Elucidating the underlying chemo-mechanical coupling mechanisms of acid-supercritical carbon dioxide (scCO₂) reactions in shale reservoirs is essential for advancing the efficacy and safety of CO₂ geo-storage and enhanced oil recovery (EOR) techniques. Multi-scale experiments-including X-ray diffraction (XRD), scanning electron microscopy (SEM), nuclear magnetic resonance (NMR), atomic force microscopy (AFM), and nanoindentation-were used to characterize changes in mineralogy, pore structure, and mechanical properties under hydrochloric acid (HCl) pretreatment, followed by scCO₂ exposure. Results demonstrate that acid pretreatment acts as an efficient method to dissolve carbonate minerals, effectively boosting secondary porosity and scCO₂ accessibility. Subsequent exposure to scCO₂ further modifies the mineral composition and induces a marked enhancement in pore-fracture connectivity, as revealed by advanced imaging and topographical analysis. The synergistic effect of acid and scCO₂, validated by nanoindentation and NMR, leads to considerable mechanical softening and superior hydrocarbon displacement efficiency. These findings establish the synergy between acid and scCO₂ in dynamically reshaping the shale pore-fracture system, thereby simultaneously enhancing hydrocarbon recovery and long-term CO₂ storage. This integrated approach offers a novel paradigm for optimizing reservoir stimulation within carbon capture, utilization, and storage CCUS frameworks.

Horizontal well with multistage hydraulic fracturing (HWMHF) optimization is crucial for the economic and efficient development of unconventional oil and gas resources. In this paper, fracture propagation modeling, production simulation, and automatic search algorithms are coupled to establish an integrated workflow for the optimization of HWMHF. Intelligent optimization of high-dimensional parameters of HF is conducted to obtain the best matching set of HWMHF parameters, which maximizes the value of the economic index in the global range. The fracture propagation simulation employed a self-pro-grammed boundary element fracture propagation simulator. The production simulation employed a CMG component simulator. The automatic search algorithm employed the Genetic algorithm (GA) and the Bayesian optimization algorithm. The whole optimization process is automated, i.e., the overall fracture morphology is automatically imported into CMG, the fracture model is automatically established, the oil and gas water production is predicted by the CMG HWMHF model, and the objective function value is calculated automatically. Further, the automatic search algorithm updates the next generation of input by measuring the relationship between the objective function values and the HWMHF parameter values, and re-performs fracturing simulation until the ideal fracturing parameters are obtained. The fracturing optimization process and results show that: (1) the combination of boundary element fracture propagation simulator and CMG component simulator can achieve fast fracturing simulation and satisfy the high simulation times for intelligent search; (2) the economic index has been improved 55% though the intelligent optimization; (3) the GA searches for better HWMHF parameters, while the Bayesian optimization algorithm performs a less number of iterations, while it can better embrace the domain knowledge; (4) both methods are suitable for solving the “black box” question of HWMHF optimization, and each has its advantages and obvious potential for field application promotion.

