High-amplitude events in seismic reflection profiles of oil and gas reservoirs are the exploration“bright spot”target,as have also been observed in recent borehole acoustic reflection imaging applications.This paper...High-amplitude events in seismic reflection profiles of oil and gas reservoirs are the exploration“bright spot”target,as have also been observed in recent borehole acoustic reflection imaging applications.This paper uses the cracked porousmedium elastic wave theory to analyze the cause of this phenomenon.We model the reservoir as layered cracked porous media and analyze the effect of wave-induced squirt flow in a multi-crack system on acoustic reflection.The propagator matrix method can generally be used to calculate wave reflection and transmission through the layered reservoir model.In particular,for a single fracture layer,assuming that the layer thickness is small compared with the wavelength,the linear slip interface theory can be used to solve the problem.The insight gained from this analysis is that,in addition to acoustic impedance,crack abundance(i.e.,crack density)and fluid properties(water,oil,or gas)significantly affect the wave reflection amplitude.Higher crack density and lighter fluid(i.e.,gas)will substantially increase the reflection intensity,generating“bright spots”in reflection surveys.The theoretical result has been verified by a deep well acoustic reflection imaging case study.The results can be used to provide a“bright spot”exploration technology for fractured oil and gas reservoirs.展开更多
Sedimentary strata typically exhibit the characteristics of transverse isotropy(VTI)with a vertical axis of symmetry.However,fractures in sedimentary strata tend to produce anisotropic closure due to horizontal in sit...Sedimentary strata typically exhibit the characteristics of transverse isotropy(VTI)with a vertical axis of symmetry.However,fractures in sedimentary strata tend to produce anisotropic closure due to horizontal in situ stress,resulting in pronounced orthorhombic anisotropy in VTI media under such stress conditions and influencing the propagation behavior of seismic waves.Previous studies have primarily focused on the elastic wave velocity anisotropy induced by applied stress in isotropic background media,neglecting the impact of VTI background media on the anisotropy induced by horizontal in situ stress and the response characteristics of seismic wave propagation.To address these gaps,we first establish the effective elastic stiffness tensor of VTI media under horizontal in situ stress using nonlinear acoustoelastic theory.Then,we derive the accurate and linearized approximate equations for P-wave seismic reflectivity of VTI media under horizontal in situ stress,based on wave equations and scattering theory,respectively.Finally,we compare and analyze the characteristics of orthorhombic anisotropic seismic response induced by horizontal in situ stress at various types of elastic reflection interfaces.Our results demonstrate that the linearized approximation of the seismic reflection response characteristics closely aligns with the accurate equations under conditions of small stress below 10 MPa,effectively capturing the azimuth-dependent orthorhombic anisotropy induced by horizontal in situ stress in VTI media.The results of this study also provide a novel theoretical approach and valuable insights into the seismic prediction of in situ stress.展开更多
基金supported by the National Natural Science Foundation of China(Grant Nos.U21B2064,42174145&41821002)。
文摘High-amplitude events in seismic reflection profiles of oil and gas reservoirs are the exploration“bright spot”target,as have also been observed in recent borehole acoustic reflection imaging applications.This paper uses the cracked porousmedium elastic wave theory to analyze the cause of this phenomenon.We model the reservoir as layered cracked porous media and analyze the effect of wave-induced squirt flow in a multi-crack system on acoustic reflection.The propagator matrix method can generally be used to calculate wave reflection and transmission through the layered reservoir model.In particular,for a single fracture layer,assuming that the layer thickness is small compared with the wavelength,the linear slip interface theory can be used to solve the problem.The insight gained from this analysis is that,in addition to acoustic impedance,crack abundance(i.e.,crack density)and fluid properties(water,oil,or gas)significantly affect the wave reflection amplitude.Higher crack density and lighter fluid(i.e.,gas)will substantially increase the reflection intensity,generating“bright spots”in reflection surveys.The theoretical result has been verified by a deep well acoustic reflection imaging case study.The results can be used to provide a“bright spot”exploration technology for fractured oil and gas reservoirs.
基金supported by the National Natural Science Foundation of China(Grant Nos.42130810,42004107)the Science and Technology Innovation Program of Hunan Province(Grant No.2022RC1238)。
文摘Sedimentary strata typically exhibit the characteristics of transverse isotropy(VTI)with a vertical axis of symmetry.However,fractures in sedimentary strata tend to produce anisotropic closure due to horizontal in situ stress,resulting in pronounced orthorhombic anisotropy in VTI media under such stress conditions and influencing the propagation behavior of seismic waves.Previous studies have primarily focused on the elastic wave velocity anisotropy induced by applied stress in isotropic background media,neglecting the impact of VTI background media on the anisotropy induced by horizontal in situ stress and the response characteristics of seismic wave propagation.To address these gaps,we first establish the effective elastic stiffness tensor of VTI media under horizontal in situ stress using nonlinear acoustoelastic theory.Then,we derive the accurate and linearized approximate equations for P-wave seismic reflectivity of VTI media under horizontal in situ stress,based on wave equations and scattering theory,respectively.Finally,we compare and analyze the characteristics of orthorhombic anisotropic seismic response induced by horizontal in situ stress at various types of elastic reflection interfaces.Our results demonstrate that the linearized approximation of the seismic reflection response characteristics closely aligns with the accurate equations under conditions of small stress below 10 MPa,effectively capturing the azimuth-dependent orthorhombic anisotropy induced by horizontal in situ stress in VTI media.The results of this study also provide a novel theoretical approach and valuable insights into the seismic prediction of in situ stress.
