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Recently, spherical nanoparticles have been studied to enhance oil recovery (EOR) worldwide due to their remarkable properties. However, there is a lack of studies of nanosheets on EOR. In this work, we synthesize the amphiphilic molybdenum disulfide nanosheets through a straightforward hydrothermal method. The octadecyl amine (ODA) molecules were grafted onto the surfaces of molybdenum disulfide nanosheets due to the presence of active sites over the surfaces of MoS2 nanosheets. The synthesized amphiphilic molybdenum disulfide nanosheets (ODA-MoS2 nanosheets) are approximate 67 nm in width and 1.4 nm in thickness. The effects of ultralow concentration ODA-MoS2 nanosheets on the dynamic wettability change of solid surfaces and emulsion stability were also studied and discussed. Besides, the core flooding experiments were also conducted to reveal the adsorption rules and the oil displacement effects of ultralow concentration ODA-MoS2 nanosheets. Experimental results indicate that the oil-wet solid surface (a contact angle of 130°) can transform into the neutral-wet solid surface (a contact angle of 90°) within 120 hrs after 50 mg/L ODA-MoS2 nanosheets treatment. In addition, micro-scale emulsions in size of 2 µm can be formed after the addition of ODA-MoS2 nanosheets by adsorbing onto the oil-water interfaces. The desorption energy of a single ODA-MoS2 nanosheet from the oil-water interface to the bulk phase is proposed. When the concentration of ODA-MoS2 nanosheets is 50 mg/L, the emulsions are the most stable. Core flooding results demonstrate that the ultimate residue of ODA-MoS2 nanosheets in porous media is less than 11%, and the highest increased oil recovery of around 16.26% is achieved. Finally, the production performance of ultralow concentration of ODA-MoS2 nanofluid (50 mg/L) in the application of Daqing Oilfield is summarized and discussed.

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Microemulsion with excellent properties has shown significant potential in enhancing oil recovery from tight formations by spontaneous imbibition. High temperature resistant and permeable flooding nanofluid (HTPF-NF) was prepared through microemulsion dilution method. Firstly, an evaluation process of HTPF-NF properties was evaluated through the particle size and high temperature resistant experiments, interfacial tension (IFT) and wettability measurements, as well as the static adsorption experiment. Meanwhile, Transmission Electron Microscope (TEM) was utilized to characterize the particle size and morphology of HTPF-NF before and after solubilization of the crude oil. Optical Microscope (OM) was also used to observe the morphological characteristics of emulsified oil droplets. Then, the spontaneous imbibition oil recovery (SIOR) mechanisms of- HTPF-NF were proposed ulteriorly by adopting imbibition tests and CT scanning technology. Moreover, the field application effects of HTPF-NF micro-fracture huff and puff in Shengli Oilfield were also analyzed. The results show that the optimum concentration of HTPF-NF is 0.2wt% with excellent comprehensive performances. Under this concentration, the particle size is 7.5 nm, the IFT value reaches ultra-low IFT, the wettability alteration ability is remarkable and the adsorption loss is minimized. Afterwards, adhesion work reduction factor (AWRF), solubilization coefficient (SC) and the self-driving force (SDF) are defined and calculated based on the IFT and contact angle for quantitatively analyzing the SIOR mechanisms. Furthermore, the data and images of spontaneous imbibition and CT scanning present that the longer the imbibition distance, the higher SIOR of HTPF-NF. In conclusion, the SIOR mechanisms of HTPF-NF are summarized as follows: remarkable property of solubilizing crude oil, brilliant ability of reducing oil/solid adhesion work by synergistic effect of IFT reduction and wettability alteration, and excellent capability of increasing imbibition distance by larger self-driving force. After soaking for two months, YB well was produced with the dramatic increase of the liquid production from 9.55 bbl/d to 34.5 bbl/d, the oil production from 5.15 bbl/d to 19.1 bbl/d and the water cut from 27.3% to 40.8%.

