Numerical Simulation Of Seismic Response Of The Gas Hydrate Reservoir In Muli, Qinghai And Its Application

As the first site of discovering gas hydrate resources on land in China, Muli area plays a significant role in gas hydrate exploration. This area locates at permafrost zone which is a natural geological condition for the formation of gas hydrate accumulation. Based on the available geological and geophysical data, this thesis studied and explored the seismic response features of the gas hydrate reservoir in Muli area by using seismic simulating technique.The author summarized the finite-difference schemes of acoustic, elastic and viscoelastic wave equations. These finite-difference schemes were used to simulate several simple models and the wave snaps and seismic records were obtained. The author analyzed the differences of the above seismic records. Besides, the author also simulated the AVO responses of different elastic parameter combinations and the changes caused by varying elastic parameters were summarized. The above researches were treated as theoretical and methodological basics for this thesis’ following seismic simulation.To give a reasonable setting of the models’ elastic parameters, the author analyzed the traits of the logging data(Vp and density) of well DK-9. The author found that BGTL equation(Biot-Gassmaan theory modified by Lee) can be used to express the relationships between gas hydrate saturation and elastic parameters of the hydrate reservoir in Muli. By analysis, the hydrate reservoir exhibits a relatively high P-wave velocity and a slightly low density. The sand clay ratio of the reservoir is about 1:2. Then geological-geophysical models with different hydrate saturation were established by using BGTL theory. The author then acquired viscoelastic seismic records by using viscoelastic wave equation. To get more frequency and amplitude features of the seismic responses, time-frequency analysis technique(Generalized S transform, GST) was performed to the simulated records. The results illustrated that the instantaneous amplitudes of higher hydrate saturation reservoir were bigger than the lower ones. Whereas the seismic responses of gas hydrate saturated reservoir exhibited relatively higher frequency and lower amplitude than non-saturated reservoirs, which were caused by geological structures.Based on the above researches, the author then analyzed and processed the field seismic and logging data. The synthetic trace was very close to real seismic trace. The author also found the amplitudes of far seismic traces were bigger than the near ones. The author extracted a limited stacked CDP gather and it illustrated that the reflection seismic amplitudes of hydrate saturated reservoir become bigger and bigger as the increase of offset. It should be concluded that the gas hydrate saturated reservoirs’ P-wave velocity and Poisson’s ratio were all bigger than the non- hydrate saturated reservoirs, which conformed to BGTL theory.In this thesis, the author started from numerical simulation and ended with field data analysis. By the combinations of simulation results and features of real seismic data, the author summarized the seismic responses of gas hydrate reservoir in Muli area. The results of this thesis had some theoretical values for gas hydrate exploration in this area.