Horizontal well drilling is a powerful means of exploring and developing unconventional oil and gas reservoirs. However, cuttings are easily deposited at the bottom of the annulus of the horizontal well section. They, thus, form a cuttings bed and affect the wellbore flow and heat transfer during drilling. Currently, no wellbore heat transfer model considers the impact of cuttings beds. Therefore, a coupling model of wellbore heat transfer and cuttings bed height was established and validated using field data in this study. Despite the model's accuracy being dependent on the precise distribution of the cuttings bed height, it better aligns with the wellbore heat transfer characteristics during horizontal well drilling. Using this model, the influences of the circulation time, circulation flow rate, drill pipe rotation speed, rate of penetration, and inlet temperature on the wellbore heat transfer were investigated. Subsequently, measures to reduce the downhole temperature were proposed. In a case study, in comparison with models that do not consider the cuttings bed, the downhole temperature calculated by the new model was 1.3 °C higher. By increasing the circulation flow rate, controlling the drill pipe rotation speed and rate of penetration, and reducing the drilling fluid inlet temperature, we lowered the downhole temperature by 13.3 °C. This provided support for achieving one-trip drilling in a 215.9 mm borehole.

The drilling fluid loss or lost circulation via fractures is one of the critical engineering problems in the development of deep oil and gas resources. The conventional treatment is to introduce granular lost circulation material (LCM) into the drilling fluid system to plug fractures. In this work, a method incorporating the fracture surface scanning technique and coupled Computational Fluid Dynamics-Discrete Element Method (CFD-DEM) numerical simulation is proposed for the first time to investigate how the LCM particles plug rough fractures. The rough fracture model is built utilizing a high resolution and high precision measurement system. The LCM particle transport and plugging process in rough fractures are captured in the CFD-DEM numerical simulations. The results show that the local fracture aperture has a significant influence on LCM particle transport and the distribution of the plugging zone. The drilling fluid loss rate will decrease, and the drilling fluid pressure will redistribute during the accumulation of LCM particles in the fracture. The fracture plugging efficiency of nonspherical LCM is improved as a result of formation of multi-particle bridges. This study provides a novel approach and important theoretical guidance to the investigation of LCM particle transport in rough fractures.

Deep and ultra-deep resource development is increasingly constrained by extreme downhole temperatures, which compromise drilling safety, tool integrity, and drilling-fluid performance. Conventional cooling measures—including drilling-parameter optimization, adjustment of drilling-fluid properties, surface cooling, and improved drillstring insulation—have achieved partial success; however, under deep/ultra-deep conditions, they often yield limited temperature reductions at high cost and with restricted adaptability. Phase-change materials (PCMs) offer an alternative approach by moderating wellbore-fluid temperature through heat absorption during melting and heat release during solidification. Nevertheless, prior studies have emphasized PCM development, whereas wellbore-scale heat-transfer modeling and the associated latent-heat coupling mechanisms remain insufficiently studied. To address this gap, a latent-heat-coupled wellbore heat-transfer model is developed by incorporating an enthalpy–temperature relation obtained from single-particle PCM simulations as a latent-heat source term. The model is validated against measurements from a self-built drilling-fluid circulation test rig, achieving an average prediction error below 5%. A case study for an 8000 m ultra-deep well indicates that adding 15% erythritol reduces the bottom-hole temperature by 16.6 °C. The proposed model establishes a sound theoretical basis for cooling design and evaluation using PCMs during drilling.

Casing deformation is frequent during hydraulic fracturing in the Weirong shale gas field, which impedes shale gas production. We investigated the effect of shale elastic modulus on casing deformation based on shale swelling in this work. According to field data, the silicon content in the Weirong shale gas field has a considerable impact on casing deformation during hydraulic fracturing. Furthermore, the results of the experiment show that the silicon content is positively related to the elastic modulus of shale. We conducted numerical simulations to analyze the casing deformation based on shale swelling. We observed that the shale elastic modulus was negatively correlated with the casing stress without shale swelling; however, the shale elastic modulus was positively correlated to casing stress with shale swelling. Furthermore, when the elastic modulus remained constant, casing stress was positively correlated with shale swelling. The numerical simulation results were validated using field data from the WY23-5 shale well. Moreover, factors such as injection pressure, formation pressure, and cement sheath elastic modulus can increase the impact of the shale elastic modulus on casing deformation with shale swelling. Casing deformation is greatly influenced by the distribution of the stimulated reservoir volume (SRV) area. Different SRV area behaviors result in different types of casing deformation. The symmetric distribution of the SRV area causes casing extrusion deformation, whereas the asymmetric distribution of the SRV area causes casing bending deformation. Moreover, casing stress is positively correlated with the length of the SRV area.