Seismic Wave Attenuation In Hydrate-bearing Sediments And The Estimates Of Attenuation Coefficient

Gas hydrates as a new kind of clean energy have attracted worldwide attention. With enhancement of exploration technology for gas hydrate resources, researchers have found that seismic wave presents abnormal attenuation characteristics in hydrate-bearing sediments, which makes the seismic wave attenuation a potential attribute for gas hydrate identification. At present, the explanations of the seismic wave attenuation phenomena in hydrate-bearing sediments cannot reach an agreement. Hence, the main purpose of this thesis is trying to study the attenuation characteristics of seismic wave in hydrate-bearing sediments theoretically, and apply the seismic wave attenuation attribute to the identification of gas hydrates.Permeability plays a crucial role in the analysis of attenuation of elastic wave energy in the porous media. Hence, in this thesis, the relationship between gas hydrates and formation permeability was studies first. Nuclear magnetic resonance measurements in hydrate-bearing sandstone samples from the Shenhu area, South China Sea show that the pore-filling hydrates greatly affect the formation permeability while depending on many factors that also bear on permeability. The Masuda model was used to formulate the empirical equation that describes the relation between relative permeability and hydrate saturation for the Shenhu area samples.Hydrate-bearing sediments show typical porous medium characteristics. Based on the BISQ and White porous medium models, the seismic wave attenuation characteristics in hydrate-bearing sediments at the microscopic scale and mesoscopic scale were detailed studied. It is found that at ultrasonic frequency band the seismic wave attenuation increases with the increasing hydrate saturation, whereas it decreases at seismic frequency band. The conclusions agree well with the existing seismic wave attenuation phenomena, laying a theoretical foundation for gas hydrate identification by seismic wave attenuation attribute. The statistical tests indicate that seismic wave attenuation in hydrate-bearing sediments is mainly influenced by the decrease of formation permeability due to gas hydrates. On this basis, a relationship can be established between seismic wave attenuation and formation permeability, which provides tentative exploration for extracting permeability from seismic data.Since the existing methods for Q-factor estimates all have disadvantages, in this thesis, the frequency-weighted-exponential(FWE) function is introduced to propose a new method for Q-factor estimates. When source wavelets propagate in absorbed media, the FWE function can be used to fit the amplitude spectrum of source and received wavelets respectively and the Q-factor can be estimated by using the symmetry indexes bandwidth factors. This method is not restricted by the type of the source wavelet. It has high accuracy and good noise tolerance. For seismic reflection data, by using this method, the Q-factor can be directly estimated without eliminating tuning effect.The Shenhu area, South China Sea is a key area of gas hydrate exploration in our country. In this thesis, the seismic wave attenuation attributes are calculated from seismic data by using the new method for Q-factor estimates. Combined with the seismic wave attenuation characteristics in hydrate-bearing sediments, the distribution of hydrate-bearing sediments is identified. The identification agrees well with the seismic reflection characteristics and drilling results, which lays a good foundation to the wide application of seismic wave attenuation attributes in the identification of gas hydrates.