Complex Resistivity Of The Borehole-surface Differential Electromagnetic Method With Applications To Reservoir Oil Saturation Evaluation

The evaluation of reservoir saturation is of great significance to petroleum exploration and development.At present,the work is mainly completed based on the resistivity information through the electrical well logging tools.For complex and heterogeneous reservoirs,its application is restricted by the location of the borehole,and the accuracy is not yet satisfactory.Due to the influence of dispersion characteristics of electrical parameters,differences are presented in reservoir resistivity data obtained under different measurement frequencies.However,the reservoir evaluation models,such as Archie's law,Waxman-Smits model,and "dual-water" model,are generally used to calculate the oil saturation through the real part of complex resistivity,without considering the dispersion effect of the electrical parameters.In addition,the complex resistivity equivalent model has not been considered in the inversion of field electromagnetic data due to the multiple solutions of the parameter inversion.This thesis first carried out the three-dimensional forward modeling for the borehole-surface differential electromagnetic(BSDEM)method,and based on the experimental analysis of rock complex resistivity,further studies the characteristics and parameter inversion of complex resistivity models in different frequency bands.A new oil saturation evaluation model based on the dispersion rate(polarizability)is presented,and proposed the constrained inversion method of complex resistivity for the BSDEM.Finally,a new evaluation method of complex resistivity based on the BSDEM is formed,which provides a new idea for petroleum exploration and development.Through the three-dimensional forward modeling software of the BSDEM,the electromagnetic response of the abnormal body under different frequencies was simulated,as well as the changing laws of the abnormal body under the conditions of different reservoir scales,polarizabilites,and time constants were studied.Then the necessity of considering the resistivity dispersion effect in electromagnetic exploration and reservoir oil saturation evaluation was analyzed.Based on the previous analysis of the conductive mechanism of rocks,the micro-conductive mechanism was summarized by the analysis of the phase spectrum characteristics of 62 kinds of rocks,including 12 kinds of non-conductive minerals and6 kinds of conductive minerals.It is concluded that the conductive mechanism in the rocks at low frequency is dominated by ion diffusion polarization,and it switches to capacitive polarization at medium frequency,then to inductive polarization at high frequency.The peak and trough of the phase spectrum are the turning point of the polarization mechanism.The contents of conductive minerals and clay can affect the formation position of the peak and trough of the phase spectrum,which further explains why different types of rocks have different characteristics on the curve.The reason why the resistivity and dispersion increase slowly or sharply with the variation of oil saturation in different stages was explained,and the empirical relationship between oil saturation and dispersion characteristic parameter was established.The applicability of different complex resistivity equivalent models was analyzed,and a constrained genetic simulated annealing hybrid algorithm suitable for the Cole-Cole model and improved low-frequency Dias model was established.The new algorithm can be rapidly converged to the global optimal solution through continuous constraints on the range of the parameters of the model,and the multiple solution problems in complex resistivity parameter inversion are improved simultaneously.Based on the analysis of the dispersion mechanism of rock,a model for evaluating the oil saturation was established on parallel conductance between formation water and induced polarization.The new model incorporates Archie's law and the resistivity generated by the induced polarization.The model parameters can be obtained by the conventional method of the complex resistivity experiment that fitting Archie parameters.Using Archie's law and the improved low-frequency Dias model,the resistivities of rocks under fully saturated water and different oil saturation states were simulated,and the resistivities calculated by the new model were compared to verify the effectiveness of the new model.A typical study area was selected to deploy the BSDEM method,and the difference anomalies of amplitude and phase of the electromagnetic field were quantitatively obtained.The resistivity and polarizability information of the target reservoir was obtained by the constrained inversion that combines seismic data with well log information.The quantitative relationship between dispersion rate and oil saturation was calculated by petrophysical experiment analysis,and the new oil saturation evaluation model was used to evaluate the reservoir oil saturation.The new method overcomes the shortcomings in the radial sounding ability of the traditional logging method,which intuitively shows the variation of reservoir oil contents in the survey area,and provides a new method for petroleum exploration and development.