The thermal behavior of the Huadian oil shale during low-temperature co-current oxidizing pyrolysis was studied by lab-scale experiments under different basic pyrolysis temperatures and input gas flow rates. The results showed that, in the process of oil shale co-current oxidizing pyrolysis, the increasing input gas flow rates under the same basic pyrolysis temperatures can significantly enhance the heat generation of oil shale. Meanwhile, it can be seen from the temperature variation characteristics of oil shale that the heat released by the oxidation reaction of semicoke and oxygen is enough to support the thermal decomposition of organic matter without supplemental heating. Moreover, it can be concluded from the 92.06% effective recovery of shale oil that a high yield of oil without a significant loss can be achieved. Finally, compared with increasing basic pyrolysis temperatures, the increased input gas flow rates have a more obvious effect on improving the effective recovery of shale oil.

The porous structure of oil shale plays a vital role in heat transfer and mass transport. In this study, the pore evolution of oil shale samples during sub-critical water extraction was investigated by scanning electron microscope (SEM), N₂ adsorption/desorption, and low field nuclear magnetic resonance (NMR). The following results were obtained: (1) With increased extraction time and extraction temperature, the yield of bitumen increased, pores in spent samples obviously developed and extended to the inner of the shale matrix, and their pore size gradually increased from the nano to micron size; (2) Pore volume and surface area of mesopores increased with increasing yield, indicating that the extraction of organic matter improves the development of organic matter pores distributed in mesopores; (3) The formation of secondary organic matter pores primarily contributes to the increment of pore volume in oil shale samples. The diameter of kerogen may range from 100 to 1600 nm; (4) Fractures probably propagated parallel to the bedding direction, and their evolution led to an initial increase in the total pore volume followed by a decrease. This is likely because fractures will be strongly compacted by pressure due to the weakening of inner support after more organic matter is extracted.

Clean fracturing fluid has the characteristics of being environmentally friendly and causing little damage to reservoirs. Meanwhile, its backflow gel-breaking fluids (GBFs) can be reutilized as an oil displacement agent. This paper systematically evaluates the feasibility and EOR mechanism of a GBF based on a polymer surfactant as an oil displacement system for reutilization. A rotating interfacial tensiometer and contact angle measuring instrument were used to evaluate the performance of reducing the oil–water interfacial tension (IFT) and to change the rock wettability, respectively. Additionally, a homogeneous apparatus was used to prepare emulsions to evaluate GBF’s emulsifying properties. Finally, core flooding experiments were used to evaluate the EOR effect of GBFs, and the influence rules and main controlling effects of various properties on the EOR were clarified. As the concentration of GBFs increases, the IFT first decreases to the lowest of 0.37 mN/m at 0.20 wt% and then increases and the contact angle of the rock wall decreases from 129° and stabilizes at 42°. Meanwhile, the emulsion droplet size gradually decreases and stabilizes with increases in GBF concentration, and the smallest particle size occurs when the concentration is 0.12–0.15 wt%. The limited adsorption area of the oil–water interface and the long molecular chain are the main reasons that limit the continued IFT reduction and emulsion stability. The oil displacement experiment shows that the concentration of GBF solution to obtain the best EOR effect is 0.15 wt%. At this concentration, the IFT reduction and the emulsification performance are not optimal. This shows that the IFT reduction performance, reservoir wettability change performance, and emulsification performance jointly determine the EOR effect of GBFs. In contrast, the emulsifying performance of GBFs is the main controlling factor for the EOR. Finally, the optimal application concentration of GBFs is 0.15–0.20 wt%, and the optimal injection volume is 0.5 PV.

A downhole electric heater, which reduces heat loss along a heat insulation pipe, is a key apparatus used to ignite oil shale underground. Downhole heaters working together with packers can improve the heating efficiency of high-temperature gases, while different packer locations will directly affect the external air temperature of the heater shell and, subsequently, the performance and total cost of the downhole heaters. A device was developed to simulate the external conditions of heater shells at different packer locations. Then, the effects of external air temperature on the performance of a downhole heater with pitches of 50, 160, and 210 mm were experimentally studied. In the test, results indicated that the heater with a packer at its outlet had an accelerated heating rate in the initial stage and decreased temperature in the final stage. Additionally, the lowest heating rod surface temperature and highest comprehensive performance were achieved with minimal irreversible loss and lower total cost when using a downhole electric heater with a packer set at its outlet. In addition, the downhole electric heater with a helical pitch of 50 mm and a packer at its outlet was more effective than other schemes in the high Reynolds number region. These findings are beneficial for shortening the oil production time in oil shale in situ pyrolysis and heavy oil thermal recovery.

The hot exhaust gas generated by a downhole combustion heater directly heats the formation, which can avoid the heat loss caused by the injection of high-temperature fluid on the ground. However, if the temperature of the exhaust gas is too high, it may lead to the carbonization of organic matter in the formation, which is not conducive to oil production. This paper proposes the use of low-temperature catalytic combustion of a mixture of methane and air to produce a suitable exhaust gas temperature. The simulation studies the influence of different parameters on the catalytic combustion characteristics of methane and the influence of downhole high-pressure conditions. The results show that under high-pressure conditions, using a smaller concentration of methane (4%) for catalytic combustion can obtain a higher conversion efficiency (88.75%), and the exhaust temperature is 1097 K. It is found that the high-pressure conditions in the well can promote the catalytic combustion process of the heater, which proves the feasibility of the downhole combustion heater for in situ heating of unconventional oil and gas reservoirs.