| Efficient exploitation of coal-bed methane(CBM)is of great significance to ensure energy security and reduce the external dependence of natural gas.The use of radial jet drilling technology to extract CBM is an attractive study area,and the design of a small size self-propelled water jet bit with high drilling efficiency and strong drilling extension is the key to the success of this technology.The straight-swirling mixed jet bit(SSMJ)has the advantages of simple structure and round drilling,but the research on the rockbreaking mechanism of SSMJ and the optimization of bit structure is not comprehensive.In this thesis,based on theoretical analysis,numerical simulation and experiments,the rock-breaking mechanism,flow field velocity distribution characteristics and broken coal rock characteristics of the SSMJ bit for radial horizontal drilling were studied,and to optimized the structural parameters of the bit.(1)The breaking mechanism of straight-swirling mixed jet was revealed.Using LSDYNA software,a numerical model of water jet impact on coal rock was established to analyze the coal rock damage process.Under the jet impact,the effective stress on the coal rock unit gradually increases,resulting in the plastic strain starting to accumulate,and the coal rock unit is damaged after reaching the threshold value.Erosion experiments were carried out for three types of jets.The SSMJ punching characteristics combine the advantages of straight and swirling jets,and the erosion hole have a large diameter and no tabs at the center.Analyzing the characteristics of broken coal rocks in different jet zones of the SSMJ,combined with the above coal rock destruction mechanism.Results show that the strong cyclonic zone has large tangential and radial velocities,and the tensile and shear effects on the coal rock are enhanced,plus the jet flow pressure takes the lead in completing the crushing.The straight jet zone has a large axial velocity and positively impacts the coal rock,which is also broken under the action of jet flow pressure.After the fissures produced by the two crushing zones penetrate each other,the coal rock corresponding to the weak cyclonic flow zone forms an independent nucleus,resulting in a decrease in the tensile strength and shear strength of the coal rock in this region,and this region is finally crushed under the impact of the weak cyclonic flow zone,which has a smaller velocity in all three directions.(2)Combined with the theoretical analysis,the inlet flow ratio affects the distribution ratio of direct and rotating jets,the groove aspect ratio mainly affects the rotational shear energy of rotating jets,while the mixing chamber aspect ratio and straight lines aspect ratio affect the mixing of the two jets and the decay of jet energy.Changing the drill structure can be used to regulate the flow field with different characteristics,while there is little research on the above structure.FLUENT simulation software was used to simulate the SSMJ flow field,to study the axial velocity variation law of the flow field under different structural parameters and the distribution law of three-dimensional velocity along the radial direction at a dimensionless standoff distance of 5.56.According to the velocity distribution characteristics,the optimal values of the structural parameters are initially determined: the inlet flow ratio was 0.46,the groove aspect ratio was 3.88,the mixing chamber ratio was 0.67,and the straight lines ratio was 0.56.The pressure loss of the jet in the internal flow field was also studied,and the results showed that the larger the proportion of the rotating jet in the mixed jet,the more obvious the energy decay.(3)Part of the SSMJ bit selection obtained from the simulation was processed,and a SSMJ jet impact crushing coal rock test was carried out,and similar material of coal rock(coal dust: gypsum: cement = 5:1:2)with similar physical properties to the original coal as the object of erosion.Firstly,impact tests with different target distances were carried out to obtain the relationship between the velocity distribution characteristics of the jet and the fragmentation characteristics of the coal rock.The greater the axial velocity,the greater the fragmentation depth at the center of the erosion hole,and the greater the tangential and radial velocities,the greater the fragmentation depth around the corresponding erosion hole and the hole diameter will be enhanced.Under this test condition,the best erosion effect is achieved when the target distance was 10 mm,and the correctness of the flow simulation was verified.Then the impact tests of bits with different structural parameters were conducted to compare and analyze the coal rock breaking characteristics(hole diameter,hole depth,breaking volume,flatness of hole bottom)under different structural tragedies.Combining with the simulation results,it was obtained that the bit with an inlet flow ratio of 0.46,a groove aspect ratio of 3.88,and a mixing chamber aspect ratio of 0.67 had the best rock breaking effect.(4)The sensitivity of hydraulic parameters and nozzle structure parameters to the rock-breaking capacity of the jet was investigated by using gray correlation analysis with the erosion pit crushing volume as the evaluation sequence,ranked from the highest to the lowest as: target distance,mixing chamber aspect ratio,groove aspect ratio and inlet flow ratio.The research results of this thesis can provide reference and guidance for determining the reasonable parameters of the SSMJ drill bit and the guidelines of bit optimization design,which has certain engineering significance.Figure [55] Table [6] Reference [92]... |
PDF Full Text Request