| Gas drilling has a remarkable effect on the production of large scale low-grade and deep-buried oil and gas resources,relying on its advantages of significantly improving penetration rate,enhancing single well production,effectively detecting and protecting reservoirs.However,due to the absence of high-efficiency measurement while drilling(MWD)techniques applicable to gas drilling,the application and development of gas drilling technology is greatly limited by security risk and path control conundrums.For this purpose,this dissertation proposes to transmit down-hole data via acoustic waves in drill-string during the process of gas drilling,and the main achievements are listed as follows:(1)Taking account of internal viscous dissipation of gas phase medium and pipe wall's shear friction,the effects of acoustic frequency,system pressure,and temperature,gas flow rate on acoustic wave transmission in drill-string was analyzed by establishing a mathematical model of acoustic wave transmission and attenuation and performing the subsequent numerical calculation.The preliminary analysis of the feasibility to transmit operational mode parameters measured down-hole via acoustic wave was also accomplished by analog computation of the actual gas drilling data.(2)Based on waveguide theory,acoustic waveguide characteristics of drill-string channels used in acoustic signal transmission,including generating mechanisms of plane sound wave and high-order wave propagation properties,were discussed in-depth,and cut-off frequency of cylindrical acoustic waveguide was obtained by developing and solving a mathematical model.Also,the research about the mechanism of variable cross-section acoustic attenuation at drill pipe coupling was carried out.In addition,this study provided theoretical basis for the carrier frequency selection by figuring out influencing factors,determined by analog computation,which shall be eliminated during selection.(3)A large-scale experiment platform was set up for simulating acoustic signal transmission in drill-string during the process of gas drilling,revealing acoustic signal transmission law in experimental pipelines and the influences of different operational mode parameters on acoustic signal transmission,as well as validating the accuracy of the theoretical model,thus would provide experimental conditions and evidences for the research and development of acoustic signal transmission technology in drill-string during the process of gas drilling.(4)In accordance with necessary requirements of down-hole source for acoustic signal transmission,hydrodynamic acoustic generators were studied from aspects of sound principle,structural feature,performance indicator,and the applicability under the working condition of gas drilling,etc.,in order to select the acoustic generator type optimized for emitting down-hole acoustic signal and form the design scheme of down-hole acoustic generator.In this study,six hydrodynamic acoustic generators were developed,each performance of which was tested by experiments.(5)Acoustic signal transmission process was analyzed,while features of several digital signal modem techniques and their applicability under the working condition of gas drilling were examined through comparison between acoustic signal transmission environments.The method of processing acoustic signal in drill-string was obtained after taking both acoustic signal modulation complexity and signal demodulation success rate into consideration.(6)On the basis of gas drilling technical characteristics,feasibility of the whole set of acoustic waveguide signal transmission technology was tested by carrying out analog computation on the actual drilling data,and a set of technique scheme,including the concrete operating procedure and the matching technology,was provided.This study provides a new solution for MWD problems during the process of gas drilling,and contributes to filling gaps in the engineering measurement and control fields of gas drilling. |
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