Complex fracture technology is key to the successful development of unconventional oil and gas reservoirs, such as shale. Most current studies focus on how to improve the complexity of the fracture network. It is still unclear whether proppant can enter the branch fractures at all levels after the formation of complex fractures. The effects of construction displacement, proppant particle size, proppant density, fracturing fluid viscosity, sand ratio, and other factors on proppant migration in single fractures and complex fractures were studied using an experimental device independently developed by the laboratory. The results show that the lowest point height of the sandbank and the equilibrium height of the sandbank are directly proportional to the particle concentration and density, respectively, and inversely proportional to the displacement and fracturing fluid viscosity. The equilibrium time of the sandbank is inversely proportional to the displacement, particle concentration, and density, respectively, and proportional to the viscosity of the fracturing fluid. Under the same experimental conditions, the larger the branch angle, the smaller the height of the main/secondary fracture sandbank. In the design of the fracturing process, fracturing fluid with varying viscosities and proppant with different densities should be selected according to the formation conditions and fracturing targets. In the face of long fracture lengths, the combination of low-viscosity fracturing fluid with an appropriate viscosity and low-density proppant can meet the goal of placing proppant over long distances and effectively supporting fractures over extended lengths. Subsequently, high-density proppant or reduced construction displacement are adopted to usefully support fractures in the near-wellbore area. The results of this paper can provide theoretical support for proppant selection and fracturing program design.

The microbial communities associated with alkaline–surfactant–polymer (ASP)-flooded reservoirs have rarely been investigated. In this study, high-throughput sequencing was used to analyse the indigenous microbial communities in two different blocks, the water flooding after the alkaline–surfactant–polymer flooding block and the alkaline–surfactant–polymer flooding block, and to ascertain the optimal stage for the implementation of indigenous microbial oil recovery technology. The different displacement blocks had significant effects on the indigenous microbial community at the genus level according to an alpha diversity analysis and community composition. In water flooding after alkaline–surfactant–polymer flooding, the dominant genus of Pseudomonas exceeded 30%, increasing to 52.1% in alkaline–surfactant–polymer flooding, but alpha diversity decreased. Through a co-occurrence network analysis, it was found that the complexity of the water flooding after alkaline–surfactant–polymer flooding was higher than that of alkaline–surfactant–polymer flooding. This means that the water flooding ecosystem after alkaline–surfactant–polymer flooding was more stable and less susceptible to external environmental influences. In addition, there were significant differences in the functional redundancy of microbial communities in different blocks. In summary, the optimal stage for implementing local microbial oil recovery technology may be water flooding after alkaline–surfactant–polymer flooding.

To investigate the settling behavior and drag characteristics of particles in fiber-containing non-Newtonian fluids, a series of systematic single-particle settling experiments were conducted. Power-law and Herschel–Bulkley fluids were prepared as base media, into which polyester fibers of various concentrations and lengths were introduced. The effects of fiber structural parameters on fluid rheology and terminal settling velocity were thoroughly evaluated. First, the rheological changes induced by fiber addition were quantitatively analyzed, revealing a nonlinear increase in both viscosity and yield stress with increasing fiber concentration and length. Subsequently, the total drag force was decomposed into viscous and fiber-induced components, and a predictive model for the fiber-induced drag coefficient was developed based on fiber structural parameters. A power-law fitting approach was employed to characterize the nonlinear relationship between the fiber drag coefficient and the particle Reynolds number. Furthermore, a parametric coupling strategy was employed, in which fiber concentration and length were embedded into the model coefficients to construct a unified and continuous predictive model for the total drag coefficient. Experimental validation demonstrated that the mean relative errors (MREs) of the proposed model were within 5.17% for power-law fluids and 9.95% for Herschel–Bulkley fluids, indicating strong predictive accuracy and applicability. The findings of this study provide a robust theoretical and experimental basis for optimizing fiber-enhanced cutting transport systems and modeling particle transportation under complex drilling conditions.

Microbial-enhanced oil recovery (MEOR) is a promising technology for oilfield development. To improve MEOR efficiency, two functional strains—Bacillus mucilaginosus ZZ-8 and Bacillus amyloliquefaciens ZZ-11—were isolated and purified. The growth characteristics, biosurfactant production, and crude oil emulsification performance of these strains were systematically evaluated through single-strain cultures and a co-culture system (ZZ-8: ZZ-11 = 1:1). The results demonstrated that the co-culture system exhibited superior growth and functional performance compared to monocultures. The cell-free supernatant significantly reduced oil–water interfacial tension, decreasing the contact angle from 53.56 ± 1.3° to 28.78 ± 0.82°, thereby enhancing crude oil detachment from rock surfaces and improving oil displacement efficiency. Gas chromatography (GC) analysis further confirmed the co-culture system’s pronounced degradation of long-chain alkanes (C17–C35). In oil sand washing experiments, the 1:1 mixed-strain fermentation broth achieved a crude oil elution rate of 84.39%, representing an 89.80% increase over uninoculated medium. This study not only validates the synergistic effect of the B. mucilaginosus–B. amyloliquefaciens co-culture system in enhancing oil recovery but also provides a theoretical foundation and innovative strategy for its practical application in MEOR technology.

Further exploitation of the residual oil underground in post-polymer flooded reservoirs is attractive and challenging. Microbial-enhanced oil recovery (MEOR) is a promising strategy to enhance the recovery of residual oil in post-polymer flooded reservoirs. Identifying and selectively activating indigenous microorganisms with oil displacement capabilities is an urgent requirement in the current design of efficient microbial-enhanced oil recovery technologies. This study combines high-throughput sequencing with functional network analysis to identify the core functional microbes within the reservoirs. Concurrently, it devises targeted activation strategies tailored to oligotrophic conditions through an analysis of environmental factor influences. The feasibility of these strategies is then validated through physical simulation experiments. With nutrient stimulation, the overall diversity of microorganisms decreases while the abundance of functional microorganisms increases. The core displacement results showed that the oil recovery factor increased by 3.82% on the basis of polymer flooding. In summary, this research has established a system for the efficient activation of functional microorganisms under oligotrophic conditions by utilizing bioinformatics, network analysis, and indoor simulation systems. This achievement will undoubtedly lay a solid foundation for the practical implementation of microbial enhancement techniques in the field.

In extended-reach wells, cuttings bed formation in high-deviation sections presents a major challenge for hole cleaning and borehole stability. This study analyzes the morphological and mechanical behavior of cuttings beds, focusing on particle size distribution and repose angle as key indicators of accumulation behavior. The modeling approach considers dominant interparticle forces, including buoyancy and cohesion, while neglecting secondary microscale forces for clarity. A theoretical model is developed to predict repose angles under both rolling and sliding regimes and is calibrated through laboratory-scale experiments using simulated drilling fluid with field-representative rheological properties. Results show that cohesive effects are negligible when cuttings are of similar size but exhibit higher densities. Laboratory measurements reveal that the repose angle of cuttings beds varies between 23.9° and 31.7°, with increasing polyacrylamide (PAM) concentration and particle size contributing to steeper repose angles. Additionally, the rolling repose angle is found to be relatively stable, ranging from 25° to 30°, regardless of fluid or particle property variations. These findings provide a predictive framework and practical guidelines for optimizing hole cleaning strategies and designing more effective models in extended-reach drilling.