| Evaluation of reservoir fracture method and its application is one of the important research topics in coal field,oil and gas exploration and development.In recent years,great progress has been made in the evaluation of rock fracture and permeability by Complex Resistivity.Applying Complex Resistivity to evaluate the development of coal seam fracture and to investigate the effect of hydraulic fracturing is of great significance to mine gas control and coal safety production.In this paper,firstly,a laboratory test method for complex resistance of coal with different water saturation(Sw)was established,the conductive mechanism of complex resistance of coal was studied,then the problems in different complex resistance models analysed.Finally,the dielectric theory is used to establish the orthogonal fracture resistance-capacitance model and verified the reliability of the model.This study lays a solid foundation for the application and development of Complex Resistivity in coalfield.The main research results are as follows:(1)The test method of complex resistance of coal with different Sw in laboratory Establish was established.Four-terminal impedance measuring instrument was used in the experiment,and the measuring frequency was 0.1-100 kHz could greatly reduce electromagnetic interference;using 2 mm thick copper plate can reduce the influence of the plate itself on the complex resistance;measuring the smoothness of coal sample surface should not be more than 0.02%,and coating conductive solution on the interface between coal sample and plate can greatly reduce the surface effect;the exposed part of coal sample should be wrapped with fresh-keeping film in order to reduce the influence of moisture evaporation on the measurement results;measuring continuously three times under sweep mode to ensure data stability and reliability.(2)The orthogonal fracture resistance-capacitance model of complex resistance of coal seam was established.By using the Cole-Cole model and the double Cole-Cole model which are widely used in rock to fit the measured data of coal samples,it was found that the calculation of the two complex resistance models were complex,and the relationship between model parameters and Sw of coal samples were weak,so the two models were not suitable for the coal field.Using the capillary model and the dielectric theory,the water-bearing connected fracture channel in coal is equivalent to resistance,and the disconnected fracture channel is equivalent to capacitance.A complex resistance orthogonal fracture resistance model of coal sample was established.Because coal itself has conductivity,the influence of R_C and C_C on coal are added into the model to modify the model.The model has the advantages of clear parameter meaning and simple calculation.(3)The orthogonal fracture resistance-capacitance model can predict the physical parameters of coal samples.The reliability of the orthogonal fracture resistance-capacitance model was verified by measuring different capacitance-resistance parallel experiments and complex resistance experiments of coal samples.Through the orthogonal fracture resistance-capacitance model,the equivalent resistance and equivalent capacitance were converted into physical parameters of connected and non-connected fracture channels,and the porosity was calculated,which matched well with the porosity measured by density method,gravimetric method and NMR method.The dispersion curves of coal samples with different Sw were simulated by the model,which are smaller than the measured results at low Sw,but closer to the measured values at high Sw.The reason is that when the Sw is low,the influence of the interface electric double layer structure is not taken into account. |
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