Research On Fracture Prediction Using Seismic Methods

Fracture anisotropy exists widely in underground media.Fractures can be used as oil and gas reservoir space and basic passageway of oil and gas,controlling the formation and distribution of reservoirs.Accurate understanding of fractures plays an important role in reservoir prediction.At present,there are many methods for fracture prediction using seismic data.Among them,the widely used methods are mainly divided into three types: post-stack P-wave attribute extraction,prestack P-wave azimuth anisotropy inversion and shear-wave split parameter-scan.In this paper,based on the elastic wave theory of anisotropic media,the shear wave time difference,polarization characteristic and velocity characteristic of P wave in HTI medium are explained.By simulating the P and S wave field in the azimuth HTI medium,the conditions and variation characteristics of the shear wave splitting phenomenon and the azimuth anisotropy of the P-wave velocity with the fracture parameters are explained from numerical simulation.Based on the orthogonality assumption of split shear wave,a new method for predicting fracture parameters using hodogram analysis is proposed.Through vector iteration calculation of R component and T component,the trajectory rectangle indicating fracture orientation is found,and the prediction of fracture parameters is realized.The model data tests with double scanning method and hodogram analysis method show that the hodogram analysis method has smaller error when it is close parallel to fracture orientation or vertical fracture orientation.The results of three-dimensional and three-component synthetic records show that when P-wave reaches wide azimuth coverage,the fracture orientation can be predicted by ellipse fitting.However,the ellipse fitting based on amplitude azimuth variation of P-wave may indicate the direction of fracture or the vertical direction,which brings uncertainty to the analysis of amplitude anisotropy of P-wave.Several fracture prediction methods with P and S wave have been successfully applied to 3D3 C data in Xinchang tight sandstone gas field in Sichuan Province and walkaround VSP data in Toputai area in Tahe Oilfield.Through the comparison of the application results,the identification ability and applicability of P and S wave for fracture detection are summarized.The post-stack attribute extraction method of P-wave can indicate the trend of fracture development to some extent.Three double scan methods based on shear-wave splitting analysis,hodogram analysis and P wave azimuth anisotropy analysis can be used to calculate and obtain consistent results under the condition that the data requirements are satisfied.However,the azimuth ellipse fitting of P wave amplitude is uncertain,and the prediction method of shear-wave splitting fracture is not limited by the number of azimuth coverage,which is the advantage of shear-wave method over P wave method.The hodogram analysis method in shear wave method has higher accuracy and stability so that it can obtain more accurate results in application.In actual application,the hodogram analysis should be the main method.P-wave and S-wave methods should be combined to verify each other in order to obtain more comprehensive results.