Rock-physics Modeling Of Dispersion Characteristics And Frequency-dependent AVO Inversion In Shale Gas Reservoirs

As one of unconventional resources,shale gas gains more and more attention in the research on oil and gas exploration in China.Comparing with conventional reservoirs,shale gas reservoirs has more complex pore space,enriched storage and rock compositions,and stronger heterogeneity,which makes the characterization and identification of shale gas reservoirs more difficult.The study of shale gas reservoirs in China is still in its infancy.At present,the research mainly focuses on reservoir accumulation mode,reservoir geology and resource potential evaluation and so on.However,there are few work about the rock physics modeling and sweet spot prediction of shale gas reservoirs.The gas identification of shale reservoirs is an important step in sweet spot prediction.When pore space in shale is saturated by fluid,velocity dispersion and attenuation can occur in seismic frequency band due to the wave-induced fluid flow mechanism.Also,the velocity dispersion and attenuation is very sensitive to fluid types saturated in reservoirs.The dispersion charateristics of gas-saturated reservoirs is more much higher than those of water-saturated reservoirs.In this paper,we consider the pore space and fracture system in shale reservoirs,we carry out the rock physics modeling based on Chapman’s model and analyze the sensitivity of dispersion attributes to fluid types in shale reservoirs.Considering horizontal aligned fractures in shale reservoirs,we model and analyze P-wave velocity dispersion and attenuation for different fluid types in shale reservoirs.Then,considering vertical fractures in shale reservoirs,we investigate the sensitivity of dispersion of P-wave velocity,S-wave velocity and anisotropy parameters to fluid types.Through rock physics modeling,we provide the theoretical support for applying the dispersion charateristics to the gas identification of shale reservoirs.Frequency-dependent AVO inversion technique quantitatively characterizes the velocity dispersion of P-wave and construct fluid identification factor to detect fluid in reservoirs.Presently the research on frequency-dependent AVO inversion mainly focuses on sand-mudstone reservoirs,tight sandstone reservoirs and carbonate reservoirs.Few researches are on shale reservoirs.In this paper,we use anisotropic propagator matrix theory to conduct seismic modeling,calibration between seismic and well data and analysis of seismic responses of shale gas reservoirs.Based on Chapman’s model,we build a theoretical model to test the frequency-dependent AVO inversion method.Calculated results indicate that the dominant frequency for fluid detection in shale reservoirs is not always the dominant frequency of wavelet,because of the tunning effect and interference due to layer structure.We apply the frequency-dependent AVO inversion method to Longmaxi shale gas reservoirs in Sichuan basin and the inverted dispersion attributes can be regarded as gas identification factor in shale reservoirs.When vertical fractures exist in shale gas reservoirs,in this paper,we investigate the effect of fluid types on the dispersion of P-wave velocities and anisotropy parameters.And we discuss the feasibility of applying the dispersion attribute to detect gas-saturated fractures in shale gas reservoirs.We give the frequency-dependent AVAZ inversion theory and the workflow of the inversion and test the method using modeling results.Lastly,we apply the frequency-dependent AVAZ technology to Longmaxi shale gas reservoirs and identify the gas-bearing fractures.We compare the inverted results and well data to vertify the validity of our method.In our paper,the frequency-dependent AVO inversion and frequency-dependent AVAZ method are both proposed considering the single-interface reflection model.When there are thin layer or thin interbedded layer structures in reservoirs,the dispersion attributes are affected by the layering effect.Thus,we propose the gas identification method based on anisotropic propagator matrix theory.Taking anisotropic propagator matrix theory as forward engine,considering the thin-layer structure in shale gas reservoirs,we use the simulated annealing particle swarm optimization algorithm to invert the frequency-dependent P-wave velocity and reservoir thickness.Also,we validate our method by modeling test and field data application.As an extension of the theory of effective dispersive medium,we give the equations for effective frequency-dependent velocity and anisotropy parameters.Based on Chapman model,we derive the equations for frequency-dependent velocity and anisotropy parameters due to fractures at low frequencies.Then,we combine the fracture-induced and layer-induced anisotropy at low frequencies and obtain the frequency-dependent parameters of fractured and layered media.We also compare the contributions of the two effects on the global dispersion of the media.As an extension of theory of AVO forward modeling,we derive the PP and PS frequency-dependent AVO equations in fractured media.Carrying out the Taylor expansion,we can obtain the low-frequency velocity and anisotropy parameters.Introduce this intrinsic dispersion to conventional AVO equations,we can obtain the equations for frequency-dependent AVO attributes and AVO equations.We analyze the effects of the fracture parameters on frequency-dependence of AVO attributes,reflection coefficients and critical angle.