Local stress field inverted for a shale gas play based on focal mechanisms determined from the joint source scanning algorithm

The joint source scanning algorithm(SSA)scans locations and focal mechanisms of microseismic events simultaneously.Compared to the traditional source scanning algorithm,it yields much more events with extra information of focal mechanisms.The availability of more events and focal mechanisms make it possible to invert for a 2D gridded stress field.As a byproduct of hydrofracturing monitoring,the method offers a new way to extract stress field as a substitute to other more expensive technologies.This method is applied to a hydraulic fracturing dataset collected from one shale gas production field in the southeast of the Sichuan basin.A damped stress inversion is conducted to obtain a 2D stress field.five hydraulic-fracturing induced fractures can be determined from the result.The events associated with these fractures generally have relatively low stacked energy and are limited to the depth of horizontal well.One existing fault(possibly associated with the axis of the central Sichuan uplift)is also determined and the events associated with the existing fault generally have higher stacked energy and are more densely populated.The existing fault may also serve as a structural boundary where the rocks to the NW side are easier to be fractured while events on the other side are sparse with low stacked energy.The existing fault also divides the stress field into two regimes:the maximum compressional stress field to the NW and SE of the fault line are dominantly in NW-SE and N-S directions,respectively.

Full moment tensor inversion constrained by doublecouple focal mechanism for induced seismicity

In this study,we propose a new method to determine full moment tensor solution for induced seismicity.This method generalizes the full waveform matching algorithm we have developed to determine the double-couple(DC)focal mechanism based on the neighbourhood algorithm.One major difference between the new method and the former one is that we adopt a new misfit function to constrain the candidate moment tensor solutions with respect to a reference DC solution in addition to other misfit terms characterizing the waveform matching.Through synthetic tests using a real passive seismic survey geometry,the results show the new constraint can help better recover the DC components of inverted moment tensors.We further investigate how errors in the velocity model and source location affect the moment tensor solution.The synthetic test results indicate that the constrained inversion is robust in recovering both the DC and non-DC components.We also test the proposed method on several real induced events in an oil/gas field in Oman using the same observation system as synthetic tests.While it is found that the full moment tensor solutions without using the DC constraints have much larger non-DC components than solutions with the DC constraints,both solutions are able to fit the observed waveforms at similar levels.The synthetic and real test results suggest the proposed DC constrained inversion method can reliably retrieve full moment tensor solutions for the induced seismicity.