Study On Well Logging Evaluation Method Of Fracture Parameters In Coal-bearing Strata

Fractured reservoirs,the main types of deep and unconventional oil-gas reservoirs,are important targets for oil-gas exploration and development.Fractured reservoirs are more common in coal-bearing strata,but the logging evaluation methods for fracture parameters in coal-bearing strata are still relatively limited.Seismic methods can be used to invert anisotropy information of formations and effectively identify fractured reservoirs.However,seismic inversion needs fracture elastic parameters which can be obtained by well logging data as the basis for the establishment of initial model.As so far,well logging evaluations of fracture parameters are mainly limited to the fracture identification,the calculation of width,porosity and intensity of fracture.The fracture elastic parameters are difficult to obtain by well logging data.Based on the detailed analysis of the reservoir features and fracture characteristics of coal-bearing strata,this paper theoretically studies the conventional well logging response characteristics,fracture logging response characteristics,and the logging evaluation methods of conventional parameters for coal-bearing strata.Based on the theory of anisotropic media,the study of logging evaluation methods of fracture elastic parameters was carried out,and the calculation methods of elastic parameters of the vertical fractures and the horizontal fractures by well logging data were proposed.Finally,the application verification of the innovative methods was carried out in coal-bearing strata.Firstly,the logging evaluation methods in coal-bearing strata were studied.This paper analyzed the main rock types,reservoir pore and fracture characteristics of the coal-bearing strata and studied the conventional logging responses and identification methods with the cross-plots of different lithology and gas-bearing reservoirs.Logging evaluation methods were proposed for lithology analysis,coal quality analysis,gas content calculation,and elastic parameters calculation in the coal-bearing strata.Then a logging evaluation and interpretation system for coal bed methane reservoirs was developed based on the CIFLog well logging platform.This system also includes full-wave sonic logging and ultrasonic imaging logging interpretation methods.It has been applied in the coal bed methane logging project and has a good effect.Secondly,the fracture parameters calculation and evaluation methods were studied with conventional logging and image logging data.This paper analyzed the responses of conventional logging and studied the fracture parameters calculation methods using double lateral resistivity logging data.The logging evaluation methods for fracture parameters in the coal formation were mainly discussed.The calculation methods of fracture width,fracture porosity and fracture permeability were proposed based on the analysis of the coal seam fracture model.In addition,this paper studied the data processing methods for ultrasonic imaging logging data,which include image magnetic north rotation,depth correction,bad track interpolation,eccentricity correction,data filtering and image enhancement.The ultrasonic logging responses of lithology and borehole structure were studied and the fracture parameters calculation methods with image logging data were introduced.This paper also analyzed the responses of array sonic logging and studied the wave separation method and reflection coefficient calculation method of stoneley wave.The methods mentioned above provide an effective technique for the conventional evaluation of fracture parameters,which lay the foundation for fracture elastic parameters evaluation.Thirdly,the logging calculation methods of the elastic parameters of vertical fracture were studied.This paper analyzed the three commonly used fracture equivalent medium theories of Hudson,Thomsen,and Schoenberg,and the conversion relationships between them under the assumption of HTI medium.The formula was derived between density,compressive velocity,fast and slow shear velocity and fracture elastic parameters,and the method to analyze the fast and slow shear velocity was studied.The fracture density e of Hudson theory,ZN and ZT of Schoenberg theory,and?^?v?,?^?v?,?^?v?,?^?v?of Thomsen theory were calculated with logging data under the assumption that the shear wave velocity obtained from dipole array sonic logging is equivalent to the shear wave velocity in HTI media.The fracture elastic parameters obtained by this method are consistent with the anisotropy development and electric imaging logging data.Finally,the waveform inversion method for horizontal fracture elastic parameters was studied.Based on the waveform inversion theory,the calculation methods of horizontal fracture elastic parameters were proposed.This paper studied finite difference method of elastic wave numerical simulation,linear sliding interface theory,solid-liquid boundary conditions and CFS-PML absorption boundary,then realized the numerical simulation method of array sonic logging in formations with horizontal isolated fracture.The basic theory and flow of genetic algorithms were discussed,the genetic coding part was improved to adapt to the fracture parameters of linear sliding theory,a fitness evaluation function for evaluating the observed wave field and the simulated wave field was established,and finally a parallel optimization and inversion trial calculation was performed.The inversion result showed that the genetic algorithm can obtain a better result when a small number of individuals and populations are set.Based on these studies mentioned above,the author finally established a complete methodology of fracture parameters evaluation by well logging data and applied the methods to the evaluation and interpretation of reservoirs in a study area including coal-bearing strata.The lithology,porosity,saturation,permeability,coal seam industrial components,coal seam Gas content,fracture distribution,fracture width,fracture porosity,Hudson fracture density e,Schoenberg fracture compliance Z_N,Z_T,Thomsen fracture parameters?^?v?,?^?v?,?^?v?in coal-bearing strata were evaluated using geological data,coal sample experimental data,conventional well logging data,array sonic logging data and electrical image logging data.The conventional logging evaluation effects,the correlation between fracture parameters obtained from different methods,and the relationship between the vertical and horizontal fracture elastic parameters were discussed.The application results showed that:?1?The logging evaluation methods in this paper can accurately evaluate the lithology,porosity,saturation,permeability,coal seam industrial composition,and coal bed gas content in coal-bearing strata.?2?The methods proposed in this paper for the evaluation of conventional fracture parameters have a good effect on the evaluation of fracture development and fracture parameters in coal-bearing strata.?3?The conventional fracture parameters calculated by logging data have a good correlation with the elastic parameters calculated in this paper.?4?It is found that horizontal fractures are more developed where vertical fractures are more developed.?5?The fracture intensity calculated by electric image logging data has a good correlation with the Hudson fracture density calculated in this paper.?6?The elastic parameters of vertical and horizontal fractures calculated in this paper are accurate and reasonable,which can provide an accurate initial model for the seismic inversion of fracture properties.