Study On The Calculation Method Of Hydrate Saturation In Porous Media Based On A Joint Electrical-Acoustic Detection Approach

The particle size,clay content and hydrate saturation of sediments significantly affect the electrical and acoustic physical properties of sediments.It is of great significance for the fine quantitative evaluation of gas hydrate reservoirs to study the joint electrical and acoustic response characteristics under complex sediment conditions and to establish a joint model to evaluate the hydrate saturation by making full use of the information of electrical and acoustic parameters.On the basis of previous experimental devices,some improvements have been made on the usage rights,data security,error handling and blockage of gas supply pipeline in the reaction kettle.Experiments show that the method of extending the gas supply pipeline into the reactor is helpful to solve the problem of extra-low hydrate saturation due to the insufficient gas supply in the reaction kettle.Four kinds of simulated sediment systems with different porous media sizes and clay contents have been designed according to the characteristics of field marine sediments,and gas hydrate simulation experiments and test studies were carried out using a simulated sediment system.The effects of temperature,effective pressure and other external factors on the impedance modulus and ultrasonic velocity of sediment systems were investigated through the data preprocessing of electrical and acoustic signals and calculation of characteristic parameters.The impedance modulus in the frequency range of 10 k Hz～500k Hz can be used as an effective electrical characteristic parameter to indicate the variation of the hydrate saturation in sediments,which provides a reference for establishing the hydrate saturation model.The modulus maximum method was used to filter the ultrasonic signal.Then the modulus maximum line method was used to extract the wave arriving time of the ultrasonic signal.This method can effectively improve the accuracy of the calculation of the ultrasonic propagation speed.Based on impedance modulus,sound velocity and hydrate saturation of the sediment,the calculation models of hydrate saturation based on Archie’s formula and Weight equation were established,and the effects of the sediment particle size and clay content on the parameters of the model were analyzed.(1)The particle size and clay affect the cementation coefficient and saturation index of Archie’s formula.When the particle size of sediment ranges within 0.188-0.850 mm and the clay content is in the range of 0～20%,the cementation coefficient and saturation index increase with the increase of particle size and clay content.(2)Particle size and clay content affect the sound velocity of hydrate sediments,and then the weight factor and saturation constant.As the particle size and clay content increase,the sound velocity of the sediment increases and the corresponding weighting factor decrease and saturation constant increase.Based on the Archie’s formula and Weight equation,a joint electrical-acoustic model was established.The accuracy of the joint electrical-acoustic model is verified by the measured data,and the spatial distribution evaluation method of hydrate was explored preliminarily.(1)A joint electrical-acoustic model can be established by combining Archie’s formula and Weight equation.The accuracy of the hydrate saturation calculated by the joint model is higher than that calculated by using Archie’s formula or Weight equation alone.(2)During the hydrate formation experiment,the hydrate saturation was calculated by using the impedance modulus and sound velocity data of different measurement areas of the sediment.The results show that there are differences in the rate of hydrate saturation increase in different areas of the reaction kettle,and the spatial distribution of the hydrate in the reactor is not uniform.