Optimization Of Special-shaped PDC Bit For Deep Hard Formation In Dehui Gas Field

At present,the application of the "three high and two big" development mode in the Dhui block is effective in increasing production,but the drilling encounters deep hard and brittle rocks in the block,and problems such as drill bit friction and slow mechanical drilling speed occur.The conical PDC cutter has achieved good results in the field hard rock experiments at domestic and overseas,providing a new way to break the hard and brittle rocks,but the force,mechanism and regulation in the process of breaking the rocks have not been completed.In this paper,to address the above problems,the study object is to crush hard brittle rocks by conical PDC cutter,and through experimental,theoretical and numerical simulation studies,the force model of rock breaking by conical PDC cutter is established,and its rock breaking mechanism and influence regulation are elucidated.The mechanical parameters of the rocks in Dhui block were measured by triaxial compression,Brazilian splitting,hardness and drillability tests.The test results show that the deep rock of Dhui block is hard and brittle,with high strength,high hardness and low plasticity,but very low tensile strength.The conventional compression-shear action cannot break the block completely,and only a shear fracture penetrates the rock.This is the reason why it is difficult to drill into such rocks with conventional drill bits.Based on the rock strength Moore Coulomb criterion and elastic mechanics,the force model of rock breaking by conical PDC cutter was established.The conical PDC cutter,cylindrical PDC cutter and double cutter joint rock breaking models were established using finite element software to analyze the crack emergence and expansion as well as the stress and strain distribution state of the rock during the rock breaking process.The study shows that: the main mode of rock breaking by conical PDC cutter is rock fragmentation in horizontal direction dominated by tensile stress;multiple conical cutter drilling into the rock at the same time can form rock ridges to break the rock faster;conical PDC cutter can break the rock with less cutting force than cylindrical PDC cutter;the optimal design parameters of conical cutter and cylindrical cutter are derived from the analysis and simulation results,and the optimal cutter angle of cylindrical cutter are derived;in the joint rock breaking,the conical cutter should be minimized.In joint rock breaking,the inter-cutter spacing of conical cutter should be minimized,the cutting speed is greater than 2m/s and the cutting force rises significantly,and the cutting force increases linearly with the difference of cutting depth between cylindrical cutter and conical cutter.The research work in this paper provides a theoretical basis for the force in the broken rock of conical PDC cutter and the cracking process of hard rock breaking,and provides a reference and basis for how to optimize the design of drill bits by using conical cutter and cylindrical cutter,which has certain guiding significance for the design of hybrid PDC drill bits for drilling into hard strata.