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Objective Shale generally contains water, and it is highly important to clarify the occurrence characteristics of adsorbed water in shale to improve the drainage effect of shale gas. Methods This study analysed shale cores from the JY11-4 and JY41-5 wells in the Jiaoshiba area of Fuling. The influence of high-pressure conditions on the occurrence characteristics of adsorbed water in shale was investigated using custom-designed frozen nitrogen adsorption experiments and nuclear magnetic resonance experiments. Results and Conclusion The results show that (1) under atmospheric pressure, the average volume of adsorbed water per unit mass calculated by the "weighing method" is 0.017 3 mL/g. The volume proportion of water in the micropores and mesopores (average of 90.94%) significantly exceeds that in the macropores (average of 9.06%). This may be because water molecules cannot occupy the adsorption sites in all the shale pores when the relative pressure is relatively low. Most water molecules condense in micropores and mesoporous pores, and only a few water molecules enter the macropores. In addition, the small and medium pores in clay-rich shale become "filled and blocked" by water molecules in the process of water adsorption. (2) Under a saturated water pressure of 30 MPa, the average adsorbed water volume per unit mass of the sample calculated by the "weighing method" is 0.021 6 mL/g. The volume proportions of water in the micropores and mesopores (average of 40.26%) is lower than that in macropores (average of 59.74%), which may be related to the fact that water molecules can occupy more adsorption sites on the inner surface of micropores and mesoporous pores under the action of capillary forces when the relative pressure is significantly increased. (3) Compared with normal pressure, high-pressure conditions increase the volume of water adsorbed per unit mass of shale (approximately 25% increase in the experiment), and the proportion of water volume in large pores is greater than that in micropores and mesoporous pores. (4) During hydraulic fracturing, the relative pressure of the reservoir increases significantly. Under the action of capillary forces, fracturing fluid may enter large pores that cannot enter under the pressure of the original reservoir to "relieve" the "unsaturated state" of the original shale reservoir. After fracturing is completed, the pressure around the reservoir is gradually released, and the fracturing fluid that originally enters the adsorption pores of the shale may have difficulty overcoming the capillary resistance at the pore, making it difficult to return.

The Sichuan Basin’s Liangshan Formation shale is rich in oil and gas resources, yet the recovery rate of shale oil reservoirs typically falls below 10%. Currently, gas injection huff-n-puff (H-n-P) is considered one of the most promising methods for improving shale oil recovery. This study numerically investigates the application of the CO2 huff-n-puff process in enhancing oil recovery in shale volatile oil reservoirs. Using an actual geological model and fluid properties of shale oil reservoirs in the Sichuan Basin, the CO2 huff-n-puff process was simulated. The model takes into account the molecular diffusion of CO2, adsorption, stress sensitivity effects, and nanopore confinement. After history matching, through sensitivity analysis, the optimal injection rate of 400 tons/day, soaking time of 30 days, and three cycles of huff-n-puff were determined to be the most effective. The simulation results show that, compared with other gases, CO2 has significant potential in improving the recovery rate and overall efficiency of shale oil reservoirs. This study is of great significance and can provide valuable references for the actual work of CO2 huff-n-puff processes in shale volatile oil reservoirs of the Sichuan Basin.

The rapid expansion of reservoir fractures and the enlargement of the area affected by working fluids can be accomplished solely through fracturing operations of oilfield working fluids in geological reservoirs. Supercritical CO2 is regarded as an ideal medium for shale reservoir fracturing owing to the inherent advantages of environmental friendliness, excellent capacity, and high stability. However, CO2 gas channeling and complex propagation of fractures in shale reservoirs hindered the commercialization of Supercritical CO2 fracturing technology. Herein, a simulation method for Supercritical CO2 fracturing based on cohesive force units is proposed to investigate the crack propagation behavior of CO2 fracturing technology under different construction parameters. Furthermore, the shale fracture propagation mechanism of Supercritical CO2 fracturing fluid is elucidated. The results indicated that the propagation ability of reservoir fractures and Mises stress are influenced by the fracturing fluid viscosity, fracturing azimuth angle, and reservoir conditions (temperature and pressure). An azimuth angle of 30° can achieve a maximum Mises stress of 3.213 × 107 Pa and a crack width of 1.669 × 10−2 m. However, an apparent viscosity of 14 × 10−6 Pa·s results in a crack width of only 2.227 × 10−2 m and a maximum Mises stress of 4.459 × 107 Pa. Additionally, a weaker fracture propagation ability and reduced Mises stress are exhibited at the fracturing fluid injection rate. As a straightforward model to synergistically investigate the fracture propagation behavior of shale reservoirs, this work provides new insights and strategies for the efficient extraction of shale reservoirs.

