Abstract
The lower rate of penetration (ROP) is one of the key technical difficulties during drilling of shale reservoirs. Percussive-rotary drilling (PRD) is crucial for increasing ROP. One of the core problems of ROP optimization for PRD are the dynamic damage characteristics of rock fragmentation. By considering the dynamic drilling parameters, a new model for estimating the PRD with a full-scale polycrystalline diamond compact (PDC) bit is established. The mechanical parameters of shale are measured by a wave velocity method. Rock damage characteristics are simulated by using the finite element method. The numerical simulation model is verified by the actual drilling case in LMX shale reservoir in Sichuan (China). The results indicate that rock element damage occurs along the direction of maximum principal stress. The order of decreasing rock damage rate is impact-static load, static load and impact load. When the impact load has the same peak value, and the rock elements in contact with the cutters obtain more energy with load frequency increasing. The rock fragmentation efficiency under a sine wave is higher than rectangular and pulse waves. The rock can obtain more energy to be broken with the increasing impact load duration and peak values. When the impact-static load goes over the rock damage threshold value, the higher the peak value of the impact load is, the more energy the rock will obtain. The higher the lateral vibration amplitude of the drill bit, the lower the efficiency of rock fragmentation. Repetitions of drill bit axial vibration at one indentation point will reduce the ROP, and the axial vibration energy of the drill bit is consumed. Therefore, a small lateral movement and reasonable axial vibration frequency increase the rock breaking efficiency. The ROP was increased through the suppression of drill string and the application of vibration. The study results can be used in the optimization designs of bit trajectory and ROP for PRD tools.
Highlights
The lower rate of penetration (ROP) is one of the key technical difficulties during drilling of shale reservoirs
This paper primarily focuses on the shale dynamic damage efficiency based on damage mechanics, strength theory, experimental and field statistic results, as well as considering dynamic parameters, including load combinations forms, impact frequency, impact wave shape, duration of impact load and impact load peak values
Drill bit trajectory is an important characteristic of drilling engineering, which has a close relationship with ROP and borehole quality
Summary
The lower rate of penetration (ROP) is one of the key technical difficulties during drilling of shale reservoirs. The modeling and simulations effects of borehole pressure on bit-cutter induced rock fragmentation was studied by DEM. The impact fragmentation of static and dynamic loading was studied [16]; this numerical model focused on the effect of two drill bluestone was studied by using the FEM [12], which focused on multi-type bits. Many studies of PRD in hard rock do not Energies 2018, 11, 1326 consider the shale formation and the complex shape of full-size PDC bits. The mechanical properties and dynamic characteristics of shale differ from those of other rock types; the dynamic drilling parameters of PRD are coupled in the drilling process, and the need for complex experimental instruments and field tests increase the experimental and test costs dramatically. This paper primarily focuses on the shale dynamic damage efficiency based on damage mechanics, strength theory, experimental and field statistic results, as well as considering dynamic parameters, including load combinations forms, impact frequency, impact wave shape, duration of impact load and impact load peak values
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