Spontaneous imbibition is an effective method of tight sandstone reservoirs development. However, their underlying mechanisms are still unclear due to the representative issue caused by strong heterogeneity and complex oil-brine properties. In this research, physical properties and thin-section images were systematically analyzed, and the spontaneous imbibition efficiency was examined from core tests. The results show that quartz and feldspar predominantly contribute to mineralogy. Low brine salinity, high oil viscosity, and high interfacial tension corresponded to strong spontaneous imbibition ability and fast process. High interfacial tension is the dominant controlling factor in this field. Furthermore, the parameter simulation model established by the dimension reduction method could catalyze the prediction of spontaneous imbibition efficiency, which is confirmed by higher reliability.

The strong reservoir heterogeneity and complex microscopic pore structure in the Linxing area make it prone to water block damage during imbibition development. In order to explore the influence of reservoir microscopic characteristics on imbibition efficiency, taking the tight sandstone gas reservoir in the Linxing area of Ordos Basin as an example, the heterogeneity of the tight sandstone reservoir in the study area is characterized in terms of physical and chemical characteristics as well as the microscopic pore structure. Using nuclear magnetic resonance, high-pressure mercury pressure, and other testing methods, spontaneous seepage experiments in real sandstone were carried out to study the distribution law of different pore structures and seepage characteristics at different times and to systematically evaluate the microscopic pore characteristics of dense sandstone reservoirs and the factors affecting seepage and suction. The results show that due to the strong microscopic heterogeneity of tight sandstone, the macroscopic properties cannot directly reflect the microscopic characteristics, and the response to imbibition efficiency is stronger. The pore size is the main controlling factor affecting imbibition, and the contribution rate of the micropore and mesopore mainstream pore size spaces is higher than that of the macropore. Micropores provide imbibition power, and mesopores provide an imbibition interval. High-porosity and high permeability reservoirs are more conducive to imbibition replacement. The intercrystalline pores have a great influence on the imbibition efficiency, and the influence of intergranular pores and dissolution pores on the imbibition cannot be underestimated. The smaller the relative sorting coefficient, interfacial tension, and contact angle, the better the imbibition effect of fracturing fluid. Research results have theoretical guiding significance for spontaneous imbibition to improve oil recovery.

Nanoscale pore-throat systems are widely developed in the pore-throat of tight reservoirs. The pore-throat structures of different microscales are complex and diverse with obvious microscale effects. Taking the Chang 6₃ tight sandstone reservoir of the Huaqing area in Ordos basin as an example, under the guidance of information entropy theory, the quantitative characterization model of pore-throat micro-scale heterogeneity in a tight oil reservoir is established based on casting thin sections, physical properties analysis, constant velocity mercury injection, and NMR technology. Moreover, the correlation between pore-throat heterogeneity and porosity, permeability and movable fluid saturation is analyzed. The results show that there are obvious differences in pore-throat heterogeneity between different reservoirs, and the throat uniformity of macro pore-fine-throat reservoir, macro pore–micro throat reservoir, and macro pore–micro throat reservoir becomes worse, successively. There is a negative correlation between porosity uniformity and porosity, permeability and movable fluid saturation. However, there is a positive correlation between throat uniformity and combined pore throat uniformity and porosity, permeability and movable fluid saturation. Therefore, the uniformity of the throat controls the seepage capacity and fluid mobility in the pore system of the Chang 6₃ tight sandstone reservoir in the study area. This has important theoretical and practical significance to enhance oil recovery and promote the efficient development of a tight oil and gas reservoir.

The feasibility of the autothermic pyrolysis in situ conversion (ATS) process for low-grade oil shale (OS) has not been determined. In this research, the pyrolysis and combustion properties of Tongchuan OS, with a 4.04% oil yield, were systematically analyzed. The findings revealed that temperatures between 350 and 425 °C favored oil production, while temperatures from 450 to 520 °C resulted in a higher rate of gaseous generation. At 300 °C, the volume expansion and ignition coking caused by the large amount of bitumen generated resulted in severe pore plugging, which significantly increased the combustion activation energy of the residue, while the presence of substantial flammable bitumen also significantly decreased the ignition and combustion temperatures. From 300 to 520 °C, the combustion performance of residue decreases continuously. In addition, pyrolysis residues of Tongchuan exhibited a slightly higher calorific value, between 425 and 520 °C, owing to its higher fixed carbon content (10.79%). Based on the ideal temperature screening method outlined for Tongchuan OS, the recommended preheating temperature for Tongchuan OS was 425 °C, while the optimum temperature for the retorting zone should be 510 °C, considering a heat utilization rate of 40%. These findings contribute valuable insights for the application of the ATS process to low-grade OS.

In situ conversion is the most potential technology for efficient and clean development of oil shale, and a downhole electric heater is key equipment for clean, efficient, and low-carbon in situ conversion. Three electric heating rods with different diameters are used to explore their influence on heater performances. The simulation results indicate that increasing the diameter of the heating rod helps to increase the minimum and maximum velocity of shell-side air, and the maximum velocity of H110-24 is 16.34 m/s, which is 1.25 and 1.13 times those of H110-16 and H110-20, respectively. In addition, the location of the local high temperature zone coincides with the area with low air flow velocity, and increasing the diameter of the heating rod can effectively reduce the heating rod surface temperature during high-power heating. Moreover, at the same heat flux, the heat transfer coefficients of H110-24 and H110-20 are 44.82-48.49% and 87.52-95.48% higher than those of H110-16, respectively. With the same heating power, the heat transfer coefficients of heaters have the same trend, indicating that the heat transfer coefficient of the heating rod can be effectively improved by increasing the diameter of the heating rod. Finally, the newly defined comprehensive performance is used to evaluate the heaters with different heating parameters. Increasing the heating power can improve the comprehensive performance of the heater, but the most effective way is to increase the diameter of the heating rod. With the same heating power, the new comprehensive performance of H110-24 and H110-20 is 48.38-52.34% and 87.29-95.19% higher than that of H110-16, respectively, and the electric heating rod with the diameter of 20 mm has the best performance.

A digital core obtained without damage and contact can provide intuitive and quantitative analysis data for sandstone pore structure analysis. However, pore analysis results based on digital cores are quite different from experimental results due to limitations of scanning resolution, quality, pore extraction method, and experimental errors. In this research, the influence of pore geometry on pore analysis is studied from pore angles and pore resolution. First, sharp angles are extracted by image processing, and the influence of angles on pore radius distribution is analyzed. Then, by up–down sampling, high- and low-resolution digital cores are reconstructed, and the effects of resolution on pore connectivity and throat structure are discussed.