地震属性参数在识别天然气水合物和游离气分布模式中的应用

DETERMINING THE DISTRIBUTION MODEL OF HYDRATE AND FREE GAS OCCURRENCE IN SEDIMENT WITH SEISMIC ATTRIBUTE PARAMETERS

为了准确估算沉积层天然气水合物和游离气饱和度,必须确定沉积层水合物和游离气的分布模式。基于Biot三相介质波传播理论,研究了水合物和游离气呈不同沉积类型时,沉积层的泊松比、纵波速度、横波速度和反射振幅随饱和度的变化。结果表明,综合分析含水合物和游离气沉积层的泊松比、纵波速度和AVA特征,可以识别天然气水合物和游离气在沉积地层中的分布模式。与不含水合物沉积层相比,BSR上纵波和横波速度比较高,而泊松比略微偏低且出现PP-波反射系数的AVO负异常表明沉积层含有水合物。BSR下纵波速度和泊松比都比较低表明沉积层游离气呈均匀分布;纵波速度低而泊松比不太低表明沉积层游离气呈块状分布。

Gas hydrates is a major energy resource potential of economic values that currently under the exploitation. Saturation is a key parameter for locating gas hydrate and free gas in marine sediment. To quantify the real saturation, the distribution models must be determined. The estimated saturations could be inconsistent if different distribution models are applied. Based on Biot＇ s theory of three-phased medium, the authors calculated some physical parameters such as Poisson ratio, compressional-wave velocity and shear velocity versus the saturation. Moreover, compressional-wave velocity and Poisson ratio are calculated with distributions of free gas in uniform-saturation and patchy-saturation models respectively. PP-reflection coefficients versus incidence angles are also calculated with different hydrate and gas saturations based on Zoeppritz equations. Compressional-wave velocity and Poisson ratio increase when gas hydrate exists in sediment. However, PP-reflection coefficients of different distribution model also differ from each other. The decrease in velocity and Poisson ratio is more distinct when free gas exists in patchy-saturation model. Compressional-wave velocity in uniform-saturation model at low free gas saturation decreases rapidly, while the decrease is small with the increase of free gas saturation （about 30% in the pore space）. But the Poisson ratio in patchy-saturation model is higher than that of uniform-saturation model at the same free gas saturation. Having analyzed the physical parameters comprehensively, we determined the distribution patterns of gas hydrate and free gas in marine sediment. Generally, in contrast with nearby unhydrated sediment, gas hydrate shows higher compressional-wave and shear velocity but slightly lower Poisson＇s ratio above BSR that associated with a negative AVO anomaly for PP-reflection coefficients, indicating the existence of gas hydrate in the sediments. Low compressional-wave velocity and Poisson＇s ratio below BSR indicates that free gas is distributed uniformly in pore space. On the other hand, low compressional-wave velocity and intermediate Poisson＇s rate below BSR is the sign of patchy distribution of free gas.