Research On Multi-function CPTU Data Interpretation And Clustering Algorithm Based On Natural Gas Hydrate Exploration

The 21th century is the century of the ocean.The construction of marine engineering and the development of marine mineral resources are the engines of the marine economy.As a new mineral,the potential of natural gas hydrate is huge.Accurate identification and precise quantitative evaluation of natural gas hydrate reservoir is the basis for the development and utilization of hydrate in nature.At present,geophysical logging is the most effective in situ identification and evaluation method except earthquake and drilling coring.Well logging can not be separated from drilling,and ocean drilling is a complex and costly work.The advantages of logging technology and conventional Piezocone Penetration Test?CPTU?technology have been conlected by multi-function CPT technology,with the development of CPTU technology,its role is not limited to the evaluation of characteristics of rock and soil,in the identification of abnormal formation,is also useful,the richness of the CPTU probe will undoubtedly improve the recognition effect.If the multi-functional CPTU technology can be used to identify the characteristic stratigraphy or reservoir of natural gas hydrate,then we can use the drilling core or logging technology to verify the natural gas hydrate reservoir,which will greatly improve the exploration efficiency and reduce the exploration cost of hydrate reservoirs.In the mechanism of penetration,the existing various research methods for CPTU were compared,and the research ideas of cavity expansion theory combined with finite element method was proprosed.the CPTU data like the resistance were explained by using the cavity expansion theory,while using the finite element method to simulate the effects of the soil stiffness index,in situ stress state and the roughness on the cone tip resistance in clay soil,the effects of strain softening and strain rate on the cone resistance in marine clay,the effects of soil expansion stress,shear modulus,friction angle,the expansion angle and other factors on cone resistance in sand,and the relationship of the deformation of soil and the extension of the plastic zone around the probe with cavity expansion theory.In terms of application,based on the comparison of the existing soil behavior classification methods based on CPTU test data,a hierarchical clustering algorithm is introduced,and a clustering flow chart based on CPTU test data is proposed.The cluster flow chart is converted into Matlab code,and the computerized clustering process is realized.A set of rules for the division and classification of soil layers is put forward by using the cluster flow chart and the international Robertson and Campanella classification charts.Then the method is applied in the CPT test,CPTU test and offshore CPTU test projects,and the borehole histogram for the near soil,cavity expansion theory and finite element method results verified clustering and stratification effect of soil based on CPTU test data.Finally,the multi-functional CPTU response characteristics for gas hydrate reservoirs and shallow gas reservoir were researched,based on rational selection and retreatment of the initial parameters for clustering,the above clustering stratification method is applied to the identification of gas hydrate reservoir and the recognition of shallow gas reservoir,and good results have been achieved.The main work and research results are summarized as follows:?1?The research ideas of cavity expansion theory combined with finite element method was proprosed.Through the large strain finite element analysis of the CPTU cone tip resistance,the cone factor expression of clay considering the stiffness index of soil,the in situ stress state and the roughness of the probe is obtained,and the cone factor expression of marine clay considering the stiffness index,strain softening and strain rate is also presented.The deformation of soil around the probe and the extension of plastic zone confirme confirmed the rationality of interpretation of CPTU results using the cavity expansion theory.?2?A clustering flow chart based on CPTU test data is proposed by introducing the hierarchical clustering algorithm,through the reasonable selection of initial data,data standardization methods,distance functions and clustering effect evaluation methods,the definition of maximum clustering number N_cmax and final cluster number N_cf.The cluster flow chart is converted into Matlab code,and the computerized clustering process is realized.On the basis of this,a set of classification and classification rules of soil layer is put forward in combination with the general Robertson and Campanella classification charts.?3?The main layer and the secondary layer can be successfully divided from the soil layer by using the hierarchical clustering method through a series of CPTU test and adjacent soil drilling.The mixed layer,interlayer and transition layer which can not be reflected by borehole histogram are separated,and the purpose of subdividing the soil layer,identifying the transition layer,mixing layer and abnormal data is s reached.And using the cavity expansion theory and finite element method results verified the effect of hierarchical clustering based on the test data of CPTU.?4?When clustering and stratification is applied to identify gas hydrate reservoirs,the clustering stratified boundary based on resistivity,acoustic time difference,density and natural gamma logging parameters has high matching degree with lithologic pillar boundary and got good layering effect.Among them,two mudstones are identified by the clustering algorithm of resistivity,interval transit time and density as initial parameters,but the specific location of gas hydrate is not identified.Location and thickness of gas hydrate are identified by the clustering algorithm based on the initial parameters of resistivity,interval transit time and natural gamma.?5?When clustering and stratification is applied to shallow gas reservoir identification,CPTU data can only be used to identify the characteristics of gas bearing soil layer different from other soil layers,and roughly judge the location and thickness of gas bearing soil layer.Only by comprehensively utilizing the multi-functional CPT data(non normalized soil behavior classification index I_SBT,resistivity Rt and acoustic velocity AC)can we identify the specific location and thickness of the gas part in the gas bearing soil layer,so as to achieve the purpose of identifying shallow gas.The clustering and stratification method based on multi-functional CPTU data aggregate the advantages of CPTU technology and logging technology.It eliminates the complexity of manual layering,refines the effect of stratification,and improves the ability of identifying formation anomalies.In a word,when the multi-functional CPTU data is used for clustering and stratification of soil,the effect of clustering can be improved by reasonablely selecting the initial parameters,standardizing the test data,eliminating the interference signal of the test curve,increasing clustering effect evaluation coefficient,and reaching the purpose of refining the soil layer and identifying the abnormal formation and the characteristic formation of gas hydrate.