Seismic Dispersion in Pierre Shale
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In applied geophysics, there is a trend to progress from a qualitative interpretation towards a quantitative interpretation.4D seismic (time-lapse seismic) is a common tool used in reservoir characterization, by which we can extract fluid type and saturation, as well as stress changes. It is known that elastic properties are related to the fluid distribution inside the sample, the fluid mobility and the frequency at which the measurement is conducted. Due to seismic dispersion, the models assuming low frequency limits are not always suitable. Especially in shale, which has a low fluid mobility, the proper flow mechanism should be proposed for modelling seismic responses. This study focuses on measurements of Pierre Shale investigating seismic dispersion as a function of different water saturations and stress levels. Two samples with RH=33% and RH=75% are involved in the experiment. Seismic dispersion of Pierre Shale is measured in a specially designed biaxial compaction cell allowing dynamic elastic measurements in the low frequency range (1-155 Hz), ultrasonic measurements (500 kHz) and quasi-static deformation. Measurements conducted covering a large frequency spectrum will help us build a better understanding of the dispersion and attenuation mechanism in shale and assist in predicting and modelling these effects.A significant dispersion in Pierre Shale is observed. The results show a larger dispersion for higher saturated sample and a significantly smaller dispersion for the drier sample, possibly due to rock-fluid interaction. The sample tends to become softer with increasing water saturation, probably caused by suction effects. The experiment is conducted under various stress levels to investigate the stress dependency. There is a trend of decreasing dispersion with increasing stress in the drier sample. While for the higher saturated sample, the dispersion is not affected by stress. This is likely due to the presence of more water in the low aspect ratio pores and micro-cracks, which makes it difficult to compress the sample. An increase in capillary pressure due to increasing water saturation can also explain why the dispersion is not affected by stress.As velocity dispersion and attenuation are always coupled, an estimated attenuation can be obtained. Cole-Cole equation (Cole & Cole, 1941) is applied to couple dispersion and attenuation for Young s modulus in this study.The drier and higher saturated samples have modelled relaxation peaks at frequencies of around 250Hz and 650 Hz respectively, which are supposed to be in the range between seismic frequency and logging frequency. This might be an indication that they are dominated by the same dispersion mechanism.Clay- bound water is also supposed to be a main mechanism controlling seismic dispersion in Pierre Shale. A larger dispersion is found in Pierre Shale compared with Mancos Shale, which is probably caused by the presence of large amounts of clay minerals, especially smectite.