Articles techniques et présentations

This study focuses on the design and optimization of a logging while drilling (LWD) tool with look-ahead detection capabilities, which is widely used in measuring formation resistivity and detecting formation boundaries ahead of the drill bit during oil and gas drilling operations. The tool provides valuable information on formation structure and fluid distribution, thereby aiding in the optimization of drilling paths and enhancing the efficiency of hydrocarbon extraction. The tool features a single transmission dual-reception structure, comprising one three-component transmitting antenna and two three-component receiving antennas, each situated on separate subs. These subs are connected via a non-magnetic drill collar. Each antenna is protected by a metallic shield, which is made of the same material as the drill collar and is slotted with a specific width. This study employs COMSOL for modeling and simulation to evaluate the impact of various factors such as the number of slots on the shield under different source distances and transmission frequencies on transmission efficiency, the effect of antenna wire diameter on signal strength, and the influence of the magnetic core on the received signal, including the effects of core thickness and relative permeability. In COMSOL, the low-frequency magnetic field module is primarily used for calculations. Due to the inclusion of the detailed antenna slot structure and magnetic core, the model is more complex compared to the structure of a bare antenna. The model construction requires the use of geometric components such as rotations and matrices. Additionally, the large scale variation in the model—ranging from millimeter-level antenna wire diameters to computational domains on the order of hundreds of meters—would typically lead to convergence issues in subsequent calculations if conventional modeling methods are applied. Therefore, an equivalent coil interface is used, significantly reducing the model's complexity. This approach also allows for detailed mesh refinement in subsequent steps. When meshing the antenna, special attention must be paid to the relationship between the frequency, material properties, and the minimum mesh size, particularly considering the skin depth. Ensuring that the minimum size is less than half of the skin depth often yields better computational results. The results have been calculated to show that with the increase of the number of slots, the reception efficiency is slightly enhanced, for the transmission frequency of 2kHz, before and after the increase of the number of slots, the reception efficiency increases from 75% to 83%, and from 18% to 28% for 96kHz. Considering the size of the metallic shield should not exceed 25cm, and the surrounding need to leave the location of the screw holes, the number of grooves between 24-28 is more appropriate, the width of the grooves is 3.2mm. Overall the horizontal antenna seems to have a stronger signal and the individual parameters respond to a simpler pattern, making it easier to adjust in terms of circuitry and mechanics.