Flow field simulation of a hydrogeological exploration drill bit for switching coring drilling and non-coring drilling

Abstract

Abstract Drilling is one of the most commonly techniques in hydrogeological exploration, which is employed to obtain rock samples and build boreholes. During conventional drilling, it needs to raise all drilling tools in the borehole, when switching coring drilling and non-coring drilling, which will cause large auxiliary operation time consumption and poor drilling efficiency. According to the structure of wireline coring tools, a large diameter modular drill bit is designed to switch coring drilling and non-coring drilling without lifting the whole drilling tools. In the Comsol simulation environment, a simulation model of the modular bit is constructed using computational fluid dynamics (CFD) method. Drilling fluid velocity and pressure characteristics flowing through the modular bit were studied, with different values of flow rate at the bit inlet, clearance between external bit and internal bit for coring drilling, and layout of nozzles at the bottom of the internal bit for non-coring drilling setting respectively. According to the analysis results, velocities of drilling fluid flowing upward at the clearance between external bit and borehole wall increases linearly, and the pressure at bit inlet increases in parabola, with the flow rate increasing. With the same input flow rate, similar velocities and lower pressure loss can be obtained in the non-coring drilling compared with the coring bit, and thus drilling cuttings can be removed effectively even if there are more drilling cuttings produced in non-coring drilling than in coring drilling, when a borehole is drilled with the same diameter. When the outside diameter of the modular bit is 216mm, the recommended clearance value is 9mm or 10mm in order to obtain lower pressure loss and larger diameter core. To generate low pressure loss and ensure bit strength, the layout with 4 nozzles on the internal non-coring bit is recommended. The modular bit enables fast switching coring drilling and non-coring drilling without raising drilling tools, and the simulation model can be used for drilling parameters selection and drill bits optimization.