The Ductile-brittle Failure Mechanism And Its Influence Factors Investigation Of Bottom Hole Rock

The low drilling efficiency is the key factor influence the oil&gas reservoir exploration and development cost;how to improve the drilling efficiency and reduce the drilling cost is a long-term difficulty problem met in oil&gas drilling process.At this stage,the mechanical rock breaking is the main method in oil&gas drilling,the rock fragmentation mechanism under drill tooth indentation and cutting directly decide the drilling efficiency and cost.This project aims at the scientific problem of rock fragmentation mechanism,the ultimate goal is to improve the drilling efficiency;the problems of crack initiation and propagation,damage evolution,chip formation,ductile-brittle failure transition,mechanical specify energy,plasticity energy dissipation ratio and fragmentation efficiency etc are studied based on the research approaches of theoretical analysis,computer numerical simulation and laboratory experiment.This paper mainly carried out the following research work:(1)Domestic and foreign literature review.A comprehensive and detailed review of the relevant literature about the interaction between drill tooth and rock at domestic and abroad is maked and its achievement and shortcomings are analysised and summarized which providing an important guidance for problems which put forward at the later chapters.(2)The experimental study on rock indentation.the formation process of the rock core,plastic zone,elastic zone and the condition of crack initiation and propagation in rock indentaion is analysied based on the cavity expansion model;the computer controlled electro-hydraulic servo universal testing machine,acoustic emission measurement system and strain analyzer are used to investigate the deformation and displacement,crack initiation and propagation,damage evolution,chip formation,ductile-brittle failure transition,mechanical specify energy,plasticity energy dissipation ratio and fragmentation efficiency etc in rock indentation;the mechanism of rock fragmentation in indentation process is revealed systematically;the concept of plastic energy dissipation ratio is proposed,and the new method of using plastic energy dissipation to evaluate rock fragmentation efficiency is put forward,and the fragmentation efficiency under different conditions are evaluated.(3)Theoretical and experimental study on the rock cutting.A new method to determine the critical cutting depth i5 proposed using MSE as an index based on roek mechanics,rock fragmentation thoery,energy dissipation theory etc,the calculation formula of critical cutting depth and plastic energy dissipation ratio are derived.A rock cutting experiment equipment is designed and processed;the crack initiation and propagation,chip formation,ductile-brittle failure transition,cutting force,mechanical specify energy,plasticity energy dissipation ratio and fragmentation efficiency etc are studied.The correctness of the formula is verified by the experiment and the relevant parameters in the formula are determine(the dependent variables and independent variables coefficient);the plasticity energy dissipation ratio is analyzed and caculated combined with the experimental data and calculation formula and the fragmentation efficiency is evaluated under different conditions.(4)The discrete element simulation of rock indentation and cutting.The uniaxial compressive strength(UCS)and Brazilian tensile strength(BTS)tests are performed to calibrate the relations between micro-properties and macro-properties of the rock specimens based on the uniaxial compression and tensile experiments tests.Then,the model of interaction between tooth and rock is established;and the crack initiation and propagation,damage evolution,chip formation,ductile-brittle failure transition,mechanical specify energy,plasticity energy dissipation ratio and fraglentation efficiency etc are studied under different conditions(lateral the liquid column pressure,rock bedding,tooth spacing,cutting depth,cutting speed,incline angle,etc.);the micro process of rock fragmentation in rock cutting is revealed systematically.(5)The analysis of other influencing factors on bottom-hole rock fragmentation.The fully coupled simulation model is established which considering the overburden pressure,horizontal in-situ stresses,drilling mud pressure,pore pressure and temperature,and then the solutions are obtained through the finite element method.The numerical solution is obtained under different mud pressure,well depth,formation temperature,permeability coefficient;and its influence on rock fragmentation is analyzed.The mechanism of rock fragmentation under indentation and cutting is systematically described in this paper.The research results are helpful for researchers to understand the rock fragmentation in oil&gas drilling profoundly;At last,the computing methods of indentation and cutting critical depth,plasticity energy dissipation ratio and the evaluation methodology of rock fragmentation are formed,which provide theoretical basis for the bit optimization design or custom design,drilling parameter optimization and the formation of low-cost drilling speed technology.