Depressurization mechanism and design principles of vortex drilling-bit

Key words:  drilling  vortex bit  bottom-hole pressure difference  reverse nozzle  numerical simulation

Abstract:

The drilling practice of oil and gas wells has indicated that reducing the bottom-hole pressure difference can significantly improve the rate of penetration (ROP). Vortex drilling-bit can reduce the bottom-hole pressure difference via a reverse nozzle to generate a vortex flow due to the high rotary speed of the drilling-bit. The depressurization mechanism of the vortex bit was analyzed. By using a hybrid mesh method, the key factors that affect the depressurization capacity of the vortex bit were investigated, which include axial angle, radial angle, reverse flow ratio, rotation speed, bit clearance, and distance between reverse nozzle outlet and the bottom-hole wall. The results show that backflow below the reverse nozzle outlet is the main reason that can decrease the depressurization effect of the vortex bit. A guideline for the design of the vortex bit is given to maximize the depressurization effect. The optimum axial and radial angles are 150°-180° and 60°-75° respectively, and the flow ratio of the reverse nozzle is best above 30%. The bit clearance should be kept below 3mm, and the distance between the reverse nozzle outlet and the bottom-hole surface can be 100-140mm. The rotation speed of the bit should be as large as possible.