Frequency Spectrum Response Characteristics Of Complex Electrical Property Of Coal Before And After Hydraulic Fracturing

There is a close relationship between coal seam fractures and gas migration,so evaluation method of coal seam cracks and fractures is an important research content of coalbed gas geology.In recent years,as a typical non-invasive method which can be used in field and laboratory,complex resistivity measurement has made great progress in material damage monitoring and evaluation.The application of complex resistivity method to evaluate the fracture development degree and hydraulic fracturing effect of coal seam after hydraulic fracturing is of great significance for coal mine gas control,coal mine safe mining and improving coal mine gas drainage efficiency.In this paper,the hydraulic fracturing experiment platform and complex resistivity test platform are built to carry out the experiment.In this paper,dielectric polarization theory and induced polarization theory are used to analyze the dispersion mechanism of complex resistivity of fractured coal.It lays a theoretical foundation for coal seam spectrum induced polarization method in evaluating coal seam fracture and fracturing effect.The main research results are as follows:（1）The hydraulic fracturing experiment platform and complex resistivity test platform was built.The hydraulic fracturing equipment consists of gripper,confining pressure loading pump and water injection pump.IM3533-01LCR measuring instrument is used to measure the complex resistivity,and the measuring parameters are the amplitude、phase、real part and imaginary part of the complex resistivity.By measuring the complex resistivity of thick copper plate,thin copper plate and conductive paper,it is found that the thick plate has obvious dispersion phenomenon and the polarization of the plate itself is serious.After using thin plate and conductive paper as conductive materials to measure the complex resistivity of anthracite and fat coal,it is found that the imaginary part of anthracite measured by thin plate has a rising stage at low frequency,while the imaginary part of fat coal has no extreme point,and the fitting parameters of Cole Cole model have large errors;The measurement results of conductive copper paper are better,and the measurement results are more consistent with the induced polarization theory.This is due to the poor fitting degree between the plate and the end face of coal sample,which leads to large errors.By measuring the complex resistivity of anthracite with conductive paper of 1cm,2cm,3cm,4cm and 5cm in diameter,it is found that only the amplitude of the real part and imaginary part curves changes greatly,and the variation law and the extreme frequency point are almost unchanged.After fitting the real and imaginary parts with Cole Cole model,it is found that the model parameterρ₀is decreasing gradually.This is because the resistivity is inversely proportional to the area and the effective conductive area of coal sample increases,so the parameterρ0 decreases with the increase of area,and the variation range of the model parameters m,c andτis very small,which can be ignored in the measurement.（2）The difference of electrical characteristics of coal body before and after fracturing is found.The anthracite is made into coal pillar,and the coal pillar is drilled.The epoxy resin is used as coagulant to insert the fracturing pipe full of coagulant into the hole wall to seal it tightly;After the gel solidifies,the electrical parameters are measured,and then the hydraulic fracturing experiment is carried out,and the data are saved.Through the measurement of coal samples,it is found that the complex resistivity phase of fractured coal samples with water content presents double peaks with the change of frequency,and the imaginary part presents ladder shape,which is related to the change of internal pore structure of coal samples after hydraulic fracturing;No matter before or after fracturing,the capacitance of coal decreases with the increase of frequency.Under the loading condition,with the change of pump injection pressure,the structure of coal sample also changes.After fracturing,No.1 coal sample has obvious cracks,and No.2,No.3 and No.4 coal samples are fractured.（3）The complex electrical dispersion response characteristics of loaded coal samples before and after hydraulic fracturing are revealed.The dispersion mechanism of complex resistivity of coal before and after hydraulic fracturing is analyzed.The phase of fractured coal presents a bimodal state in water bearing state,and its physical mechanism is space charge accumulation effect.The complex resistivity amplitude of No.2 dry coal sample after fracturing is greater than that before fracturing,which is due to the cutting of conductive channel caused by coal sample fracture.The reason for the existence of double peaks in the phase imaginary part data of No.3 and No.4 coal samples after fracturing in dry state is the superposition effect.The difference of capacitance data between No.2,No.3,No.4 coal sample and No.1 coal sample before and after fracturing is due to the increase of capacitor plate spacing.