Numerical simulation of rock-breaking and influence laws of dynamic load parameters during axial-torsional coupled impact drilling with a single PDC cutter

Axial and torsional impact drilling technology is used to improve the drilling efficiency of hard rock formation in the deep underground.Still,the corresponding theory is not mature,and there are few correlative research reports on the rock-breaking mechanism of axial and torsional coupled impact drilling tools.Considering the influence of the impact hammer geometry and movement on the dynamic load parameters(i.e.,wavelength,amplitude,frequency,and phase difference),a numerical model that includes a hard formation and single polycrystalline diamond compact cutter was established.The Riedel-Hiermaier-Thoma model,which considers the dynamic damage and strength behavior of rocks,was adopted to analyze the rock damage under axial and torsional impact loads.The numerical simu-lation results were verified by the experimental results.It was found that compared with conventional drilling,the penetration depths of axial,torsional,and axial-torsional coupled impact drilling increased by 31.3%,5.6%,and 34.7%,respectively.Increasing the wavelength and amplitude of the axial impact stress wave improved the penetration depth.When the bit rotation speed remained unchanged,increasing the frequency in the axial and circumferential directions had little effect on the penetration depth.However,as the frequency increased,the cutting surface became increasingly smooth,which reduced the occurrence of bit vibration.When the phase difference between the axial and circumfer-ential stress waves was 25%,the penetration depth significantly increased.In addition,the bit vibration problem can be effectively reduced.Finally,the adjustment of engineering and tool structure parameters is proposed to optimize the efficiency of the axial-torsional coupled impact drilling tool.

EXPERIMENTAL STUDY OF ROCK BREAKING EFFECT OF STEEL PARTICLES

Particle impact drilling is an efficient drilling technology for deep-well hard formation, With this technology, the rock is cut mainly by high-speed spherical particle impact under hydraulic action. In this article, the influence of jet flow factors, hydraulic factors and abrasive factors on rock breaking is studied through indoor experiments of impact by steel particles. The results indicate that the particle water jet has an optimal standoff distance and particle concentration; the rock breaking effect declines with the increase of the confining pressure and the decrease of the pump pressure and particle diameter. This study will provide some food of thought for the development of particle impact drilling technology.

Global case studies of soft-sediment deformation structures(SSDS): Definitions,classifications, advances, origins, and problems

Soft-sediment deformation structures（SSDS）have been the focus of attention for over 150 years.Existing unconstrained definitions allow one to classify a wide range of features under the umbrella phrase＂SSDS＂.As a consequence,a plethora of at least 120 different types of SSDS（e.g.,convolute bedding,slump folds,load casts,dish-and-pillar structures,pockmarks,raindrop imprints,explosive sandegravel craters,clastic injections,crushed and deformed stromatolites,etc.）have been recognized in strata ranging in age from Paleoproterozoic to the present time.The two factors that control the origin of SSDS are prelithification deformation and liquidization.A sedimentological compendium of 140 case studies of SSDS worldwide,which include 30 case studies of scientific drilling at sea（DSDP/ODP/IODP）,published during a period between 1863and 2017,has yielded at least 31 different origins.Earthquakes have remained the single most dominant cause of SSDS because of the prevailing＂seismite＂mindset.Selected advances on SSDS research are：（1）an experimental study that revealed a quantitative similarity between raindrop-impact cratering and asteroid-impact cratering;（2）IODP Expedition 308 in the Gulf of Mexico that documented extensive lateral extent（〉12 km）of mass-transport deposits（MTD）with SSDS that are unrelated to earthquakes;（3）contributions on documentation of pockmarks,on recognition of new structures,and on large-scale sediment deformation on Mars.Problems that hinder our understanding of SSDS still remain.They are：（1）vague definitions of the phrase＂soft-sediment deformation＂;（2）complex factors that govern the origin of SSDS;（3）omission of vital empirical data in documenting vertical changes in facies using measured sedimentological logs;（4）difficulties in distinguishing depositional processes from tectonic events;（5）a model-driven interpretation of SSDS（i.e.,earthquake being the singular cause）;（6）routine application of the genetic term＂seismites＂to the＂SSDS＂,thus undermining the basic tenet of process sedimentology（i.e.,separation of interpretation from observation）;（7）the absence of objective criteria to differentiate 21 triggering mechanisms of liquefaction and related SSDS;（8）application of the process concept＂high-density turbidity currents＂,a process that has never been documented in modern oceans;（9）application of the process concept＂sediment creep＂with a velocity connotation that cannot be inferred from the ancient record;（10）classification of pockmarks,which are hollow spaces（i.e.,without sediments）as SSDS,with their problematic origins by fluid expulsion,sediment degassing,fish activity,etc.;（11）application of the Earth＇s climate-change model;and most importantly,（12）an arbitrary distinction between depositional process and sediment deformation.Despite a profusion of literature on SSDS,our understanding of their origin remains muddled.A solution to the chronic SSDS problem is to utilize the robust core dataset from scientific drilling at sea（DSDP/ODP/IODP）with a constrained definition of SSDS.

Study on the fragmentation of granite due to the impact of single particle and double particles

Particle Impact Drilling(PID)is a novel method to improve the rate of penetration(ROP).In order to further improve the performance of PID,an investigation into the effect of single and double particles:(1)diameter;(2)initial velocity;(3)distance;and(4)angle of incidence was undertaken to investigate their effects on broken volume and penetration depth into hard brittle rock.For this purpose,the laboratory experiment of single particle impact rock was employed.Meanwhile,based on the LS-DYNA,a new finite element(FE)simulation of the PID,including single and double particles impact rock,has been presented.The 3-dimensional(3D),aix-symmetric,dynamicexplicit,Lagrangian model has been considered in this simulation.And the Elastic and Holmquist Johnson Cook(HJC)material behaviors have been used for particles and rocks,respectively.The FE simulation results of single particle impacting rock are good agreement with experimental data.Furthermore,in this article the optimal impact parameters,including diameter,initial velocity,distance and the angle of incidence,are obtained in PID.