The Research On The Evaluation Of Hydraulic Fracturing Based On Multiple Microseismic Events

Microseismic monitoring,as an important method of monitoring hydraulic fracturing,has been widely used.On the one hand the number and location of microseismic events is one of the important factors for evaluating hydraulic fracturing effect.Now the methods used for the detection and location of microseismic events mostly are for microseismic events with high SNR.The microseismic events with low SNR,as most part of microseismic events is very important to explain the hydraulic fracture.On the other hand,in the process of hydraulic fracturing,the formation of hydraulic fracturing fractures leads to changes in formation properties,such as changes in formation velocity and anisotropy,which can reflect the effect of hydraulic fracturing.Microseismic events have been proved to be clustering in space and time.Based on this feature of microseismic events,this thesis studies the above two aspects.We first introduce an improved matching and locating technique to detect and locate microseismic events(-4<ML<0)associated with hydraulic fracturing treatment.We employ a set of representative master events to act as template waveforms and detect slave events that strongly resemble master events through stacking cross correlograms of both P and S waves between the template waveforms and the continuous records of the monitoring array.Moreover,the residual moveout in the cross correlograms across the array is used to locate slave events relative to the corresponding master event.In addition,P wave polarization constraint is applied to resolve the lateral extent of slave events in the case of unfavorable array configuration.In order to reduce the influence of backazimuth errors of P wave to the location of low S/N(signal to noise ratio)microseismic events generated by hydraulic fracturing,we introduce a method called maximum projection which computes the backazimuth difference between the reference event and the target event by searching the angle to rotate the reference event and finding the maximum projection value of the target event onto the rotary reference event.We have proved in theory that the incoherent noise has no effect on the maximum projection method but can affect the computation precision of the particle motion method easily.And when the data is recorded by several sensors,we can also stack the projection value of different sensors to improve computation precision.At last we applied our improved matching and locating technique to the downhole microseismic data set during one treatment stage along a horizontal well within the Weiyuan shale gas play inside Sichuan Basin in SW China and we try to analysis the spatial and temporal variations of anisotropy during the fracturing process by means of shear wave splitting.We get the following conclusions:(1)We detected about 10 times as many events as traditional methods;(2)The microseismicity is generally characterized by two distinctly different clusters,both of which are highly correlated with the injection activity spatially and temporarily.The distant and well-confined cluster is featured by relatively large-magnitude events,whereas the cluster in the immediate vicinity of the wellbore(cluster B)includes two apparent lineations of seismicity with a NE-SW trending,consistent with the predominant orientation of natural fractures.We speculate that the distant cluster is triggered by reactivation of a preexisting critically stressed fault,whereas the two lineations are induced by shear failures of optimally oriented natural fractures associated with fluid diffusion;(3)We observed a significant increase of the delay time of slow wave with depth,which may be due to the fact that the deeper the depth,the more fracture zones the ray path traverses.