Due to the long-term development of water and gas injection in Tahe Oilfield, the production increase effect decreases year by year. Based on the understanding of the treatment difficulties of water injection and gas injection in the development stage of the fractured vuggy reservoir in Tahe Oilfield, this paper combined with the theoretical analysis of Nano-sheet technology at this stage, such as super amphiphilic interface performance, super emulsification and viscosity reduction ability, ultra-low core adsorption, and super oil washing ability, and has carried out the field application of 2-D black nano-sheet oil displacement in the fractured vuggy reservoir in Tahe Oilfield for a total of 15 wells. Through the oil increase evaluation and comparison of well groups with different inter-well displacement types and different karst reservoir spaces after the application of Nano-sheet, it is found that the well groups with good inter-well connectivity composed of weathered crust underground river karst reservoir space had a high degree of effectiveness. It is the best-selected area for field application. The mine receiving efficiency was as high as 85.7%, and the oil increase is significant, with a cumulative oil increase of more than 5000 tons. It provides practical guidance and a theoretical basis for the follow-up development of 2-D black nano-sheet oil displacement technology in the fractured vuggy reservoir of Tahe Oilfield.

This paper summarizes the change rule of production performance and the EOR efficiency from the micro-dispersed gel foam injection in the fractured-vuggy carbonate reservoir of Tahe Oilfield. The TK722CH2 well group injected gas from August 2014 to September 2018. During the gas injection stage, the effect of periodic gas injection decreased obviously, the effective direction of gas injection was single and the risk of gas channeling increased greatly. The field pilot test f micro-dispersed gel foam was carried out on September 20, 2018. The fluid is injected into well group in three slugs: micro-dispersed gel foam, normal foam and nitrogen gas. As a part of the foam pilot test monitoring, a gas tracer study was performed before and after the injection of gel foam in the reservoir. After the pilot test was carried out in the TK722CH2 well group, the subsequent injection gas swept new fractures and vugs, and a new dynamic connectivity has been established. The connectivity of well group changed from 1 injection well connects with 1 production well to 1 injection well connects with 4 production wells. Through the field pilot test of micro-dispersed gel foam, this paper verifies the effect of improve gas flooding and increase sweep volume of micro-dispersed gel foam. By analyzing the results of the field pilot test, the relevant technical mechanism of micro-dispersed gel foam in fractured-vuggy reservoir is revealed. As a result, the field pilot test in this paper provides theoretical basis and technical support for the efficient development of fractured-vuggy carbonate reservoir.

Spontaneous imbibition is an effective method to improve the oil recovery of tight reservoirs.The development and research of nano-emulsion provide a new technical direction for improving spontaneous imbibition oil recovery (SIOR) in tight reservoirs.The salinity has a significant effect on the properties and spontaneous imbibition of nano-emulsion, but the effect on the imbibition mechanisms is still unclear.In the paper, thelower nano-emulsion system with core-shell structures, called HS-LNE,is prepared by nano-emulsion dilution method.Afterwards,the properties and imbibition mechanisms of HS-LNE under different salinitiesare studied.Firstly,the particalsize andstability of the HS-LNE system are evaluated by centrifugation and spectroscopy. What's more, the interfacial tension (IFT), wettability,and adsorption experiments under diffierent salinitiesare carried out, and the salinity influences on the physicochemical properties of the HS-LNE system is systematically analyzed.At the same time, the SIORvariationlaw of the HS-LNE system under different salinitiesis comprehensively explored through static spontaneous imbibition experiments.The experimental results show that the HS-LNE system has excellent stability at high salinity. Moreover, as the salinity increasing, the IFT of the HS-LNE system decreased, and the wettability alternationincreased.However, the adsorption on solid interfaceswas not significantly changed and SIOR reached the highest value of 50.27% at the optimal salinity (50×103mg/L). Finally, the HS-LNE system with the optimal salinity was selected for the field test forpermeability enhancement and flooding in Well Gan 128 in Jilin Oilfield. The oil production was increased by 450 t after 5 months, with an estimated validity period of 24 months and a cumulative oil production increase of 1900 t.