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.

The negative environmental impact of carbon emissions from fossil fuels has promoted hydrogen utilization and storage in underground structures. Hydrogen leakage from storage structures through wells is a major concern due to the small hydrogen molecules that diffuse fast in the porous well cement sheath. The second-order parabolic partial differential equation describing the hydrogen diffusion in well cement was solved numerically using the finite difference method (FDM). The numerical model was verified with an analytical solution for an ideal case where the matrix and fluid have invariant properties. Sensitivity analyses with the model revealed several possibilities. Based on simulation studies and underlying assumptions such as non-dissolvable hydrogen gas in water present in the cement pore spaces, constant hydrogen diffusion coefficient, cement properties such as porosity and saturation, etc., hydrogen should take about 7.5 days to fully penetrate a 35 cm cement sheath under expected well conditions. The relatively short duration for hydrogen breakthrough in the cement sheath is mainly due to the small molecule size and high hydrogen diffusivity. If the hydrogen reaches a vertical channel behind the casing, a hydrogen leak from the well is soon expected. Also, the simulation result reveals that hydrogen migration along the axial direction of the cement column from a storage reservoir to the top of a 50 m caprock is likely to occur in 500 years. Hydrogen diffusion into cement sheaths increases with increased cement porosity and diffusion coefficient and decreases with water saturation (and increases with hydrogen saturation). Hence, cement with a low water-to-cement ratio to reduce water content and low cement porosity is desirable for completing hydrogen storage wells.