| Petrochemical industry is a pillar industry in China,and its development not only promotes the economic development and the construction of national defense industry,but also brings serious organic petroleum hydrocarbon pollution.Establishing an accurate,rapid,simple and qualitative and quantitative detection scheme for hydrocarbon pollutants is the premise to carry out the research work of petroleum hydrocarbon pollutants in soil,and it is also the key to realize the evaluation of soil environmental quality in petroleum hydrocarbon contaminated sites.Conventional hydrocarbon pollution detection methods,such as ultraviolet fluorescence method and gas chromatography,are all small-range detection methods,which are difficult to give consideration to both detection range and efficiency,and cannot achieve efficient field in-situ pollution delineation.Magnetic resonance technology has many advantages,such as accuracy,rapidness,in-situ and direct analysis of hydrogen protons in fluid.It has been widely used in logging and other fields,and can realize the measurement of pore distribution of porous media containing hydrocarbon substances and the identification of fluid characteristics in pores.Based on this,this paper studies the relaxation characteristics of hydrocarbon pollutants by using low-field magnetic resonance technology.In the experimental scheme of hydrocarbon pollution characteristics research,firstly,all kinds of sample models simulating real hydrocarbon pollution in the field are prepared by indoor laboratory method,and then one-dimensional pulse sequence and two-dimensional pulse sequence are selected for the sample models,and the acquisition parameters of pulse sequence are optimized.Then,the ability of spectrum resolution of two inversion algorithms applied to magnetic resonance data is studied,and the accuracy of the two algorithms for spectrum resolution of experimental samples in this paper is compared.Finally,the one-dimensional T2spectrum and two-dimensional imaging relaxation spectrum of different hydrocarbon pollutants are analyzed,and the magnetic resonance signals of water and hydrocarbon pollutants in pores are qualitatively and quantitatively distinguished under certain conditions.The specific research contents are as follows:（1）Firstly,the basic principle of magnetic resonance is introduced based on the response process of pollutant hydrogen atom.Accord to that influence of the occurrence state of field pollutants on the pollution response,the response mechanism of bulk relaxation,surface relaxation and diffusion relaxation of fluid in pores are studied,the phenomenon of multi-exponential attenuation of fluid in pores is explore,and the formula of magnetic resonance signal response of multiphase fluid is derived.（2）Combined with the theoretical analysis results,the detection sequence and interpretation method of hydrocarbon pollutants by low-field magnetic resonance are studied.Contaminated samples were prepared by common hydrocarbon pollution types and their occurrence media.On the basis of spin echo response theory,one-dimensional CPMG and two-dimensional IR-CPMG excitation sequences for longitudinal and transverse relaxation time measurement were designed,and the acquisition parameters of pulse sequences were optimized.Aiming at the typical hydrocarbon pollutant model,SVD and BRD inversion algorithms for one-dimensional and two-dimensional response signal interpretation are studied.At the same time,aiming at the problem of low inversion efficiency caused by the large amount of 2D response data,the 2D data compression technology is studied,which effectively improves the stability and efficiency of spectral algorithm.The validity of the one-dimensional CPMG and two-dimensional IR-CPMG measurement sequences and the spectral decomposition algorithm in this paper are verified by comparing the spectral decomposition results of typical contaminated samples.（3）Two hydrocarbon pollution models of loose sediment,uniform pore and complex pore,are established.On the basis of one-dimensional T2spectrum inversion interpretation and magnetic resonance two-dimensional imaging method,a comprehensive analysis of the relaxation characteristics of pollutants and water two-phase fluid is completed.Firstly,the relaxation characteristics of hydrocarbon pollutants and water two-phase fluid are accurately analyzed by the information of characteristic peak relaxation time,peak value and peak area of T2spectrum of hydrocarbon pollutants.Then,the two-dimensional imaging method is used to solve the problem that the signals of pollutants in one-dimensional T2spectrum of some samples and water two-phase fluid overlap indistinguishable.（4）Aiming at the problem that the presence of clay in quartz sand will affect the overall pore size distribution of medium,firstly,the influence of clay content in complex pores on one-dimensional T2relaxation spectrum was studied qualitatively and quantitatively,and then the deviation of characteristic peak curve of T2spectrum,the change of the number of characteristic peaks,and the corresponding relationship between the relaxation time of characteristic peaks and clay content were further discussed.Then,the influence of water saturation in complex pores on relaxation mechanism is studied.The occurrence of water in complex pores is qualitatively and quantitatively analyzed by one-dimensional T2spectrum,and the phenomenon of water absorption of clay is further put forward and verified.To sum up,in this paper,through the preparation and measurement of typical petroleum hydrocarbon samples,the qualitative and quantitative detection of hydrocarbon pollutants by magnetic resonance technology has been effectively realized.The design of magnetic resonance pulse sequence,data interpretation algorithm and analysis conclusion of typical contaminated samples in this paper will provide research ideas and reference for the application of magnetic resonance method in field hydrocarbon pollution detection in unsaturated zone. |
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