Research On Focal Mechanism And In-situ Stress Inversion Of Fracturing Microearthquakes Based On The Moment Tensor Model

Unconventional oil and gas resources will play an important role in achieving the carbon peaking and carbon neutrality goals,which is often extracted through hydraulic fracturing.Hydraulic fracturing alters reservoir stress and fractures rock,creating microseismic events.By studying the focal mechanism of microearthquakes,we can reveal the mechanism of microearthquakes and stress variation of underground reservoir,and provide information support for fracture structure.During the fracturing process,microseismic signals from the ground need to be collected continuously.For a large number of continuous signals,accurate and efficient P-wave arrival picking technology becomes very necessary in real time fracturing monitoring.Based on this application,this paper firstly proposes an automatic picking method based on the combination of short and short time window of damping energy and AIC rule,and presents a solution for automatic identification of microseismic events in the process of hydraulic fracturing.It is proved that the proposed method is more accurate than the traditional AIC method,which provides a basis for the subsequent source mechanism inversion.Secondly,in terms of focal mechanism inversion,the moment tensor model is used to characterize the fracture microseismic source.The information of initial polarity of P-wave and amplitude ratio are used as the constraint condition for focal mechanism inversion.Meanwhile,the mean square error is used as the objective function to measure the error between theoretical amplitude and observed amplitude.In order to solve the problem of polarity misjudgment when the initial dynamic polarity of P wave is automatically picked up,the percentage of polarity mismatch is tested,and it is proved that the P wave picking algorithm proposed in this paper can further improve the accuracy of the solution in the focal mechanism inversion.Then,based on the parameters obtained from the focal mechanism solution,the damping stress inversion method is adopted in this paper.The study area was divided into grids,and the directions of principal compressive stress,intermediate stress,advocate stress and relative stress factors were obtained by inversion respectively.According to the directions and magnitude of the four,the ground stress trend was analyzed.Then,by collecting the microseismic event waveform in the construction process of a hydraulic fracturing well in northwest China,the focal mechanism solution characteristics of the study area are obtained by inversion,and the damping stress tensor inversion is carried out by the focal mechanism solution to study the stress field characteristics of the area,and to explain the formation and development of fracture network.It is found that most of the microseismic events in the process of hydraulic fracturing in this area are caused by newly opened fractures(most of the events have high CLVD component),and a few of them are caused by the slip of the existing faults caused by stress conduction.The stress distribution in this area has some complexity around the wellhead,which is inferred to be related to high pressure fluid injection during hydraulic fracturing.Finally,the paper summarizes the whole paper,and prospects the next research ideas and directions in the study of focal mechanism of microseismic events generated by fracturing and geostress inversion.