Study On Characteristics Of Acoustic Propagation In Gas-liquid Two-phase Flow Within Marine Riser

Due to the complexity of marine geologic environment,the higher difficulty and risk of drilling operation,the deepwater drilling set a higher demand on well control.How to detect gas-cut earlier and more accurately has an important significant on well safety and blowout prevention,however,most of traditional monitoring methods can not meet the requirement of deepwater well control.Gas-cut can be detected earlier by using noninvasive ultrasonic to monitor the flow characteristics of drilling fluid in riser.Therefore,this paper's objective is to study the acoustic response of gas-liquid two-phase flow in the riser and provide theoretical basis for early and accurate detection of gas-cut.Theoretical analysis,numerical simulation and experiment are carried out in this paper.First of all,according to the acoustic scattering theory,the relationship between scattering coefficient and void fraction is obtaioned by studying the influence of single air bubble and bubble populations on sound field.Based on the derivation of the sound propagation equation in the bubble liquid,the key parameters affecting the sound propagation in gas-liquid two-phase flow have been revealed,which can provide guidance for following numerical modeling and experimental verification.Secondly,a numerical model has been established to simulate the ultrasonic propagation within the annulus of marine riser and the credibility of the numerical model is verified by the experiment using the facility we established to simulate gas-cut monitoring.Then the influence of the existence and eccentricity of drill pipe on annular sound field has been studied and the best frequency of exciting pulse has been selected though processing numerical signals.Using coda wave interferometry(CWI),the characteristic of coda wave velocity change has been researched under the condition of low void fraction and the applicability of CWI also has been discussed.At the same time,the varying tendency of attenuation coefficient,frequency shift and the arrival time of first wave in gas-liquid two-phase flow with the change of void fraction has been analyzed.Finally,the quantitative regression relation of void fraction has been developed based on synthetical consideration.Theoretical study shows that the bubble scattering coefficient,acoustic attenuation coefficient and equivalent acoustic velocity are influence by void fraction,bubble size and the driving frequency.Numerical simulation results show that the sound pressure on the riser boundary near transmitter increases and the sound pressure far from transmitter decreases greatly because of the existence of drill pipe,the different eccentric angles of drill pipe cause a big change to the re-distribution of the sound field.By introducing effective sensitivity to optimize the emission frequency,0.2MHz is selected as the best frequency under the research environment.Under the condition of low void fraction,the coda wave velocity linearly decreases approximately with the increase of the void fraction,and the suitable void fraction should not exceed 0.4%.Through the study we found that,with the void fraction increasing,the acoustic attenuation coefficient increases,the dominant frequency decreases and the arrival time of first wave lags behind in gas-liquid two-phase flow.Based on the multi-factor regression analysis method,the quantitative relationship of void fraction is obtained,which can be used to predicted void fraction according to the characteristic parameters of acoustic signals.