The Study Of Identifying The Gas Hydrate Occurrence Type Via Singularity

Pressure coring demonstrates that gas hydrate can be divided into two forms: invisible pore-filling hydrate,which is filled in pores,and visible fracture-filling hydrate,which is filled in small cracks.Therefore,to mine reasonably and safely,we must identify the occurrence type of hydrate prior to mining.Moreover,the improper exploitation of fracture-filling hydrate may lead to the collapse of the seabed and methane leakage.Based on the differences between the occurrence characteristics of these two types,we propose a method for identifying the occurrence type via singularity analysis.For pore-filling GHBS,hydrates uniformly fill the pores of the medium,their physical properties are relatively homogeneous and the singularity is weak.For fracturefilling GHBS,hydrates are randomly distributed in the cracks of the medium,their physical properties have strong mutability and the singularity is strong.Therefore,there are differences between the singularity characteristic information of two types of GHBS.If these singularity differences are within the frequency band of seismic wave,which requires that seismic exploration can detect differences between the two formations,the information will be transmitted to the seismic records.Then,we can identify these differences via singularity analysis and distinguish between the hydrate occurrence type.We verify the correctness of the method in the following ways: 1.Studying whether the waveform characteristics can detect hydrate filling types through the forward modeling.2.Using actual seismic data to verify whether the results of forward modeling are correct.According to the distribution characteristics of hydrate obtained from the actual hydrate samples,two kinds of two-dimensional models of occurrence types are established,and then forward seismic records are made to obtain their singularity characteristics.The numerical experiments demonstrate that the fracture-filling hydrate zone has a strong singularity,while the pore-filling hydrate zone has a weak singularity.Then,we identify the occurrence type of the hydrate zone using the actual seismic data of the South China Sea,Blake Ridge in the United States and Hydrate Ridge.Through comparison with the actual core data,it is demonstrated that the proposed method is effective and practical.Some factors may affect the accuracy of the method in the process of establishing the model and processing the actual data.Thus,we have made supplementary research on the feasibility and applicability of the method.We have studied the influence of hydrate scale,hydrate distribution density,formation porosity,hydrate saturation,hydrate layer thickness and noise on the identification of hydrate occurrence types via singularity,and improved the systematicness of the research.