Conventional hydraulic fracturing techniques typically consume large amounts of water when producing shale gas. Fracking fluids may cause environmental pollution. In contrast, supercritical carbon dioxide (scCO₂) (above 31.8°C, 7.29MPa) can displace CH₄ in shale reservoirs. Achieve CO₂ sequestration while increasing the shale gas production. We studied the mechanical properties and fracture characteristics of a shale under the action of scCO₂, nitrogen, helium, and water by comparing the triaxial compression tests of shale samples with seven coring angles. The results show that: (1) scCO₂ effectively reduced compressive strength of the shale and weakened the anisotropy of shale; (2) scCO₂ caused the content of dolomite, calcite, and illite to decrease by 4.713.5, respectively; (3) scCO₂ produced micropores and microfractures 10 times larger than the original size in the microstructure. These microstructures can help improve the seepage and gathering of shale gas, leading to enhanced shale gas recovery and CO₂ storage.

Fiber sand control is widely used on site due to its advantages such as easy operation, significant effects, and low cost. However, the mechanism of fiber control proppant flowback is currently unclear, and the main controlling factors affecting the critical flow velocity are not yet clear. There is a lack of relevant theoretical support to optimize the design of fiber sand control. This article uses composite material theory to analogically calculate the tensile strength of fiber-reinforced sand and friction reinforcement theory to analogically calculate the shear strength of fiber-reinforced sand. The relationship between fiber strength force and tensile strength and shear strength is proposed, combined with the different stress conditions of proppant particles before and after fracture closure, and a fiber-controlled proppant flowback strength model is established based on the force arm balance. The critical velocity of proppant was calculated and compared through the stability experiment of fiber-reinforced proppant filling layer, and the accuracy of the model was verified and the influencing factors were analyzed. The results show that the influence of tensile strength and shear strength on fiber strength at the early stage of fracture closure is 0.2 and 0.8, respectively, and that after fracture closure is 0.1 and 0.9, respectively. The critical velocity calculated by the model is basically consistent with the experimental results. The critical velocity is most affected by the particle size and closure pressure of proppant, followed by the amount of fiber, and the influence of fiber length and diameter is relatively small.

The complexity of the fracture network, formed by the interaction of hydraulic fractures and natural fractures during hydraulic fracturing, is one of the important criteria for guiding shale gas field production. In this study, we selected the deep shale of the Longmaxi formation as the object of study. Through indoor hydraulic fracturing experiments and fracture 3-D reconstruction technology, we conducted a quantitative study on the complexity of the fracture network created by the influence of hydraulic and natural fractures interacting with each other during hydraulic fracturing. The results show that the fractal dimensions, average connection numbers of topological structure branches, and relative lengths of fracture intersections for hydraulic fractures in 10 groups of specimens are 1.93-2.27, 0-1.143, and 1.02-1.98, respectively.

The lithological composition of deep coal rock reservoirs in the Ordos Block is complex. The characteristics of hydraulic fracture propagation directly impact reservoir stimulation effectiveness. Therefore, efficient development requires an in-depth understanding of the cross-layer propagation mechanisms of fractures in deep coal rock. To clarify the cross-layer patterns and explore the controlling factors in deep coal rock, large-scale laboratory true triaxial hydraulic fracturing physical simulation experiments were conducted. These experiments, combined with CT scanning and post-fracture 3D reconstruction technology, investigated Ordos Block deep coal rock under different perforation locations, and the complexity of fractures was quantitatively characterized. Due to the well-developed weak planes such as natural fractures in coal rock, perforations in coal rock significantly reduce the breakdown pressure compared to perforations in sandstone. The complexity of perforation fractures in coal rock is far greater than in sandstone. Quantitative characterization of fracture complexity shows that the number of perforation fractures in coal rock fracturing reached 450% of that in sandstone, and the fracture area ratio reached 131.7%. Under high-rate and high-viscosity fracturing conditions, dominant hydraulic fractures tend to form, while the well-developed natural fractures in the coal rock interact with each other, resulting in a complex fracture network. Perforations in coal rock can effectively connect adjacent sandstone layers through cross-layer propagation, whereas perforations in sandstone form dominant hydraulic fractures without connecting the adjacent coal rock layers. The findings can provide operational guidance for optimizing field fracturing operations.