Experimental and numerical study on the dynamic characteristics of full-size PDC bit

Abstract

Grasping the dynamic characteristics of drill bit can help optimize drilling design and improve rate of penetration (ROP). This work focuses on the nonlinear dynamic characteristics of full-size polycrystalline diamond compact (PDC) bit. One of the most conspicuous features of this work is that the nonlinear dynamic model for drillstring-bit-rock system is established in actual drilling engineering scale. In the proposed model, the drillstring dynamic model is based on the FE method combined with beam theory, and PDC bit-rock dynamic model could take the actual bit structure and rock properties into consideration. A logical and feasible coupling method is proposed to realize the coupling of the drillstring dynamic model and PDC bit-rock dynamic model. A full-size 81/2″ PDC bit and three kinds of rock samples (limestone, sandstone and artificial stone) are used to conduct systematic experimental study on dynamic response of PDC bit. The simulated and experimental bit footage, WOB, TOB, three-way vibration acceleration, and bottomhole topography after drilling are obtained. Results showed that: PDC bit drills the fastest in artificial rock, followed by sandstone, and drills the slowest in limestone. With the increase of rock strength, the MV of WOB increases and the fluctuation becomes more severe. The TOB is larger when drilling in artificial rock, followed by sandstone and limestone. The bit footage increases as initial WOB increase. With the increase of the initial WOB, the MV of WOB increases, and the WOB and TOB fluctuation become more severe. Axial acceleration, radial acceleration and tangential acceleration all increase as initial WOB increase. The bit footage increases as rotation speed increase. With the increase of rotational speed, the fluctuation of TOB tends to decrease, and the tangential acceleration decreases obviously. When PDC bit is crossing different lithologically rock, a dramatic instantaneous fluctuation will appear in WOB, TOB and vibration acceleration. The research results of this paper can provide a valuable and useful guidance for drilling engineers to improve ROP and reduce downhole accidents.