Research On Influence Factors Of Rock-breaking Efficiency And Physical Design Of WOB Self-adjusting PDC Bit

The existing problems of low efficiency,short lifespan,and premature failure caused by vibration when drilling in the complicated strata in deep wells with conventional PDC bits greatly increase the development cost.In this paper,a new step-by-step rock-breaking method is proposed from two perspectives of reducing the rock strength and improving the attack capacity of the bit,and a corresponding weight-on-bit(WOB)self-adjusting PDC bit(WSADB)is developed.To maximize its effectiveness,the factors influencing rock-breaking efficiency(RBE)and the optimized design of its structure were studied.The research results are expected to provide a theoretical basis and technical support for the field application of the WSADB and improving RBE in deep wells.To study the influence of structural parameters of WSADB on weakening rock strength,a dynamic model was established based on Biot’s poroelastic theory,and a prediction model of rock strength failure was established combined with 3D Mohr-Coulomb criterion.Furthermore,the influence of the stress distribution at bottom-hole with different bit structures on weakening anti-drilling strength of the rock is analyzed.The prediction model can be used for real-time and quantitative evaluation of the rock damage at bottom-hole or on the borehole wall.Potential weakening degree of the rock to be drilled at the bottom-hole is greatest when the diameter ratio of the pilot bit and reaming bit is equal to 0.5.The larger the extension length of the pilot bit,the greater the potential damage to the rock at the bottom-hole,but there is a significant "marginal effect" when the extension length exceeds twice the diameter of the pilot bit.To verify the feasibility of the WSADB in increasing rate of penetration(ROP),a Φ120mm PDC bit and a Φ64-120 mm WSADB are designed and processed,and comparative experiments are conducted under multiple rock samples conditions.The results show that the RBE can be significantly improved,a larger ROP can be gained with smaller elevation in torque,and the mechanical specific energy(MSE)and the threshold WOB can be significantly reduced when drilling with the WSADB.To study the potential factors that may affect the working characteristics of the full-size WSADB and their influence laws,a finite element model simulating the rock-breaking process under complex loading conditions in deep wells was established based on the extended Drucker-Prager criterion,and the influences of the structural parameters,drilling parameters,elastic coefficients and rock properties on the working characteristics of the WSADB were analyzed.The distribution ratio of WOB decreases with increasing WOB and rotational speed,and tends to increase with the increasing elastic coefficient and rock drillability deterioration.To explore the impact of drilling parameters on RBE under the guidance of MSE theory,and to address the possible problem of insufficient WOB during the application of PDC bits,drilling tests were conducted with Φ50/120 mm conventional PDC bits under different drilling parameters and lithological conditions.The test results show that: while drilling in hard formations,continuously increasing WOB and rotational speed can significantly improve ROP,increase MSE,reduce RBE,and decrease mechanical energy utilization.Based on the prediction results of the optimal WOB of the full-size PDC bit,it is recommended to gradually strengthen the parameters to the drilling mode of “bigger WOB and higher rotational speed”.Given the engineering challenges faced in the conglomerate section of Kangcun and Gidik formations in Bozi Block of Tarim Oilfield,the Φ152-311 mm WOB self-adjusting PDC bit was designed in terms of structural parameters,crown shape,cutters distribution and elastic parameters of the spring,and the dual-stage shunt system was optimized based on the coupled DEM-CFD calculation method.After strength checking,machining and assembling,and application scheme preparation,the field test on the reliability of the WSADB was carried out.Taking cuttings retention and cuttings transportation ratio as the evaluation index,the flow allocation ratio of 6:4 for the pilot and reaming bit is most conducive to chip transportation at the bottom-hole.The field test of Φ152-311 mm WSADB shows that the bit can accomplish the given task under the complex loading conditions with a high displacement of the pump,and the designed bit could work safely with high reliability.