Joint Experimental Study On Acoustic Velocity-Resistivity-Nuclear Magnetic Resonance Characteristics Of Low Porosity And Permeability Sandstones

To understand the influence of pore structure of low porosity and permeability on the properties of acoustic velocity-resistivity-nuclear magnetic resonance deeply,thereby make pore structure characterization,fluid identification and other evaluation better,the typical samples in Gaobei ramp were selected to carry out acoustic velocity-resistivity-nuclear magnetic resonance joint experiment.Based on principle of statistics and topology,microscopic pore structure parameters of mercury injection curves,T₂ spectrum and casting thin sections and their correlation were extracted and researched,then pore structure classification method based on the characteristics of mercury injection fractal curve and PCA-FCM were studied.Based on differential equivalent medium theory,bulb tube model,double pore components model and the properties of distribution of pore throat by mercury injection curves with experimental analysis,the properties of acoustic velocity,resistivity and T₂ spectrum in different pore structure sandstones were researched.As a link to pore structure,the theoretical relation of acoustic velocity-T_2gm,acoustic velocity-resistivity and T₂–I was discussed.Based on the above relation,the gas-bearing identification chart of joint acoustic velocity-resistivity-nuclear magnetic resonance.The research results show that there are four kinds of pore structure in research area,and the microscopic parameters could be used in characterization of pore structure.The result of pore structure classification method based on PCA-FCM is matched with the method of mercury injection fractal curve.Pore structure influence acoustic velocity through pore aspect ratio,greater pore aspect ratio leads to bigger acoustic velocity under condition of same porosity.Formation factor increases with the rise of tortuosity and pore-throat ratio,the fitting results of double pore components model are conformed to real situation.The change of pore structure influent the change rule of m and n.Nuclear magnetic resonance T₂ spectrum is an indicator of pore distribution,the sensitivity of T_2gm and fractal dimension(D_h1)decrease with pore structure changing worse.There is a power function relationship between acoustic velocity-T_2gm,acoustic velocity-resistivity,T2-I and the fitting index of T₂-I(n_T2-I)-thickness of water film,a linear relationship between the fitting index of T₂-I and n.The joint acoustic velocity-resistivity-nuclear magnetic resonance gas-bearing identification cross plot proves a favorable identification result,the method provides a useful reference to gas-bearing identification in low porosity and permeability reservoirs.