Natural fractures (NFs) have been recognized as the dominant factors that increase hydraulic fracture complexity and reservoir productivity. However, the interactions between hydraulic and natural fractures are far from being fully understood. In this study, a two-dimensional numerical model based on the displacement discontinuity method (DDM) has been developed and used to investigate the interaction between hydraulic and pre-existing natural fractures. The inelastic deformation, e.g., stick, slip and separation, of the geologic discontinuities is captured by a special friction joint element called Mohr-Coulomb joint element. The dynamic stress transfer mechanisms between the two fracture systems and the possible location of secondary tensile fracture that reinitiates along the opposite sides of the NF are discussed. Furthermore, the model results are validated by a series of large tri-axial hydraulic fracture (HF) tests. Both experimental and numerical results showed that the displacements and stresses along the NFs are all in highly dynamic changes. When the HF is approaching the NF, the HF tip can exert remote compressional and shear stresses on the NF interface, which results in the debonding of the NF. The location and value of the evoked stress is a function of the far-field horizontal differential stress, inclination angle of the NF, and the net pressure used in fracturing. For a small approaching angle, the stress peak is located farther away from the intersection point, so an offset fracture is more likely to be generated. The cemented strength of the NF also has an important influence on the interaction mechanism. Weakly bonded NF surfaces increase the occurrence of a shear slippage, but for a moderate strength NF, the hybrid failure model with both tensile and shear failures, and conversion may appear.

SUMMARY: Geological storage of captured CO₂ is essential for reducing anthropogenic greenhouse gas emissions and ensuring the sustainable use of fossil fuels. Understanding the influence of saturation, pressure and temperature on the elastic properties of brine-saturated sandstone flooded with supercritical CO₂ is critical for interpreting seismic and sonic logging data, which aids in monitoring and quantifying subsurface changes associated with CO₂ injection. The experimental results indicate that as scCO₂ saturation increases (0–60 per cent), the P-wave velocity decreases significantly with an average of 14 per cent drop, while the S-wave velocity increases slightly. Temperature variations (80–110 °C) have a minimal effect on both velocities (1–2 per cent), whereas elastic features show notable sensitivity to the variation of confining pressure (20–30 MPa) and pore pressure (10–20 MPa). Ignoring the effects of pore pressure might lead to the bias of interpreting seismic data for monitoring scCO₂ saturation change. The constructed rock physics models well capture the coupled effects of porosity and scCO₂ saturation on the P-impedance and P- and S-wave velocity ratio, which show good agreement with the experimental results. These findings are crucial for improving monitoring methods and enhancing the accuracy of predictive models for CO₂ geological storage.

To explore coal fracture propagation mechanisms, the supercritical CO2 fracturing of coal seams was simulated under true triaxial stress with various injection methods, injection temperatures, and injection flow rates. The spatial–temporal distribution of the fracture propagation and evolution was located by acoustic emission monitoring. The microcosmic action mechanism of supercritical CO2 fracturing to form fractures and its advantages over conventional hydraulic fracturing are discussed in detail. The results suggest that due to the high porosity of coal, CO2 in the local area is subject to multi-phase transitions, which makes the spatial distribution of fractures more discrete and more complex. For coal seam fracturing, supercritical CO2 can reach the fracture tip and penetrate the coal substrate more easily than water. In addition, supercritical CO2 can access smaller channels than that of water, and thus, it tends to induce secondary fractures and connect natural fractures far from the injection site.

Although supercritical CO₂ (SC-CO₂) fracturing has shown promise in oil and gas development with demonstrated potential, its application in shale gas extraction remains in its infancy globally. In this study, fracturing experiments were conducted with water, liquid CO₂ (L-CO₂), and SC-CO₂, as well as SC-CO₂ at varying pump rates. The results reveal that SC-CO₂ fracturing produces a highly complex fracture network characterized by fractures of varying numbers, deflection angles, and tortuosity. Analysis of CO₂ temperature and pressure data showed a sharp drop in injection pressure and temperature at breakdown, followed by fluctuations until injection stopped. Acoustic emission (AE) monitoring demonstrated that energy released during main fracture initiation significantly exceeded that from CO₂ phase transition-driven fracture extension, underscoring the dominant role of main fractures in energy dissipation. Compared to hydraulic fracturing, SC-CO₂ fracturing created a seepage area 2.2 times larger while reducing the breakdown pressure by 37.2%, indicating superior stimulation performance. These findings emphasize the potential of SC-CO₂ to form intricate fracture networks, offering a promising approach for efficient shale gas extraction.

Petroleum has become a strategic resource to the national economy, and forecasting its demand is a critical step to supporting energy planning and policy-making for carbon reduction. We first conducted a characteristic analysis of end consumption for petroleum products, and the key affecting factors are identified through an extended logarithmic mean Divisia index (LMDI) method. Afterwards, the long-range energy alternatives planning system (LEAP) was adopted to predict the petroleum products demand by considering the potential impacts of different policies on the identified key factors. Through comparative analysis of three scenarios including five sub-scenarios, the findings show that the dual carbon constraints are crucial to petroleum demand control. Under the enforcement of existing carbon peaking policies, the petroleum products demand will peak around 2043 at 731.5 million tons, and the impact of energy intensity-related policies is more significant than that of activity level. However, even if the existing policy efforts are continued, supporting the carbon-neutral target will not be easy. By further strengthening the constraints, the demand will peak around 2027 at 680 million tons, and the abatement contribution will come mainly from industry (manufacturing), construction, and transportation. Additional abatement technologies are necessary for the petroleum industry to achieve carbon neutrality.