Rockburst is still a stubborn disease in the field of engineering geology. The present research pays more attention to the influence of geological conditions on rockburst and less to the influence of type and stiffness of engineering support on rockburst. We explore the influence of support stiffness from weak to strong on rockburst and reveal the characteristics of fracture and microseismicity during rockburst through microseismic monitoring and numerical simulation. The main results and conclusions can be drawn: (1) Strong stiffness support makes the surrounding rock accumulates higher energy before rockburst. The evolution of microseismicity and its indexes can be used as precursors of potentially strong rockburst. (2) Strong stiffness support is easy to concentrate high stress under the action of surrounding rock pressure, and it is easy to fail under the disturbance of external load. This will produce a “sudden unloading effect” on the surrounding rock mass and may lead to a more serious rockburst. Numerical simulation verifies the existence of that effect and are consistent with the actual signs of failure. Our research is helpful to clarify the rockburst problem in the field of engineering geology, specifically to reveal the mechanism of rockburst and the early warning criteria of rockburst hazard under the action of supporting structure, which can provide practical data and theoretical support for scientific and reasonable prevention and control of rockburst risk in tunnel and underground engineering.
This paper describes the optimization of a hybrid bit which is a polycrystalline diamond compact (PDC) bit with rolling cone cutters to improve dynamics and durability. When drilling the hard & interbedded formations with conglomerates in the Tarim Basin in Northwest China, PDC bits break down with broken & chipped cutters in the cone, nose & shoulder sections of the profile. In addition severe torsional vibrations lead to twist-off's and stick-slip which damages the bit and drilling system. Even worse, because of the torsional vibration, less Weight on Bit (WOB) is being applied, which results in lower Rate of Penetration (ROP), less footage and more trips. Roller-cone bits generally drill faster than PDC bits, but bearing life is short in the hard and abrasive conglomerates. Due to success in similar application elsewhere in the world, a hybrid bit was proposed and tried to drill the challenging formations in the Tarim Basin. In the first run the entire conglomerate interval was drilled at higher ROP, with lower torsional oscillations and saving 16 rig days. Although this was a success there was an opportunity for further improvement by strengthening the PDC cutting structure to avoid premature cutter damage mainly in the central, cone section of the profile. An upgraded hybrid bit with split, overlapping blades and more durable Dual Chamfer cutters was deployed in the same interval. Respective performance of PDC, Roller Cone, Hybrid bit and Optimized Hybrid bit used in the same formation are compared in terms of footage, ROP, Mechanical Specific Efficiency (MSE) and dull condition. By using Optimized Hybrid bit, the customer saved multiple bit runs & drilled eight times farther with the ROP 92% faster than conventional PDC bit run in the same well.
China’s Sichuan Basin contains prolific gas fields and presents many drilling challenges due to the complex structural geology. Xujiahe is one of the most challenging formations in the basin. The interbedded and hard, abrasive nature of Xujiahe results in short intervals drilled and low rates of penetration (ROP). Polycrystalline Diamond Compact (PDC) bits have limited drilling efficiency in this formation because impact damage and abrasive wear lead to short intervals drilled and very low ROP. Seven-bladed PDC bits which replaced roller cones were once believed to be the most cost-effective solution. To expedite gas exploration, improve ROP and increase distance drilled in the Gaoshiti-Moxi block, an innovative application-specific engineering drill bit was required. To identify the solution, a cross-functional team from the drilling contractor and the service company conducted extensive technical research into drilling practices. Sophisticated software was used to analyze the rock properties, and to identify the formation drilling problems. Reviews of offset drill bits provided clues to drilling challenges. After a series of technical analyses, an application-specific hybrid bit was designed and tested in this block. Continuous optimization of drilling parameters is implemented by field engineer. The first three trials drilled through this formation, saving two PDC bits and two trips over a 500-meter interval. Post-run review and analysis of the first trials was conducted for further improvement of ROP and interval drilling using drilling parameter and bottomhole assembly (BHA) optimization. One fast run with an average of 45% better ROP was achieved on following runs. This saved seven drilling days and 2.5 runs; an average savings of 180,000USD per well. This paper describes the collaborative solution and detailed technical optimization of hybrid bit performance in Gaoshiti-Moxi Block. Based on the drilling data, economic analysis of this project is also presented. With this cross-functional team’s effort, how this success model may apply on other drilling projects will be also included.
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