The Rock-Breaking Mechanism And Experimental Study Of PDC Bits Under Torsional Impact

With the continual increase of the number of deep wells and ultra deep wells in China,the situation encountered in the drilling becomes more complex, rock breaking is often difficult,so the problem is happened for the long period, the slow drilling speed, the higher drilling cost during the drilling process.The rock-breaking technology is a core content of oil and gas drilling Engineering, the efficiency of rock breaking plays a decisive role for the speed and cost during oil and gas exploration and development, how to improve the efficiency of the rock breaking technology by means of scientific methods becames evident in the imperative.PDC bit has the advantages of high rock breaking efficiency, fast drilling speed, long service life and stable performance, which is widely used in the field drilling around the world, but so far, the theoretical research is still far behind filed practice. In order to further expand the PDC bit application scope and provides a better theoretical basis for drilling rate and performance evaluation, the paper has carried out a series of research work to seek the rock breaking mechanism and experimental study of PDC Bits under torsional impact. Specific content as follows:1.Simplified model of the drilling system is built,this paper analyses the intrusion rock process of PDC cutting teeth and gets boundary conditions and control conditions in the process of drilling, then explains the movement of the torsional shock PDC bit.2.Cutting card model is built under confining pressure, analysing stress on the cutter tip and considering the affect of a variety of parameters, the force formula of tangential and axial are derived under confining pressure. At the same time, the knowledge of metal cutting is considered in the model, then advances a theory which is called cutter ball ribbon’s shape, The model not only accounts for the friction between the cutter ball ribbon ’s shape and bottom hole but also takes into account the relative motion between the ribbon’s chip and cutter ball shape, new force formula of tangential and axial are derived.3.Starting from the single teeth cutting two-dimensional model, meanwhile, the projection of the shear plane on the rock is assumed to be elliptical, this paper builts rock cutting volume model of single teeth, getting the cutting volume formula of PDC teeth, then redefines PDC single teeth cutting efficiency.4.Using the torsional impact rock-breaking equipment, this paper simulates PDC drilling experiment under torsion impact condition for different rock, the effect factors of PDC bit drilling rate are explored by analysing the experimental data. Finally, this paper presents some basic principles for selecting bits.