墨西哥湾GC955H井天然气水合物储层声波衰减特征

墨西哥湾GC955H井钻遇两种不同的水合物储层,浅层裂隙充填型水合物和深层砂岩型水合物.浅层水合物充填在细粒泥质沉积物的裂隙中,由电阻率测井计算的饱和度平均值为25%.深层水合物充填在砂岩孔隙中,由电阻率计算的饱和度平均为65%.基于声波全波形数据,本文计算了GC955H井储层的声波衰减大小,结果发现两种水合物层对声波衰减的影响不同.泥岩水合物层中的声波衰减与上覆背景泥岩沉积层基本相当.砂岩水合物层的声波衰减大于0.1,最大0.15,远高于上覆和下伏的背景砂岩层.对声波频率的分析发现,GC955H井泥岩层中水合物对声波频率的影响不大,砂岩水合物层的声波频谱与上下背景砂岩层相比发生变化,主频增大.通过对比两种水合物储层的特征,本文初步定性分析了造成水合物对储层声波传播影响不同的原因,包括岩性、水合物饱和度、水合物赋存方式等;但对此的定量描述需要未来更详尽地讨论和研究水合物的声波衰减机制.

The characteristics of sonic wave attenuations of gas hydrate reservoirs at Well GC955H,Gulf of Mexico

The effect of gas hydrate on sonic wave/seismic attenuations is still an unresolved issue. The results vary from region-to-region and also from experiment-to-experiment within the same region. Intuitively, hydrate strengthens the sediment with the increase in elastic modulus; hence the attenuation should decrease with hydrate saturation. Alternatively, the energy loss mechanism involving the fluid flows within the pores of hydrate matrix would increase attenuation in hydrate reservoir. More studies should be conducted on the mechanism of attenuation induced by gas hydrate.#br#Two different gas hydrate reservoirs were discovered at Well GC955H from Gulf of Mexico, fracture-filling gas hydrate at the shallow interval and pore-filling gas hydrate in sands at the deep interval. Using Archie's equation, the average saturation of gas hydrate from resistivity is estimated to be 25% at the shallow clay interval while the value is 65% at the deep sand interval. However, the fractured reservoir shows anisotropy, and thus the 25% saturation of gas hydrate for the fracture-filling gas hydrate at shallow interval probably is overestimated.The sonic wave attenuations are calculated from sonic waveform data and the results show different effects at the two hydrate reservoirs.#br# The sonic wave attenuations of shallow gas hydrate interval are similar to those of overlying background clay. The attenuations of deep gas hydrate interval in sands are over 0.1, 0.15 maximally, and are much larger than the overlying and underlying background sands. The frequencies of sonic waveforms at GC955H were also analyzed. The gas hydrate bearing in clay has little influence on the frequency of sonic wave. However, the gas hydrate bearing in sands changes the sonic wave frequency and increases the dominant frequency compared to that of background sands. Based on the comparison of the two hydrate reservoirs within the same GC955H well, our preliminary analysis indicates that the different effects on the sonic wave attenuation could be explained by lithology of sediments, gas hydrate saturation and morphology. Quantitatively resolving this problem requires more studies on the mechanism of attenuations induced by gas hydrate.