Study On Measurement Method Of Hydrate Saturation Based On TDK

Simulation experiment is of great significance for the study of physical and chemical properties of hydrate bearing sediments,hydrate accumulation mechanism and exploitation scheme.However,accurate measurement of hydrate saturation in sediment samples is faced with many challenges.Time domain reflectometry(TDR)has the advantage of two parameter measurement.Using TDR technology,the dielectric constant and resistivity of hydrate bearing sediment samples can be obtained simultaneously,and then the hydrate saturation can be obtained.At present,although TDR technology is initially introduced into the field of hydrate measurement,it’s still necessary to study the optimization of structure of TDR probe and the measurement response of hydrate bearing sediments,so as to improve the accuracy and reliability of TDR measurement of hydrate saturation.Based on the travel time method and spatial weight function method,the structure of TDR probe was analyzed and optimized by numerical simulation while numerical simulation is the main research method.The influences of probe structure on the dielectric constant response,electric field distributions and electromagnetic wave signal propagation processes of TDR were studied.The structure optimization method of TDR probe based on numerical simulation is established.The results show that:(1)the spatial weight function method can simulate the porous medium on the pore scale and calculate the apparent dielectric constant of the measured medium;(2)when the probe diameter remains unchanged,increasing the number of outer probes on the circumference with the probe spacing as the radius and keeping the same distance between the outer probes can make the high electric field intensity area evenly distributed,reduce the measurement blind area in the circumference range,and detect the hydrate formation in the probe measurement area more sensitively;(3)for the three-rod TDR probe,when the frequency of electromagnetic wave excitation signal is 1GHz,the maximum spacing between the inner and outer probes should be less than 3cm,and the probe performance is the best when the ratio of probe spacing to probe diameter is 5;(4)when the probe insulation material is 3.58,it can significantly reduce the attenuation degree of voltage amplitude in the process of electromagnetic wave propagation,while the material with dielectric constant of 80 has little effect on the attenuation degree of electromagnetic wave.In the range of 0 ～ 1cm thickness,increasing the insulation thickness can improve the influence of the two types of materials on the attenuation degree of electromagnetic wave at the same time.The effect of hydrate saturation on the dielectric constant response of TDR was studied by numerical simulation.The experimental platform of hydrate formation and decomposition was built and the THF hydrate simulation experiment in natural sea sand was carried out.The apparent dielectric constant and resistivity characteristics of hydrate bearing sediments during hydrate formation and decomposition were obtained,as well as the variation trend of two TDR parameters with saturations.The results show that:(1)for the sediments with different hydrate content,the dielectric constant calculated by travel time method and spatial weight method is consistent;(2)the type and thickness of probe insulation layer structure will change the measurement results and variation trend of TDR technology on the apparent dielectric constant of sediments;(3)compared with the resistivity parameter,the apparent permittivity parameter can more sensitively reflect the phase change and experimental process of hydrate;(4)the apparent permittivity of hydrate bearing sediments decreases linearly with the increase of hydrate saturation,and the resistivity increases exponentially with the increase of hydrate saturation;(5)when the pore water is 0.50% Na Cl solution and deionized water,the two TDR apparent permittivity measurements are basically consistent.Based on the experimental data,a variety of TDR dielectric models are compared,and the dielectric model and resistivity model of hydrate saturation are established respectively.The results show that:(1)Maxwell-Deloor model and Lichteneker-Rother model(α=0.70)is in good agreement with the experimental results,and the Maxwell-Deloor model and Lichteneker-Rother equation can accurately describe the quantitative relationship between TDR apparent permittivity and hydrate saturation;the insulation structure of the probe leads to a nonlinear relationship between the apparent permittivity and hydrate saturation;(2)the sediment resistivity obtained by TDR satisfies the linear fitting of Archie’s formula in logarithmic form,and the resistivity model can effectively estimate hydrate saturation;(3)clay has obvious influence on TDR measurement results,and the parameters of dielectric model and resistivity model of hydrate saturation need to be modified;(4)the joint evaluation of hydrate saturation based on TDR measurement provides an effective way to improve the accuracy and reliability of the evaluation results.