The Method For Acoustic Impedance Inversion Of Cement And Mud Based On Lamb Wave Instruments

In oil and gas exploration,in order to prevent oil,gas and water layers from interfering with each other and affecting oil and gas production,it is necessary to block the layers.By injecting cement into the annulus between the casing and the formation to achieve effective separation between the layers,it is also possible to fix the casing and the protective casing from corrosion by the formation fluid.With the development of science and technology and the new demands of oil and gas exploration,the use of low impedance(Slow cement)cement to cementing well in the field is becoming more and more extensive,which puts forward a higher requirement for cementing quality evaluation.At present,some traditional acoustic impedance logging methods(such as CBL,VDL and SBT logging,etc.)are unable to distinguish the acoustic impedance of light cement after casing from that of liquid,so it is difficult to accurately judge the actual cementation of cement and effectively evaluate the cementing quality.In order to solve this problem,Schlumberger introduced the Isolation Scanner tool to combine the acoustic impedance measurement with the A0 flexural Lamb wave attenuation measurement,which greatly improved the cementing quality evaluation of low-impedance cement.Based on the propagation characteristics of the vertical incident resonant wave and the oblique incident Lamb wave,combined with the theoretical calculation and experimental measurement results,the response characteristics of the resonant mode wave and the flexural Lamb wave when the acoustic impedance of the medium inside and outside the casing is changed are studied in this paper.The results show that the amplitude of the resonant wave increases with the increase of the acoustic impedance of the fluid in the well and decreases with the increase of the acoustic impedance of the medium outside the casing.The attenuation rate of the flexural Lamb wave also increases with the increase of the acoustic impedance of the fluid in the well,while the influence of the medium outside the casing on the attenuation rate has a critical value of acoustic impedance: Within the critical value,the attenuation rate increases with the increase of acoustic impedance;outside the critical value,the attenuation rate decreases rapidly and remains low.n this paper,the relationship between the resonance efficiency(amplitude ratio of the resonant wave and the reflected wave in the time window),the attenuation rate of the flexural Lamb wave and the acoustic impedance of the medium on both sides of the casing was determined.After the calibration of a specific instrument,a method for jointly solving the acoustic impedance of the medium on both sides of the casing was established,and a processing software that could be applied in the field was formed.Through the processing of the data measured by the instrument in the experimental well,the acoustic impedance values on both sides of the casing are close to the actual impedance values,which verifies the reliability and accuracy of the method.The method of resonance efficiency and Lamb wave attenuation combined to invert the acoustic impedance of the medium outside the casing can solve the calibration problem of the non-free casing section and improve the applicability of the instrument and the accuracy of interpretation.