In the Jiaoshiba block of the Fuling shale gas field, the employed reserves and recovery factor by primary well pattern are low, no obvious barrier is found in the development layer series, and layered development is difficult. Based on the understanding of the main factors controlling shale gas enrichment and high production, the theory and technology of shale gas three-dimensional development, such as fine description and modeling of shale gas reservoir, optimization of three-dimensional development strategy, highly efficient drilling with dense well pattern, precision fracturing and real-time control, are discussed. Three-dimensional development refers to the application of optimal and fast drilling and volume fracturing technologies, depending upon the sedimentary characteristics, reservoir characteristics and sweet spot distribution of shale gas, to form “artificial gas reservoir” in a multidimensional space, so as to maximize the employed reserves, recovery factor and yield rate of shale gas development. In the research on shale gas three-dimensional development, the geological + engineering sweet spot description is fundamental, the collaborative optimization of natural fractures and artificial fractures is critical, and the improvement of speed and efficiency in drilling and fracturing engineering is the guarantee. Through the implementation of three-dimensional development, the overall recovery factor in the Jiaoshiba block has increased from 12.6% to 23.3%, providing an important support for the continuous and stable production of the Fuling shale gas field.

In September 2013, the National Energy Administration approved the establishment of Fuling national shale gas demonstration area (hereinafter referred to as the demonstration area), whose construction was completed in December 2015. After nearly one decade of development, it has grown into the main shale gas production base in China. In order to speed up the integration and breakthrough of China's shale gas theoretical understanding and exploration and development technology and effectively promote the great development of marine shale gas in China, this paper reviews and summarizes the development and construction history of the demonstration area, geological theory understanding, engineering technology and key equipment progress. And based on this, the future development direction of the demonstration area is predicted. And the following research results are obtained. First, shale gas exploration and development in the demonstration area is divided into three stages, i.e., exploration evaluation, phase I and II construction, and stacked development and adjustment. Second, the “binary enrichment” theory for marine shale gas and the engineering theory for efficiently developing gas reservoirs are innovatively established. Third, a series of supporting technologies are innovatively developed, such as optimized and fast drilling technology for shale gas cluster horizontal wells, differentiated network fracturing technology, high-efficiency gas production, gathering and transportation technology, and green development technology for karst mountains, and the localization of key equipment and tools is realized. Fourth, the efficient development of shallow overpressure shale gas reservoirs above 3500 m in depth and the effective development of shale gas reservoirs at the depth of 3500–4000 m are realized in the demonstration area. Fifth, the construction of the demonstration area in the future includes accelerating the development of normal-pressure deep shale gas, continuously tackling key shale gas EOR technologies, actively promoting the field application of new technologies and methods, and powerfully strengthening the construction of green demonstration areas. In conclusion, this demonstration area is the earliest one of four national shale gas demonstration areas, and its achievements will provide continuous guidance for the shale gas exploration and development in China and play a demonstrative and guiding role in promoting the development of shale gas geological theories and exploration and development technologies in China.

In order to clarify the gas slip flow effect and its influencing mechanism of shale gas reservoirs, low-temperature nitrogen adsorption measurements were performed on the Silurian Longmaxi Formation shales from the Sichuan Basin. Pore structure characteristics of shales were described based on nitrogen adsorption-desorption isotherms. Non-steady state gas flow experiments on gas shales were carried out to obtain the apparent permeability coefficients of helium and nitrogen. The effects of pore pressure, gas types, confining pressure on gas slippage were discussed. Results show that gas slippage cannot be neglected when the pore pressure is less than 2.5 MPa. The Klinkenberg corrected permeability coefficients of shales are gas-dependent due to the "molecular sieving effect". The Klinkenberg corrected permeability of helium on shales is larger than that of using nitrogen. Gas slippage factors are also related to gas types. Helium slippage factor of shales is about 1.7 times of nitrogen slippage factor. The effective transport pore diameter of helium on shales with the confining pressure being 10-40 MPa ranges from 113 to 166 nm, while that of nitrogen is between 66 and 99 nm. These values are significantly larger than that derived from low-temperature nitrogen adsorption. A power function is utilized to fit the gas slippage factor and permeability of shales and can be used to predict gas flow in shales.

Nanopore in shales is the place for hydrocarbon accumulation and migration. However, there is a lack of understanding of the nanopore structure with regard to their ultratight and multiscaled nature. Here, the porous morphology of gas shales from the Sichuan Basin of China was investigated using field emission-scanning electronic microscopy (FE-SEM) with high resolution. Low-pressure nitrogen adsorption experiments at 77 K were conducted to obtain the adsorption-desorption isotherms, BET-specific surface area, pore size distribution, pore volume, and average pore diameter values. Research results show that pores of the studied shales are at the nanometer scale, and the average pore diameter is between 3 and 5 nm. The pore structure of these shales is complicated, which is not only predominately mesopores (pore diameter at 2–50 nm) but also some micropores (pore diameter50 nm). The specific surface area of shales ranges from 13 to 30 m2/g. The micropore volume and mesopore volume occupy the total pore volume highly up to 77%–92%, which indicates that micropores and mesopores are the main storage place for shale gas. Through the analysis of adsorption isotherms and hysteretic loops, there are mainly two kinds of pores in shales, including ink-bottle-like pores and slit pores. Micropores of these shales are mainly related to organic matter, while macropores are mainly related to clay minerals. The estimation about porosity using the combined physical model shows that organic matter and clay minerals contribute about 50% and 33% to the porosity of these shales, respectively.