| The conical diamond element(CDE)has excellent impact resistance,wear resistance,and high ability to break hard rocks,which can provide an efficient rockbreaking tool for the exploration and development of deep oil & gas and geothermal resources.At present,the research on CDE cutter mainly focuses on the rock breaking law of CDE cutter and the field application of hybrid PDC bits,and there is a lack of systematic research on the mechanism of CDE cutter breaking hard rock and the design of hybrid PDC bits.Therefore,this paper adopts laboratory experiments,numerical simulations,and theoretical derivations to compare the rock breaking characteristics of CDE cutter and conventional PDC cutter,and based on this,study the rock breaking process and stress and damage evolution characteristics of CDE cutter,and establish a three-dimensional mechanical model for predicting cutting forces,and optimize the parameters of mixed cutter arrangement.The main results are as follows:1.Experiments of single cutter breaking granite with conical and conventional cutter were carried out,and the cutting force,rock chips size distribution,cutting groove morphology,and cutting fracture characteristics of the conical and conventional cutter were analyzed to further clarify the rock breaking advantages and shortcomings of this two cutters.The results show that: the average and fluctuation degree of cutting force of CDE is 46.15% and 31.85% smaller than those of conventional cutter,and the MSE of CDE is also 34.09% smaller than that of the conventional cutter;the cutting groove edge of CDE is serrated,the surface is rough,the cross-section is "V" shaped,and the cutting fracture shows the characteristics of tensile fracture,while the cutting groove edge of the conventional cutter is neat,the surface is relatively smooth,the cross-section is a regular arc,and the cutting fracture shows the characteristics of shear fracture.2.A visualization experiment system for the rock-breaking process of CDE was built,the numerical calculation model of rock breaking by CDE was established,and the rock-breaking law of CDE,rock chip formation process,cracks initiation and propagation,and rock stress response and damage evolution characteristics were studied.The mechanism model of CDE breaking granite was proposed.The results show that: the cutting force increases linearly with the increase of cutting depth and cutting angle,and increases quadratically with the increase of contact area;the MSE decreases significantly with the increase of cutting depth,decreases first and then increases with the increase of cutting angle.The best rock breaking effect with a cutting angle of about 20°;the rock breaking process can be divided into two stages: crushing and chipping;the compressive stress around the cutter tip is concentrated,and the rock undergoes compressive shear failure.An arc-shaped band-like tensile stress zone is formed around the compressive stress zone.Tensile micro-cracks are induced in the cutter tip and at the boundary of the compressive stress zone;the micro-cracks expand to the front of the cutter to form arc-shaped tensile main cracks,and block debris collapse occurs,which improves the rock breaking efficiency this time,and expands into the rock to deteriorate the rock strength and form bottom damage area,greatly improving the subsequent rock breaking efficiency.3.Based on the three-dimensional force analysis of the CDE,a three-dimensional mechanical model for predicting the cutting force was established by theoretical derivation considering the real bottom-well stress conditions,and the influences of hydrostatic pressure,in-situ stress,reservoir temperature,and rock surface heat transfer coefficient on the cutting force were predicted.The results show that: the absolute value of the model prediction error is less than 7% under atmospheric pressure;the cutting force increases linearly with depth of well and hydrostatic pressure;the thermal stress is tensile stress,which can significantly reduce the cutting force.when the formation temperature is 300 ℃ and the cutting depth is 1.0 mm,the percentage of cutting force drop caused by thermal stress is 74%;the cutting force decreases significantly with the increase of formation temperature;the cutting force decreases gradually with the increase of heat transfer coefficient.4.The mixed tool cutting experiment with a conical and a conventional cutter was carried out.The influences of the cutting sequence,spacing,and depth difference of mixed cutters on the rock-breaking effect were analyzed.The parameters of the mixed cutter arrangement were optimized,and the rock-breaking mechanism of the mixed tool was revealed.The results show that: the CDE followed by the conventional cutter can provide a higher rock breaking efficiency,and the rock-breaking efficiency gradually increases with the increase of cutter depth difference for single-track cutting;For multitrack cutting,with the increase of conical cutter spacing and cutter height difference,the cutting force and MSE decrease first and then increase,and the optimal conical cutter spacing and cutter height difference are 18 mm and 0.5 mm,respectively.Conical cutters form a large number of cracks around the cutting groove and degrade the rock strength.Conventional cutter expands the groove and peels the damaged rock around the cutting groove to improve the overall rock breaking efficiency.The research results reveal the mechanism of CDE cutter breaking granite,which can provide theoretical support for the design of hybrid PDC bits suitable for improving the rate of penetration(ROP)in deep hard rock drilling. |
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