Optimizing Parameters of Two-Stage Pdc Bit in Deep Formation Applications

Abstract

Two-stage PDC (Polycrystalline diamond compact) bit is a special drilling bit consisting of a small-diameter pilot and a large-diameter reamer. The unique rock-breaking structure is designed to improve the drilling efficiency and reduce drilling costs in deep and abrasive formations. According to previous studies, the hydraulic structure of the two-stage PDC bit can significantly affect the drilling performance of removing cuttings near the bottom of the wellbore. Therefore, this research has proposed to achieve a high efficiency of cuttings removal by optimizing the hydraulic structure. In this paper, the flow field at the bottom of the well near the two-stage PDC bit (Φ171.45 mm pilot + Φ215.9 mm reamer) has been numerically studied using the finite element method. As shown in the analysis results, both the flow rate of the pilot and the flow rate ratio of the pilot to the reamer increase significantly with the increase of the area ratio of the pilot nozzles to the total nozzles. The numerical data supports that the cuttings can be removed effectively from the bottom of the wellbore when the area ratio is between 0.75 and 0.80. Moreover, the cuttings can be removed effectively from the bottom of the wellbore when the mud viscosity is about 10 mPa·s, the rate of penetration is between 4 and 8 m/h, the drill speed is 90rpm and the cuttings particle sizes is between 1 and 2 mm. Besides, increasing the pump rate can imporve the cutting-carrying efficiency through the wellbore bottom. Field test results indicate that the average penetration rate of the test interval is increased by 30% and the footage of the test interval is increased by over 40% compared to the adjacent interval of the same well. This study can provide guidance for the optimal design of the hydraulic structure of such two-stage PDC bits.

Keywords: Two-stage PDC bit, Pilot, Reamer, Flow Rate, Hydraulic structure, Cutting carrying