| The underbalanced drilling technology was born in 1950s, after that, it was maturated and rapidly developed into a new drilling technology in 1990s. It got wide application in china and abroad, and became other development hotspots after horizontal well technology. Gas drilling was the important branch of the underbalanced drilling technology, and among them, air drilling which took the compressed air as drilling circulating fluid was the earliest developing underbalanced drilling technology. In recent years, research and application of the gas drilling technology bring a new climax at china and abroad after experiencing several high tides and low ebbs. The air hammer underbalanced drilling technology compatible with their advantages was the organic combination of gas drilling and percussive rotary drilling, and its successful application was provided wide space for the development and operation of the air hammer-down the hole hammer (DTH)and pneumatic hammer.The hollow-through DTH hammer was also called down-the-hole hammer rock drilling tools which possessed with the special structure form. The hollow-through DTH hammer with the whole process into reverse circulation drilling craft, which was relative to the hollow-through DTH hammer, was a high technology combined with three technique-DTH hammer impacting breaking rock, fluid medium full hole reverse circulation and continuous obtaining rock core sample-into one system, and had got more widely applied in the solid mineral exploration and hydrology water well drill. At present, the DTH hammer has been formed series specification. The drilling process and the outfitting tools gradually become maturation, so the application area is continuously extended and has a broad prospect.The paper supported by the project of "feasible research on air reverse circulation drilling system ", aiming at characteristics of the gas drilling used in the oil and gas exploration, carried out the relative technique research on hollow-through DTH hammer used in gas drilling in order to import the hollow-through DTH hammer with the whole process into reverse circulation drilling craft into the oil and gas exploration, exerting its unique technical advantage, to solve the efficient development problems that the low-presser, low permeability and low yield reservoir faced when it was applied in the oil and gas exploration drill field currently, and to supply the favorable technical support for enhancing the drill speed of the complex strata and deep well ground consisting of hard rock. The main research contents and conclusions are the following:1. The feasibility and basic requirements of hollow-through DTH used in oil and gas exploration and development was conducted a comprehensive analysis. The hollow-through DTH hammer which merged gas drilling, percussive rotary drilling and reverse circulation drilling into one body, had broad application prospects in the field of oil and gas exploration and development, and possessed inimitableness advantages of single technique, for example, dramatically reducing compressed air consumption, avoiding sidewall erosion disturbance, avoiding severities accident of well leak, disposing formation water production, undamaging reservoir. The hollow-through DTH hammer would confront with large diameter and deep hole work conditions when it was applied in gas drilling, so it is required to have the great air consumption and the ability of high voltage and high temperature. The requirements of the craft are to ensure the reverse circulation stability developed by fluid medium and the smooth when rock ballasts was ejected.2. The experimental study on the structural design of the private reverse circulation bit. The private reverse circulation bit is the key part for executing the hollow-through DTH hammer with the whole process into reverse circulation drilling craft, and it is not only the cutting rock tool at bottom hole, but also the function element of controlling fluid medium reverse circulation. It is found that to develop the reverse circulation bits suitable for gas drilling and good of reverse circulation performance is the more important link of the hollow-through DTH hammer with the whole process into reverse circulation drilling craft entering into the oil and gas exploration drilling field. In order to design the fluid passage and to choose the underside contour shape, the paper made a laboratory experimental study about the large diameter reverse circulation bit in gas drilling.(1) It was developed the simulated bottom-hole fluid flow tester, and its structure form of fluid passage was basically the same as the real situation when the bit was working at the bottom. With flexible and convenient features, the tester was an open platform, and it could experiment with different structure forms of fluid passage and different types of the underside contour shape bit models. It was more scientific and reasonable through measuring and comparing the air displacement of bit outer space to evaluate the formation effects of reverse circulation.(2) It was designed three type inner tubes with the different inner nozzle, and test results showed that: the pumping characteristic of the inner nozzle with new spiral model which was firstly put forward in this paper was better than the pumping characteristic of the inner nozzle with injection model which was currently used. The conclusion was conducive to improve the performance of reverse circulation drilling bit.