Wave speeds in gas hydrate and sediments containing gas hydrate: A laboratory and modeling study.

摘要

Gas hydrates are crystalline, ice-like phases of water that physically trap “guest” molecules in cages created by the water crystal lattice. Gas hydrates formed with methane have been found in many continental margin and deep-sea sediments and beneath permafrost in the Arctic. Natural gas hydrate deposits may contain an economically significant volume of natural gas and/or play a role in global climate change. The presence of gas hydrate can also affect the strength and rigidity of sediments.; Current estimates of natural gas stored in gas hydrates are uncertain because it is difficult to quantify the amount of gas hydrate present in the subsurface. Seismic data could be used to more effectively locate and quantify gas hydrate if more were known of the elastic properties of gas hydrate and sediments containing gas hydrate.; In this thesis I analyze previous models and summarize previous measurements of wave speeds in gas hydrate. One model (limited to T 0°C) is updated to account for modern laboratory measurements and extended to predicting shear wave speeds. I also report new experiments on propane and methane hydrate. Experiments on propane hydrate formed by bubbling gas through water and water saturated sediments were largely unsuccessful because the resultant hydrate was unsuitable for wave speed measurements. Subsequent experiments on methane hydrate formed from granulated ice resulted in compressional and shear wave speed measurements over a range of temperatures (−15 to 15°C) and pressures (0 to 15,000 psi). The measurements made below 0°C matched the predictions of the model. Wave speed measurements were also made on ice samples, and “hand-book” values were obtained. Methane hydrate was found to be much more resistant to compaction than ice.; The results from the methane hydrate experiments were used in rock physics models to predict the effect that gas hydrate has on compressional and shear wave speeds in sediments. The models were tested against well log data from ocean bottom sediments and sands known to contain gas hydrate. The results matched independent estimates, where available, and made it possible to infer how the gas hydrate was distributed in the pore space.