Microseismic Study For Source Location And Structure Imaging

Microseismic monitoring plays an increasingly important role in mining,reservoir fracturing,flooding as well as recovery.For example,microseismic monitoring has become the key technique to image hydraulic fractures.The simulated reservoir volume can be estimated based on the fracture network or change in rock property caused by hydraulic fracturing treatment,which helps the site engineer to evaluate hydraulic fracturing and perform next fracturing treatment.Three available approaches are mainly utilized to estimate simulated reservoir volume:first,characterize fracture geometry and size in terms of event location and moment tensors;second,image directly fractures or faults produced in stimulated areas;third,invert for the geomechanic parameters of stimulated areas,such as velocity ratio or Poisson's ratio.Therefore,the main purpose of this thesis is to analyze and improve some popular methods existed in the three available approaches.In the first approach,we aim to reduce or eliminate some uncertainties in current location techniques.Surface monitoring and borehole monitoring are two widely recording approaches.As for surface monitoring,the source scanning algorithm is usually used to locate event for the poor quality of recorded seismic data.The linear stacking condition in the algorithm hardly avoids location uncertainties resulted from polarity reversals and strong noises.We propose two correlation-based stacking conditions in order to reduce location uncertainties:the first condition is the cross-correlation,where we multiply the amplitudes of neighboring receivers before summing up all the products during the stack.In terms of high-density coverage of receivers deployed on the surface,a majority of first motions on neighboring receivers are always the same up or down except for those on focal planes,thereby the multiplying leads to the positive product.Hence,the stack condition does not only avoid the energy dispersion but enhances the spatiotemporal resolution of the energy map.The second condition is the multi-cross-correlation,in which we directly multiply amplitudes over single receiver or receiver group.In each receiver group,the linear stack condition is utilized to sum the amplitudes up.The group number is set up based on the experience and requirement of imaging stability and resolution.The stack condition degrades into the linear stack condition if all receivers are included in one group;the stack condition is unstable while it reaches the highest spatio-temporal resolution if each group only includes one receiver.As to the cross-correlation condition,the product of multiple receiver groups is independent of the polarity reversals so that the energy can be focused on the right position.Besides,the multi-cross-correlation enables both the spatiotemporal resolution and noisy tolerance higher than the first condition.Further,unlike current methods that correct polarities by estimating focal mechanisms before stacking,both proposed stack conditions are able to eliminate the effect of polarity reversals without any extra computation cost.All synthetic and real cases demonstrate the validation of the proposed stack conditions.Compared with the surface monitoring,the borehole monitoring records seismic data with much higher quality.Arrivals of borehole recordings are usually picked to locate event by traveltime inversion or grid search.Because the main effect of event location uncertainty is attributable to the uncertainty of velocities,recent studies mainly focus on the simultaneous inversion of event locations as well as velocity models.When anisotropy is taken into the account,multiple-parameters inversion has strong non-linearity and is prone to be trapped into local minimums,especially under the circumstances of poor or insufficient ray coverages.In order to improve the stability of the inversion,we propose to take advantage of vertical slowness components,which are approximately estimated by neighboring receivers,to constrain anisotropic parameters in the layers which receivers are deployed in.In the assumption of vertical transverse isotropic,we incorporate the vertical slowness component into the traveltime inversion to jointly invert for event locations and five Thomsen's parameters.In the synthetic example,we validate the effectiveness of the vertical slowness constraint under the circumstance of poor ray coverage.We also apply the proposed method into one field case and obtain a more reasonable event distribution than the inversion does only using arrival times.In the second approach to estimate stimulated reservoir volume,my purpose is to improve the imaging resolution by modifying current reverse time migration.Conventional reverse time migration has been proved as an effective means to image hydraulic fractures with the recorded diffracted(coda)waves.We propose a multi-cross-correlation imaging condition to replace conventional cross-correlation condition,and introduce the staining algorithm on the forward modeling.The staining algorithm generates one stained wavefield for imaging target fracture.In the multi-cross-correlation condition,multiple extrapolated wavefields are obtained before correlating forward wavefield and stained wavefield separately,which produces two images:the complete image and the stained image.The multi-cross-correlation condition enhances the spatial resolution of imaging results,staining algorithm improves the spatial resolution of target imaging areas.We use synthetic examples to present the effectiveness of modified reverse time migration and further analyze the effects from location uncertainty,background velocity inaccuracy and levels of noises.In the third approach of estimating stimulated reservoir volume,the 3D traveltime tomography is applied to invert for the changes or anomalies of velocity ratio or Poisson's ratio in fractured areas,for which the fractures and fluids filled changes geomechanical parameters of rocks.The conventional velocity ratio or Poisson's ratio model is determined by simply taking ratio of P-and S-wave velocities or based on the assumption of P-and S-wave raypaths.However,the heterogeneity of the velocity ratio or Poisson's ratio model results into differences of P-and S-wave raypaths.Hence,the final resolution will be affected as assuming that P-and S-wave raypaths are identical.We propose to decompose S-wave velocity into P-wave velocity and velocity ratio or Poisson's ratio.P-wave and S-wave arrival times are then utilized for jointly inverting event locations and velocity ratio or Poisson's ratio in 3D simultaneously,which does not assume the identical of P-and S-wave raypaths.Under this proposed method,the final velocity ratio or Poisson's ratio model is resolved better.In addition,we extend the proposed method into double-difference and cross double-difference tomographic versions in order to better constrain event locations and velocity ratio or Poisson's ratio models.The theories and synthetic cases present that the proposed method is better than simply taking the ratio of P-and S-wave velocities in the resolution of velocity ratio or Poisson's ratio solutions.Besides,cross double-difference tomography is able to obtain the best location and tomographic results.Further,we incorporate back azimuths into the proposed method to constrain the event locations.The results demonstrate the validation of the modified method with back azimuths in the field case.