Borehole Acoustic Reflection Imaging And Application In Horizontal Well

The single well shear wave reflection suryey can image the reflectors of several tens of meters away from borehole,and detect geological structures such as inclined formations,fractures,faults and caves around vertical wells.In recent years,there has been a breakthrough in shale gas exploration and production.Many horizontal wells are drilled through the gas shale to facilitate hydraulic fracturing and production of the reservoir.It is particularly important to apply single well shear wave reflection suryey in horizontal wells.This paper analyses the elastic wave response of the fractures using the linear slip interface theory and establish a geometric model for the double fracture system to simulate the wavefield for the borehole reflection survey.The results were applied to characterize and interpret the fracture system from the field dipole shear wave reflection survey data.Then,using the borehole acoustic technology to identify and describe the layered shale formation and vertical factures beside the borehole.The technology includes shear-wave anisotropy measurement and reflection imaging using the four-component(4C)cross-dipole logging data along the well.Finally,three application examples of borehole acoustic reflection imaging in horizontal wells are given,which provides the necessary theoretical basis and technical support for the single well shear wave reflection suryey in horizontal wells.The major work of this paper consists of three parts as follows.The first part simulates and analyses the wavefield for the borehole reflection survey with multi-fracture system.We analyse the elastic wave response of the fractures using the linear slip interface theory and investigates the effects of fracture material variation on the SH-wave characteristics.Then establish a geometric model for the double fracture system to simulate the wavefield for the borehole reflection survey.A fast and effective method was formulated for the simulation.The results were applied to characterize and interpret the fracture system from the field dipole shear wave reflection survey data.The results of this study provide a useful theoretical foundation for the field data interpretation.the elastic wave response of the fractures using the linear slip interface theory.The second part studies the imaging processing method of borehole reflection survey in horizontal well.Firstly,We analyze the mechanism of acoustic wave propagation along a horizontal wells in shale formation.Then we demonstrate the processing method of imaging the formation interface and vertical fractures using fast and slow shear waves.For the fourcomponent cross-dipole logging data,an inversion technique is used to separate the fast and slow principal waves.After extracting the up-and downgoing reflections from the data,the fk filtering is used to enhance the amplitude of weak reflection signal,and then the reflection waves from fast and slow principal waves are respectively migrated to image the the formation interface and vertical fractures.Finally,the anisotropic diffusion filter and the median filtering method are combined to improve image quality for borehole reflection imaging.The third part presents three case studies using borehole acoustic technology for reflectors characterization and imaging in a horizontal well drilled through shale gas reservoir.The first field example shows that borehle acoustic reflection survey can describe the borehole trajectory according to the formation interface imaging and determine the location of sweet spot by identifying the crossing well vertical fractures.According to the simulation results.According to the above simulation results,the filling properties of vertical fractures can be judged.Finally,Application of the technology to a high-production shale gas reservoir in the Sichuan basin of Southwest China has successfully delineated vertical fracture sets crossing the horizontal well,suggesting that the presence of the fractures is related to the high gas production of the well.