitrogen gas drive in single fracture. The displacement front of foam varies in four forms: micro-scale foam accumulation, small-scale foam accumulation, mesoscale foam slight deformation and large-scale foam serious deformation. This study could provide a theoretical basis for enhance oil recovery and further dig the potential of remaining oil in fractured carbonate reservoirs. • Designed and Fabricated similarity models and microscopic visual models of varying-aperture fractures of carbonate reservoirs. • There is a synergistic effect of gas and water flooding in fractures, which promote to oil production by regulating the position of oil-water interface in fractures. • The displacement front of foam varies in four forms: micro-scale foam accumulation, small-scale foam accumulation, mesoscale foam slight deformation and large-scale foam serious deformation. [ABSTRACT FROM AUTHOR]

Fractured-vuggy carbonate reservoir played a significant role in unconventional reservoirs, but it was difficult to be developed due to its complex reservoir types. Therefore, it was crucial to find a suitable development program to enhance oil recovery. In this work, a two-dimensional visual physical model of fractured-vuggy reservoir was designed and fabricated according to actual geological data and injection-production characteristics of a representative fractured-vuggy carbonate reservoir, Tahe Oilfield in China. Nitrogen flooding experiment, nitrogen flooding under the high gas energy experiment, nitrogen flooding under strong bottom water energy experiment and foam assisted nitrogen gas flooding experiment were carried out respectively by using two-dimensional visual model. The flow behaviors and the main mechanisms for enhanced oil recovery were revealed by analyzing the oil displacement performance in the visual physical model. Experiment results showed that the gas cap energy and the bottom water energy played a synergistic effect in the nitrogen flooding. The stronger the bottom water energy was, the larger the gas spread area was, and the increase of oil recovery would be very high. If the gas injection rate was too strong, the dominant gas flow channel would be formed prematurely, and the oil recovery was little increase. In addition, foam could effectively control the gas mobility, delay the formation of gas channeling. Meanwhile, the oil recovery of foam assisted nitrogen gas flooding was higher 12% than that of nitrogen flooding. Therefore, the mechanism for improving the degree of development was obtained mainly through gravity differentiation to form the secondary gas top and realize the development of the remaining oil. • A two-dimensional visual physical model of fractured-vuggy reservoir was designed and fabricated. • The flow behaviors and the main mechanisms for nitrogen and foam assisted nitrogen gas flooding were studied. • The oil recovery of foam assisted nitrogen gas flooding was higher 12% than that of nitrogen flooding. [ABSTRACT FROM AUTHOR]