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Imbibition after fracturing is an important way of enhancing oil recovery in shale oil reservoirs. It includes three steps: fracturing, huff and puff imbibition, and water flooding. The shale oil reservoir of the Lucaogou Formation in Jimsar Sag was taken as the research object to understand further the mobility of shale oil by imbibition and displacement. First, the porosity, permeability, and wettability of the reservoir were analyzed via experimental methods such as nitrogen permeability porosimetry, automatic mercury porosimetry, microscopic scanning electron microscopy (SEM), and nuclear magnetic resonance technology. The effects of imbibition and displacement processes on various pore sizes were comparatively studied based on the self-absorption method combined with core nuclear magnetic resonance (NMR) to test the wettability. The experimental results show that the reservoir has well-developed micro nano pores and poor pore connectivity. Moreover, the wettability is neutral to oil wetting, and the average pore throat radius distribution is between 0.01 and 0.39 μm. Small (r t < 0.09 μm), medium (0.09 μm < r t < 0.85 μm), and large (r t >0.85 μm) pores account for 42.56%, 28.64%, and 28.8%, respectively. Through pore size conversion, we can know that the lower limits of the fluid utilization radius for the infiltration and displacement processes are 10 and 40 nm, respectively. Therefore, reservoirs with pore distribution between 10 and 40 nm can be exploited through imbibition, while reservoirs above 40 nm are suitable for both imbibition and conventional displacement oil recovery.

This paper proposes a model for the electromagnetic performance of the dual air-gap liquid-cooled eddy current retarder (DAL-ECR) considering the transient permeability. First, the structure and working principle of the DAL-ECR are introduced. Next, the analysis model of the static air-gap flux density considering the flux leakage and end effect is established based on the piecewise function method and the magnetic equivalent circuit (MEC). Then, based on the skin effect of the electromagnetic field in the retarder, an iterative method for solving the transient permeability of the stator is proposed. According to Faraday’s and Ampere’s laws, the analysis model of the static air-gap flux density, and the transient permeability, the analysis model of the transient air-gap flux density is established. The braking torque of the DAL-ECR is then calculated while taking the actual path of the eddy current and the skin effect on the permeability of the stator into consideration. Finally, the calculation accuracy of the model was verified by the finite element method (FEM) and the bench test.

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