Seismic Response And Prediction Of Thin Interbedded Reservoirs With Fractures

With the flourishment of the domestic economy,the consumption of oil and gas resources also increases rapidly,while 60% of the unconventional oil and gas reservoirs are closely related to the fractures.However,there are many studies on a single group of single-scale fractures at the present stage,and there are few studies on the development of a single group of fractures in thin interbeds.Most continental sedimentary rocks have characteristics of thin interbeds in China.Affected by tectonic movement,vertical or nearly vertical high-angle fractures will develop in thin interbeds,which results in typical orthorhombic anisotropy.This paper establishes an orthorhombic media model induced by a set of vertical fractures in thin interbeds.The stiffness coefficients are constructed by Bakulin background medium parameters,fracture weaknesses,and Tsvankin dimensionless anisotropy parameters.Then,the calculation of phase velocities,polarization vectors,and reflection coefficients is presented based on Christoffel equations.Combining the strict boundary conditions of elastic wave propagation theory,continuity of displacement and stress are required.We also design the model to analyze the characteristics of the phase velocities and reflection coefficients in orthorhombic media.Taking the Ordovician strata in the Shunnan 1 and Shun 4 wells in the Tazhong area of the Tarim Basin as the research object,the zero-offset VSP data of the Shunnan 1 and Shun 4 wells are addressed.The evident shear wave splitting is visible from the obtained shear wave data and the azimuth of fractures can be predictable.According to the principle of VSP data acquisition,the corresponding velocities of fast and slow shear waves could be calculated at the placement of shear wave splitting.And combining logging data and using the scanning method can get approximate fracture weaknesses.Finally,the long axis direction of the azimuth phase velocities fitting effect is compared with the predicted fracture development direction,and the direction of predicted fractures is verified by combining the structural background.Through the above work research,the following achievements and understanding are mainly obtained:1.The modeling method of orthorhombic media is studied,and the exact equations of phase velocities,polarization vectors and reflections coefficient of PP-wave,PS1-wave and PS2-wave are derived.2.Based on the forward modeling analysis of the theoretical model,the seismic response law caused by the fracture weaknesses and background medium parameters is clarified.3.Established a technical process for quantitative prediction of fracture azimuth and fracture weaknesses based on VSP data.The development of fractures in Shunnan 1 and Shun 4wells and the distribution of fractures in Structural Belt of Fault No.1 Central Tarim Basin are predicted.