Unconventional oil and gas resources serve as vital replacement energy in China’s hydrocarbon portfolio, and their efficient development is of great significance for safeguarding national energy security. The implementation of staged multi-cluster hydraulic fracturing in horizontal wells, along with the optimization of intra-stage cluster design parameters, is critical to maximizing the production potential of unconventional reservoirs. Clarifying fracture propagation mechanisms and quantifying the relationship between fracture geometry and well productivity is key to optimize intra-stage multi-cluster fracturing strategies. In this study, a phase-field method is employed to simulate the competitive propagation morphology of multiple fractures within a fracturing stage. A fracture morphology identification technique is integrated to construct a two-dimensional equivalent fracture model, which can characterize the stimulated flow pathways. Equivalent physical parameters after stimulation are extracted and transferred-together with geometric descriptors-as input for a discrete fracture flow model. This enables automatic coupling and data transfer between the geometric and flow models, thereby facilitating quantitative evaluation of production performance under different fracturing scenarios and ultimately achieving fully coupled fracture propagation-fluid flow simulation. The accuracy and feasibility of the dual-model coupling method are verified through comparison with laboratory-scale physical simulation experiments and field fracturing data. On this basis, the effects of intra-stage cluster number and cluster spacing on fracture morphology and production response are further investigated. The results show that, as the cluster spacing increases from 15 m to 25 m, the fracture deflection point shifts farther from the wellbore, and the tip deflection angle decreases from 30° to 24°. Meanwhile, the pressure gradient around the fracture tip is reduced, weakening the fluid driving force and significantly diminishing inter-fracture fluid interference. This change leads to a decline in peak daily oil production and stabilized production rate, with daily and cumulative oil output decreasing by 35.88% and 35.89%, respectively. In contrast, when the number of clusters per stage increases from 3 to 5, the deflection angle at the tip of the outer fractures increases from 30° to 34°, while the coverage of the induced stress field expands from 36.74% to 42.46%. This results in a higher pressure gradient surrounding the fractures, enhancing the fluid driving force and significantly improving oil mobilization. Consequently, peak daily and cumulative oil production increased by 40.49% and 45.467%, respectively. Therefore, optimizing the intra-stage cluster spacing and cluster number can effectively balance the degree of fracture interference and enhance single-well productivity, thereby improving the overall effectiveness of staged multi-cluster hydraulic fracturing in horizontal wells.

Carbon dioxide pre-fracturing has shown high application potential in improving oil recovery in conglomerate reservoirs. However, the influence of CO₂ on the physical properties of reservoir rock and its diffusion behavior within the reservoir matrix have not been systematically studied. This paper integrates CO₂-saturated water soaking experiments, true triaxial fracturing experiments and field-scale tests to demonstrate that CO₂ soaking induces quartz reduction and clay mineral increase, leading to a decrease in porosity and mechanical strength. Clay-cemented conglomerates experience a greater loss in compressive strength and a higher reduction in permeability compared to calcareous-cemented counterparts under identical CO₂ soaking. In the horizontal principal stress direction, CO₂ fracturing achieves a greater fracture penetration depth than slickwater fracturing or CO₂ pre-injection followed by slickwater fracturing. CO₂ pre-fracturing reduces breakdown pressure by 15%-5% and increases fracture complexity. Field tests confirm a reduction in injection pressure and improved effective stimulation. However, dnarrower fracture width and higher tortuosity may limit proppant transportation.

Natural fractures (NFs) are common in shale and tight reservoirs, where staged multi-cluster fracturing of horizontal wells is a prevalent technique for reservoir stimulation. While NFs and stress interference are recognized as significant factors affecting hydraulic fracture (HF) propagation, the combined influence of these factors remains poorly understood. To address this knowledge gap, a novel coupled hydro-mechanical-damage (HMD) model based on the phase field method is developed to investigate the propagation of multi-cluster HFs in fractured reservoirs. The comprehensive energy functional and control functions are established, while incorporating dynamic fluid distribution between multiple perforation clusters and refined changes in rock mechanical parameters during hydraulic fracturing. The HMD coupled multi-cluster HF propagation model investigates various scenarios, including single HF and single NF, reservoir heterogeneity, single HF and NF clusters, and multi-cluster HFs with NF clusters. The results show that the HMD coupling model can accurately capture the impact of approach angle (θ), stress difference and cementation strength on the interaction of HF and NF. The criterion of the open and cross zones is not fixed. The NF angle (α) is not a decisive parameter to discriminate the interaction. According to the relationship between approach angle (θ) and NF angle (α), the contact relationship of HF can be divided into three categories (θ = α, θ < α, and θ > α). The connected NF can increase the complexity of HF by inducing it to form branch fracture, resulting in a fractal dimension of HF as high as 2.1280 at angles of ±45°. Inter-fracture interference from the heel to the toe of HF shows the phenomenon of no, strong and weak interference. Interestingly, under the influence of NFs, distant HFs from the injection can become dominant fractures. However, as α gradually increases, inter-fracture stress interference becomes the primary factor influencing HF propagation, gradually superseding the dominance of NF induced fractures.