Micro-macro fracture mechanism of heterogeneous granite in percussive drilling

The conventional rotary rock breaking method faces a technical bottleneck in improving the rate of penetration(ROP)in deep hard formations.Percussive drilling is the most potential approach to increase rock-breaking efficiency and ROP.However,the rock-breaking mechanism of percussive drilling is still unclear enough,especially the micro-fracture mechanism of rock under confining pressure(under lateral pressure and hydraulic pressure).In this paper,the impact rock breaking experiments by four kinds of Polycrystalline Diamond Compact(PDC)cutters are carried out using a drop-weight impact testing machine and an acoustic emission(AE)recording system,the influence of parameters such as cutter shape,rake angle,and impact energy on rock-breaking are systematically analyzed.This study includes a numerical simulation to examine the process of crack initiation,propagation,and cuttings formation during the impact process with the consideration of confining pressure.The results show the conicalshaped cutter is the most aggressive with high breaking efficiency.The penetration depth of the cutter is mainly influenced by the impact energy and cutter shape than the rake angle of the cutter.There exists critical impact energy makes the rock breaking efficiency the highest.The critical impact energy is about 40 J when using the conical-shaped cutter with a rake angle of 15°.The rock mainly failed in tensile mode,and the inter-grain crack is the main crack.Hydraulic pressure can inhibit the formation of horizontal cracks,while lateral pressure can inhibit the formation of vertical cracks and reduce the proportion of tensile cracks.The research results can provide some reference and basis for improving the rock-breaking efficiency in deep hard formations.

Bifurcations and the penetrating rate analysis of a model for percussive drilling

In this paper, we investigate a low dimensional model of percussive drilling with vibro-impact to mimic the nonlinear dynamics of the bounded progression. Non- holonomity which arises in the stick-slip caused by the impact during drilling fails to be correctly identified via the classical techniques. A reduced model without non-holono- mity is derived by the introduction of a new state variable, of which averaging technique is employed successfully to detect the periodic motions. Local bifurcations are presented directly by using C-L method. Numerical simulations and the penetrating rate analysis along different choices of parame- ters have been carried out to probe the nonlinear behaviour and the optimal penetrating rate of the drilling system.

Evaluation of the energy efficiency of rotary percussive drilling using dimensionless energy index

This paper sets forth a geomechanics framework for assessing the energy efficiency of rotary percussive drilling using the energy criterion, which has been proposed by Victor Oparin for volumetric destruction of high-stress rocks having nonuniform physico-mechanical properties. We review the long-term research and development in the specified area of science and technology, including research and development projects implemented at the Institute of Mining, Siberian Branch of the Russian Academy of Sciences. A new modified expression of Oparin’s dimensionless energy criterion of volumetric rock destruction k is introduced. The range of in situ values is determined for the energy criterion of volumetric rock destruction at the optimized energy efficiency of rotary percussive drilling. The temporospatial intervals of geotechnical monitoring are found to control pneumatic drilling energy efficiency at subsoil use objects in Russia. The integrated experimental, theoretical and geotechnical approach to the comprehensive investigation of real-time processes of rock fracture in rotary percussive drilling using the energy concept possesses the necessary geomechanical performance-and-technology potential to create the next level geotechnical monitoring of drilling systems for various purposes, including determination of physico-mechanical properties and the stress-strain analysis of rock mass in full-scale drilling.

