| Summary: | Includes bibliographical references. 2018 Fall. There is a need to estimate the amount of gas hydrates occurring in the subsurface to establish the potential of natural gas hydrates as, for example, gas resource, geo-hazards, or climate change factors. Controlled laboratory measurements of the seismic properties of pure hydrate and hydrate-bearing sediment are critical to calibrate seismic and more importantly well-logging methods used to estimate gas hydrate accumulations. In general, the presence of hydrates is accompanied by an increase in acoustic velocity and attenuation. The stiffening effect of hydrate formation in unconsolidated sediment strongly depends on hydrate habit: hydrate formation along the grain surfaces increases velocities already at low (Sh ~ 3%) hydrate saturation, whereas hydrate located in the pore space will increase velocity only at high (Sh > 20%) hydrate saturation. I measured ultrasonic P- and S-wave velocity and attenuation in pure tetrahydrofuran (THF) hydrate and THF hydrate-bearing sediment as functions of pressure and temperature. In addition, I measured complex electrical conductivity in a methane hydrate-bearing sandstone during multiple cycles of hydrate formation and dissociation. These combined measurements allow us to understand the effect of hydrate growth on geophysical properties, interactions between sediment – water – hydrate and hydrate – water interfaces, and provide a better understanding of why an increase in hydrate saturation in sediment is accompanied by an increase in wave attenuation in well log data. The ultrasonic measurements show that presence of liquid water between hydrate grains increases attenuation in pure THF hydrates and sand-clay mixtures with varying hydrate saturation (0, 40, 60, and 80%). The observations suggest that trapped water within the hydrate causes the heightened attenuation. A comparison with laboratory data obtained using methane as a hydrate former verifies that acoustic properties of THF hydrate-bearing sediments are comparable to the methane hydrate-bearing sediments found in nature, demonstrating that THF hydrate is an appropriate proxy for methane hydrate. After an increase in pressure from 435 to 2175psi, the loss-diagram shows that samples with various hydrate saturation (0, 40, 60, and 80%) converge to the same linear behavior; as the hydrate saturations increase, the Ki - µi ratios decrease, implying a change in loss mechanisms. These findings help make a better prediction on the effects of hydrate saturation in the subsurface on the sediment properties and can be used to interpret field seismic observations. Similar to acoustic attenuation, electrical conductivity is sensitive to the existence and amount of free water. Hydrate formation results in a thin hydrate layer along the grains. The conductivity data suggest that small layers of unreacted free water are present between the thin layer of hydrate and the sediment grains. This is very important because residual water has a significant effect on wave attenuation. Hydrate formation consumes pure H2O which, during the onset of hydrate formation, results in an increase of temperature due to the exothermic reaction. This is observed as a sudden increase in electrical conductivity. Further cooling results in the observed decrease in conductivity. The reverse effect is detected during hydrate dissociation. Combined, these two processes could be used to monitor the hydrate formation or dissociation front in the subsurface.
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