Drillstring vibration is detrimental to drilling operations. It is crucial to understand the underlying mechanisms to circumvent these vibrations and to help improve drilling performance. This paper presents a six degrees-of-freedom (DOF) finite element method (FEM) model to characterize the drillstring dynamics. In addition, a comprehensive bit-force model is developed and included as a boundary condition to the model, corresponding to the vibrations in axial, lateral, and torsional directions. This bit-force model considers the bottom hole assembly (BHA) eccentricity, mud damping, bit–rock interaction, and their coupling mechanisms. Simulation results have shown good agreement with field observations and experimental data in the literature. The utility of this modeling framework is demonstrated in the paper through case studies for normal operation, stick–slip vibration, and whirl vibration.

Summary: High bottomhole temperature can lead to decreased downhole tool life in geothermal and high-pressure/high-temperature (HPHT) oil and gas wells. The temperature increase is exacerbated when circulation stops (e.g., during connection, tripping, and well control situations). While continuous circulation technology is an appropriate solution for managing temperature, it is not yet widely adopted in HPHT and geothermal drilling practices. This work investigates factors that impact downhole temperature (DHT) and recommends strategies to better manage the temperature when continuous circulation is not available. An integrated thermo-hydraulic model was developed to capture the transient behavior of DHT and was applied here to study the transient temperature profile when there is no fluid circulation. The model was validated using the open-source FORGE field data set, with the mean absolute percentage error between 1% and 4%. In addition, hundreds of case scenarios were numerically studied to investigate the impact of several key factors on the DHT. The evaluated factors include the pumps-off time, type and physical properties of the drilling fluid, wellbore hydraulic diameter, reservoir temperature, geothermal gradient, total wellbore depth and profile, and operational parameters before stopping the circulation. The cooling effects of different drilling parameters were compared to a benchmark case of continuous circulation. A correlation map was generated to visualize the impact of those parameters on the DHT distribution when circulation stops. A logarithmic relationship between the pump stop time and the DHT was observed. For the FORGE case scenario, the DHT increases by 27°C and 48°C after the pump stops for 30 minutes and 60 minutes, respectively. It was observed that water-based mud (WBM) with a high viscosity increases fluid convection heat resistance between the formation and wellbore. Also, drilling with a higher flow rate before stopping the pump can cool the near-wellbore formation faster and reduce the DHT even after circulation ceases. Wells with high geothermal gradients, like FORGE wells, have a higher temperature buildup during circulation stoppage than wells with low geothermal gradients targeting the same reservoir (formation) in-situ temperature. This study investigates the efficacy of different cooling strategies to avoid DHT buildup when there is no circulation. It thereby facilitates the optimization of geothermal and HPHT well design and construction to prevent downhole tool failures. The developed correlation map can aid drilling engineers in understanding the impact of different drilling conditions on the DHT.

The power generated by wind turbines varies due to variations in the wind speed. A pack of rechargeable batteries could be used as a reserve power source to alleviate the intermittency in the wind turbine power. An integrated wind turbine and battery storage system is constructed where the wind turbine is electrically connected to a rechargeable battery system. Such a system can operate in two modes depending on the wind speed, power demand, and battery limit. The switching conditions for the wind turbine to operate in multi-input, single-output and single-input, single-output control mode are discussed. Linearized approximations of the closed loop wind turbine system are derived in order to analyze the switching stability between control modes. Common quadratic Lyapunov function (CQLF) is established for both control modes to prove the system stability. Simulation results demonstrating system stability are also presented.

Summary: Geothermal energy has gained much attention as a promising contributor to the energy transition for its ability to provide a reliable, environmentally friendly source of heat and baseload power. However, drilling high-temperature (HT) reservoirs presents significant technical and economic challenges, including thermally induced damage to bits and downhole (DH) tools, increasing drilling time and cost. This paper introduces drilling heat maps for proactive temperature management in geothermal wells during well planning and real-time drilling operations phases to avoid thermally induced drilling problems. This study uses a transient hydraulic model integrated with a thermal model to predict the bottomhole circulating temperature (BHCT) while drilling geothermal wells. The model is used to generate a large volume (1,000s) of case scenarios to explore the impact of various cooling and other heat management strategies on the BHCT in the Utah FORGE field, used here as an example, covering a wide range of drilling parameters. Results are captured, visualized, and analyzed in convenient heat maps, illustrating the advantages of using such heat maps in geothermal well construction and real-time operations. Model validation with FORGE 16A(78)-32 well data and a west Texas case scenario shows good agreement between the modeling results and experimental data, with a mean absolute percentage error (MAPE) of less than 4%. There is a clear logarithmic relationship between the drilling flow rate and BHCT at a constant mud inlet temperature and a linear relationship between the mud inlet temperature and BHCT at a constant drilling flow rate. Pronounced variation of BHCT in geothermal wells is observed with mud type, mud weight, and mud viscosity. In addition, insulated drillpipe (IDP) technology is found to significantly reduce BHCT (14–44% on average for FORGE scenarios) compared to conventional drillpipe (CDP), particularly in wells with extended measured depth (MD) where other heat management technologies and strategies become less effective. Drilling heat maps can alert drilling engineers to strategies with the highest BHCT-lowering impact, allowing focused technology selection and decision-making regarding optimal temperature management during the geothermal well design phase. In addition, real-time heat maps are valuable for facilitating active temperature management and providing real-time guidance for optimal drilling parameters during daily drilling operations. In general, heat maps can help to avoid drilling problems related to the combination of HT and temperature limitations of DH equipment, which will benefit the safe and cost-efficient development of geothermal resources.

The vanadium redox flow battery (VRFB) is an attractive grid scale energy storage option, but high operating cost prevents widespread commercialization. One way of mitigating cost is to optimize system performance, which requires an accurate model capable of predicting cell voltage under different operating conditions such as current, temperature, flow rate, and state of charge. This paper presents a lumped isothermal VRFB model based on principles of mass transfer and electrochemical kinetics that can predict transient performance with respect to the aforementioned operating conditions. The model captures two important physical phenomena: (1) mass transfer at the electrode surface and (2) vanadium crossover through the membrane. Mass transfer effects increase the overpotential and thus reduce the battery output voltage during discharge. Vanadium crossover causes a concentration imbalance between the two half-cells that negatively affects the voltage response particularly after long term cycling. Further analysis on the system linearity is conducted to assess the feasibility of using a linear control design methodology.