Numerical Simulation of Impact on Pneumatic DTH Hammer Percussive Drilling

The process of DTH（down-the-hole） hammer drilling has been characterized as a very complex phenomenon due to its high nonlinearity,large deformation and damage behaviors.Taking brittle materials（concrete,granite and sandstone） as impact specimens,the explicit time integration nonlinear finite element code LS-DYNA was employed to analyze the impact process and the penetration boundary conditions of DTH hammer percussive drilling system.Compared with previous studies,the present model contains several new features.One is that the 3D effects of DTH hammer drilling system were considered.Another important feature is that it took the coupling effects of brittle materials into account to the bit-specimen boundary of the drilling system.This distinguishes it from the traditional approaches to the bit-rock intersection,in which nonlinear spring models are usually imposed.The impact forces,bit insert penetrations and force-penetration curves of concrete,granite and sandstone under DTH hammer impact have been recorded;the formation of craters and fractures has been also investigated.The impact loads of piston-bit interaction appear to be relatively sensitive to piston impact velocity.The impact between piston-bit interaction occurs at two times larger forces,whereas the duration of the first impact doesn＇t change with respect to the piston velocity.The material properties of impact specimen do not affect the first impact process between the piston and bit.However,the period between the two impacts and the magnitudes of the second impact forces greatly depend on the specimen material properties.It is found that the penetration depth of specimen is dependent on the impact force magnitude and the macro-mechanical properties of the brittle materials.

Exploration and determination of the principles of rotary-percussive underground slimhole drilling

A possibility of the efficient use of rotary percussive drilling to provide drilling smaller diameter holes(40–70 mm) both in mining and prospecting is disclosed herein. A new construction designed for the nipple thread connection is described. The relative amplitude variation, change of power pulse time and energy in their propagation throughout the drilling tool are determined. A possibility of the efficient power pulse transfer along the drill string to the rock destruction tools with new nipple connections which allow automating the make-up and breakout system of drill pipe was supported by experiments.

Crack-free high-aspect ratio holes in glasses by top–down percussion drilling with infrared femtosecond laser GHz-bursts

We report novel results on top-down percussion drilling in different glasses with femtosecond laser GHz-bursts.Thanks to this particular regime of light–matter interaction,combining non-linear absorption and thermal cumulative effects,we obtained crack-free holes of aspect ratios exceeding 30 in sodalime and 70 in fused silica.The results are discussed in terms of inner wall morphology,aspect ratio and drilling speed.

A novel experimental system for performance evaluation of water powered percussive rock drill

A set of water powered excavation test system was developed for the comprehensive performance testing and evaluation of water powered percussive rock drill indoors. The whole system contains hydraulic power section, electronic control system, test and data acquisition system, and assistant devices, such as guideway and drilling bench. Parameters of the water powered percussive rock drill can be obtained by analyzing testing data, which contain impact energy, front and back cavity pressure, pressure and flow in each working part, drilling velocity, frequency and energy efficiency etc. The system is applied to test the self-designed water powered percussive rock drill SYYG65. The parameters of water powered percussive rock drill with impact pressure of about 8.9 MPa are 58.93 J for impact energy, and 8.97% for energy efficiency, which prove the effectiveness of system.

Experimental studies on rock failure mechanisms under impact load by single polycrystalline diamond compact cutter

Percussive drilling shows excellent potential for promoting the rate of penetration(ROP)in drilling hard formations.Polycrystalline diamond compact(PDC)bits account for most of the footage drilled in the oil and gas fields.To reveal the rock failure mechanisms under the impact load by PDC bits,a series of drop tests with a single PDC cutter were conducted to four kinds of rocks at different back rake angles,drop heights,drop mass,and drop times.Then the morphology characteristics of the craters were obtained and quantified by using a three-dimensional profilometer.The fracture micrographs can be observed by using scanning electron microscope(SEM).The distribution and propagation process of subsurface cracks were captured in rock-like silica glass by a high-speed photography system.The results can indicate that percussive drilling has a higher efficiency and ROP when the rock fractures in brittle mode.The failure mode of rock is related with the type of rock,the impact speed,and the back rake angle of the cutter.Both the penetration depth and fragmentation volume get the maximum values at a back rake angle of about 45°.Increasing the weight and speed of falling hammer is beneficial to improving the rock breaking effects and efficiency.The subsurface cracks under the impact load by a single PDC cutter is shaped like a clamshell,and its size is much larger than the crater volume.These findings can help to shed light on the rock failure mechanisms under the impact of load by a single PDC cutter and provide a theoretical explanation for better field application of percussive drilling.