| The reverse circulation drilling technology of cluster-type pneumatic DTH hammer combines rapid drilling with high efficient deslagging,which represents one of the development directions of advanced drilling technology for large diameter drilling on the ground in coal mine area.Due to the complex geological conditions in China's coal mining areas and the wide distribution of watey strata with different strength,the problems of discontinuous reverse circulation and low slag removal efficiency often occur in the process of air drilling,which seriously restrict the improvement of drilling efficiency.In view of the actual situation of different water abundance in the strata,a new idea is put forward to support the formation and stability of reverse circulation by using the pressure generated by the water column seal body with a certain height at the bottom of the hole.It is of great practical significance to study the formation mechanism and process control of the reverse circulation with bottom water column seal during the large-diameter cluster pneumatic DTH hammer drilling,and to explore the rapid drilling method for large-diameter drilling in watey strata with different strength.Relying on the National Key R&D Program of China"high efficiency drilling and safe tunneling technology for large diameter rescue wells in complex formation(2018YFC0808202)",the formation mechanism and process control of cluster pneumatic DTH hammer reverse circulation with water column seal were studied by means of theoretical analysis,numerical simulation,similar test and field test.Firstly,the principle of reverse circulation drilling technology with water column seal is studied,and the physical model of reverse circulation with water column seal based on?178mm double wall drill pipe and?850mm cluster DTH hammer is constructed.Based on the theory of gas-liquid two-phase flow,the theory of flow pattern identification and conversion in reverse circulation process is analyzed,which provides theoretical support for the research of this paper.Secondly,by establishing the numerical simulation model and using the computational fluid dynamics method,the orthogonal numerical simulation is completed based on the three key technical parameters including the air intake quantity Q,the liquid column height H and the inner pipe return height h in the process of reverse circulation with water column seal,which shows that a group of parameter matching simulation schemes can form the reverse circulation with water column seal and three groups not.It is concluded that the formation of reverse circulation with water column seal is a development process from double circulation at the bottom of hole to reverse circulation,and then the variation law of pressure at the bottom of double circulation channel in the formation of reverse circulation with water column seal is expounded.A set of visual similarity test device was developed,and the similarity simulation test was completed.The three key parameters including air intake Q,liquid column height H and inner pipe return height h were tested and analyzed.The critical value curve of Q-H parameter matching was obtained when h was 14m and 18m respectively.Combined with the numerical simulation results,the formation mechanism of reverse circulation with water column seal was revealed.Based on the secondary development of multiphase flow model analysis software,the flow characteristics of gas-liquid two-phase flow in vertical inner tube with liquid carrying capacity of 0.1-0.7m~3/min were analyzed,and the flow characteristics of gas-liquid-solid three-phase flow in vertical inner tube with liquid carrying capacity of 0.1-0.7m~3/min and solid content of 3%-6%were analyzed based on the improved three-phase flow model.The corresponding relationship between the bottom pressure of inner pipe reverse circulation channel and the hole depth is obtained,which provides a reference for the selection of key parameters in the process of reverse circulation drilling with water column seal.Finally,the theoretical research results of reverse circulation with water column seal are verified through the water column seal reverse circulation drainage test and drilling slag removal test.Combined with the actual drilling situation of large diameter hole,the drilling technology of reverse circulation with water column seal is improved,and the structure design of cluster pneumatic DTH hammer is optimized.The research results of this paper provide a more reasonable water column seal reverse circulation technology method for cluster pneumatic DTH drilling,which has theoretical guidance and engineering reference significance for the research and application of high efficiency drilling technology in large diameter hole. |
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