Abstract
AbstractA better understanding of the effect of methane hydrate morphology and saturation on elastic wave velocity of hydrate‐bearing sediments is needed for improved seafloor hydrate resource and geohazard assessment. We conducted X‐ray synchrotron time‐lapse 4‐D imaging of methane hydrate evolution in Leighton Buzzard sand and compared the results to analogous hydrate formation and dissociation experiments in Berea sandstone, on which we measured ultrasonic P and S wave velocities and electrical resistivity. The imaging experiment showed that initially hydrate envelops gas bubbles and methane escapes from these bubbles via rupture of hydrate shells, leading to smaller bubbles. This process leads to a transition from pore‐floating to pore‐bridging hydrate morphology. Finally, pore‐bridging hydrate coalesces with that from adjacent pores creating an interpore hydrate framework that interlocks the sand grains. We also observed isolated pockets of gas within hydrate. We observed distinct changes in gradient of P and S wave velocities increase with hydrate saturation. Informed by a theoretical model of idealized hydrate morphology and its influence on elastic wave velocity, we were able to link velocity changes to hydrate morphology progression from initial pore‐floating, then pore‐bridging, to an interpore hydrate framework. The latter observation is the first evidence of this type of hydrate morphology and its measurable effect on velocity. We found anomalously low S wave velocity compared to the effective medium model, probably caused by the presence of a water film between hydrate and mineral grains.
Highlights
Gas hydrates are naturally occurring ice-like clathrate compounds that form when sufficient gas and water coexist under low temperatures and high pressures, generally found in marine and permafrost environments (Kvenvolden, 1993).Currently, seafloor gas hydrates are being considered as a viable alternative energy resource (Boswell & Collett, 2011), and may have an important role in future climate change (Archer et al, 2009), carbon dioxide sequestration (Jung et al, 2010) and continental slope stability (Sultan et al, 2004)
Our results provide further evidence of how methane hydrate saturation relates to hydrate morphology, of how this morphology influences elastic wave velocity and electrical resistivity, two important geophysical parameters used in hydrate exploration, and of the mechanism of coexisting gas and hydrate
While our experiments show transitions of the geophysical properties at specific hydrate saturations in our experiments, it is likely that such transitions occur at different hydrate saturations depending on sediment type and hydrate formation method
Summary
Gas hydrates are naturally occurring ice-like clathrate compounds that form when sufficient gas (methane is the most common in nature ) and water coexist under low temperatures and high pressures, generally found in marine and permafrost environments (Kvenvolden, 1993).Currently, seafloor gas hydrates are being considered as a viable alternative energy resource (Boswell & Collett, 2011), and may have an important role in future climate change (Archer et al, 2009), carbon dioxide sequestration (Jung et al, 2010) and continental slope stability (Sultan et al, 2004). It is important to obtain accurate estimates of the amount and distribution of gas hydrates, largely reliant on geophysical remote sensing technologies and data interpretation Such estimates depend on knowledge of hydrate formation processes and how they affect geophysical properties. Geophysical remote sensing methods use elastic wave velocity and electrical resistivity anomalies to quantify hydrates in marine sediments, based on rock physics models that relate these anomalies to hydrate content (e.g., Collett, 2001; Cook & Waite, 2018; Doveton, 2001; Ecker et al, 2000; Edwards, 1997; Helgerud et al, 1999; Spangenberg, 2001)
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