(3) It was designed three type bit models with the different underside contour shape. To compare with the experimental results measured under the same condition was shown that: it was the concave bit model that was the best to form the reverse circulation effects, and it was also found that formation effects of reverse circulation of the convex bit model and the concave-convex bit model were almost same.(4) Based on the convex bit model and the concave-convex bit model, with the contrast experiment for bottom nozzle's arrangement mode, the results showed as following: under the same condition of the bottom nozzle's number, diameter, and total flow cross section area, the even layout in the same sphere was more favorable to form the reverse circulation than the layout in different sphere.With the experimental study, it was resolved a number of key problems about the structural design on the large diameter reverse circulation bit, and that provided the objective and reliable data foundation for the structural design on the reverse circulation bit used in the gas drilling.3. Numerical simulation analysis on local flow field of reverse circulation bitOn the basis of experimental study, it was set up the numerical calculation model of the inner nozzle local flow field and the numerical calculation model of the hole- bottom flow field, and with the large commercial CFD software- FLUENT to simulate and analyze the flow field. The inner nozzle's local flow field simulation results showed the multiple jets flow pattern formed by the two different types inner nozzles. At the same time, it also proved that the jet angle was an important factor impacting the multiple jet pumping characteristics. Among these, the multiple jets formed by the spiral inner nozzle was more sensitive to the changes of jet angle, when the jet angle is greater than 45°, the pumping performance deteriorated rapidly. The numerical simulation results of the hole-bottom flow field revealed the influence mechanism about forming reverse circulation effects of the underside contour shape and the hole-bottom nozzle layout, which was of great significance to guide and optimize the structural design on the large diameter reverse circulation bit used in the gas drilling.To ascertain the gas injection parameters of the circulation system- volume flow and pressure were critical of the hollow through DTH hammer reverse circulation in the gas drilling. It was not only the basis of the surface high-pressure gas injection system configuration, but also related to the formation of reverse circulation and the slag discharging effects. On the basis of the gas drilling theoretical study at home and abroad, it was built up the pressure plotting model of the hollow through DTH hammer twofold wall drill tools reverse circulation system, got the pressure distribution law of the circulation system, and derived the calculation formula. With the combination of the gas injection volume, the gas injection pressure, penetration rate and changes in hole depth, it was given a general calculation example of H = 2000m by programmed calculation. It was provides the basis for the hollow through DTH hammer reverse circulation gas drilling to select the gas injection parameters.The main creative point in this paper includes:(1) An inner jet nozzle with the new structure design was presented. With the lab experiments and numerical simulation results, it was proven that the new structure was more beneficial to form the reverse circulation, and it could improve the reverse circulation performance of drilling bits.(2) For the first time, it was studied the underside contour shape which was impacting the forming effects of the reverse circulation, and designed the reverse circulation bit hole -bottom flow field simulation tester. Experiments showed that the concave bit model was more favorable for the reverse circulation formation. And by means of CFD numerical simulation, the influence mechanism of the different bit underside contour shape to the reverse circulation formation was revealed.(3) By measuring the air displacement of bit outer space, it was compared and analyzed the influence of the hole-bottom nozzle even layout in the same sphere and grouped interleaving even layout in the different spheres on the reverse circulation formation, so gained the conclusion of that the even layout in the same sphere is more favorable for the reverse circulation formation.(4) On the basis of the gas drilling theoretical study at home and abroad, it was firstly built up the pressure plotting model of the hollow through DTH hammer twofold wall drill tools reverse circulation system, and got the pressure distribution law of the circulation system. It was provides the basis for the hollow through DTH hammer reverse circulation gas drilling to select the gas injection parameters.At present, the hollow through DTH hammer and the private reverse circulation bit which are special designed for the gas drilling have been completely processed and made, and are waiting for the appropriate wells to test. The above content in this paper are just the preliminary fundamental research about the hollow through DTH hammer used in oil and gas exploration field, and these are yet to be tested in practice. Further more, it is need to deeply comprehensive research based on the practice of the hollow through DTH hammer reverse circulation gas drilling.
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