Seismic Prediction Method And Application For Tight Sandstone Reservoirs

With an increasing number of oil and gas exploration and development activities and the gradual consumption of conventional resources,unconventional oil and gas reservoirs have received more and more attention.Therefore,it is necessary to study prediction methods for unconventional reservoirs.This dissertation focuses on tight sandstone reservoirs and carries out studies on rock physics modeling and reservoir prediction method.Tight sandstone reservoirs have some characteristics that are different from conventional sand and shale reservoirs,such as low porosity,low permeability,and complex pore structure.They bring difficulities for elastic parameters inversion,seismic rock physics modeling,petrophysical property prediction and fluid identification.With the help of reservoir characteristics analysis in tight sandstones,the theory of rock physics modeling,and the application of machine learning algorithm,this dissertation proposes multiple methods for the predictions of petrophysical properties and dispersion attributes,and provides the prediction process and technical route for tight sandstone reservoirs.By considering the influence of pore connectivity and pore structure,a partially connected pore model is built.It uses the contribution of connected pores to describe the permeability of the pores.It is assumed that the unconnected pores are characterized by the effective medium model and the connected pores are characterized by the Gassmann model.In addition,the soft pore content is used to describe the structural characteristics of the pores,assuming that the aspect ratio of hard pores is 1,and the aspect ratio of soft pores is 0.01.Through the application of the new model,it is pointed out that the Russell fluid term is strictly applicable to tight sandstone reservoirs only when the pores are fully connected or the pores are all hard pores.Other conditions are approximately applicable,and with the increasing of soft pores or the decresing of connected pores,the fluid term shows less applicable.If the pore fluid flow is considered,an attenuation rock physics model needs to be constructed.By introducing the influence of non-uniform fluid distribution,the Chapman model is extended,then a new rock physics model for rocks with cracks considering both squirt flow and patch saturation is given.This model is in good agreement with the existed models,which verifies the effectiveness of the new model.This model is helpful to study the wave propagation of fluid-bearing rocks,and can more comprehensively analyze the influence of reservoir petrophysical properties on elastic parameters.The analysis of reservoir characteristics shows that the velocities of tight sandstones filled with oil or gas are close to that of overlying shale or pure sandstone,and are prone to weak reflections,and these weak reflection events will suffer transmission loss and interlayer multiples.Therefore,it is considered that the reflectivity method is used to obtain the P-and S-wave velocities and density,which is more suitable for the seismic inversion of tight sandstone reservoirs than the Zoeppritz equation.With the use of machine learning algorithm,prediction methods have been conducted on petrophysical properties such as porosity,pore scale and permeability of tight sandstone reservoirs,including: an improved method for reservoir lithology prediction based on HMM and RF;the prediction of porosity,pore scale and permeability based on Bayesian discriminant.Besides,based on the effective medium theory,some fluid identification methods are proposed: a joint posterior probabilistic pore fluid identification method based on statistics and deterministic rock physics;the application of a new effective fluid modulus derived from EMM model.These studies give specific technical routes and ideas for tight sandstone reservoir prediction and have achieved good logging and seismic data application effects.Together,they have built a systematic reservoir prediction framework.Due to the existence of cracks in the tight sandstone reservoirs,the Chapman model suitable for fractured reservoirs is analyzed,and the dispersion and attenuation characteristics of different elastic parameters are given.It is found that the dispersion and attenuation of the Russell fluid term is the strongest,followed by the Laméparameter.The frequency-dependent AVO inversion also verifies this conclusion.In addition,the parameter related to the dry rock modulus in the Russell approximation formula is analyzed,and a reasonable range of this parameter is given.This dissertation provides extensive research for tight sandstone reservoirs.According to whether to consider pore fluid flow,two sets of rock physics model are proposed,and corresponding reservoir prediction methods are given.The study in this dissertation has important guiding significance for the exploration and development of unconventional reservoirs.