Analysis of the characteristics of the first-arrival wave signal of downhole microseismic under the wave impedance difference

The difference in formation wave impedance is one of the important factors affecting the quality of wellbore microseismic monitoring data. Based on Snell's law and the Zoeppritz equation, by studying the influence of wave impedance difference and incident angle on the characteristics of the first arrival wave signals of microseisms, the relationship between the energy of the first arrival waves in wellbore microseismic data, the magnitude of the formation wave impedance difference, and the incident critical angle was revealed and verified through forward modeling. It was concluded that formations with large wave impedance differences have a significant impact on the energy and propagation path of the first arrival waves. Taking Well XX in a coalbed methane field as an example, at Perforation Section 1, some geophones failed to receive the first arrival signals, which affected the three-component rotation positioning of the geophones and the monitoring interpretation. By obtaining the formation velocity information of the target interval from acoustic logging data and combining it with the changes in formation wave impedance, the reasons for the absence of the first arrival waves in the microseismic perforation records of some geophones were analyzed. The deployment depth of the geophones was adjusted in a timely manner to solve the problem that some geophones could not detect the first arrival waves when the wave impedance difference was large and the incident angle was greater than the critical angle. After the depth of the geophones was adjusted upward by 30 meters, good first arrival wave signals were obtained at Perforation Section 2. The positioning results of these signals were used for wellbore microseismic fracturing fracture monitoring. The interpretation results, when comprehensively analyzed with conventional logging and dipole acoustic logging, showed that the microseismic event points were basically consistent in height with the reservoir characteristics and the fracture height indicated by dipole acoustic logging.