Effect of CO₂ hydrate formation on seismic wave velocities of fine‐grained sediments

Abstract

[1] This study examines the effect of gas hydrate formation on seismic wave velocities of fine-grained sediments. Synthesis of gas hydrates in fine-grained sediments has proved to be challenging, and how hydrate formation would affect the seismic wave velocities and stiffness of clay-rich sediments has not yet been fully understood. In this study, CO 2 hydrate was synthesized in remolded and partially water-saturated clayey silt sediments that were originally cored from a hydrate occurrence region in the Ulleung Basin, East Sea, offshore Korea. After achieving excess water conditions, compressional wave and shear wave velocities were measured for different hydrate saturations and under different vertical effective stresses. The results reveal that the compressional wave velocity V P and shear wave velocity V S increase, and the stress-dependency of V P and V S decreases as the hydrate saturation S H increases from 0% to~60%. In particular, the V S -S H trend lies between the grain-cementing model and the load-bearing model, suggesting that gas hydrate formation in clayey silt sediments causes weak cementation from a hydrate saturation less than~28%. The weak cementation in fine-grained sediments can be explained by the breakage of hydrate bonds that are cementing grains during sediment compression and/or the innate weakness in bonding between hydrate crystals and fine mineral grains owing to the presence of unfrozen water films on clay mineral surfaces. In addition, it is found that at low S H , the cementation effect on V P is masked by the high stiffness of pore-filling phases, but it becomes pronounced at S H greater than 47%.

Effect of methane hydrate morphology on compressional wave velocity of sandy sediments: Analysis of pressure cores obtained in the Eastern Nankai Trough

“…Through the comparison with model predictions such as those of Dvorkin et al (2000), it was found that this relationship depends on the hydrate morphology, such as grain coating, cementing, pore-filling, and sediment frame component (or loadbearing) within the pore space (M. Lee et al, 1996;Berge et al, 1999;Reister, 2003;Yun et al, 2005;Priest et al, 2009;J. Lee et al, 2010;Hu et al, 2010, Li et al, 2011Best et al, 2013;Kim et al, 2013a). This suggests that the P-wave velocity obtained from logging and seismic surveys can be used to estimate the in situ hydrate saturation if the hydrate morphology in actual reservoirs is revealed.…”

Observations of pore‐scale growth patterns of carbon dioxide hydrate using X ‐ray computed microtomography

“…The second and third rounds of X‐ray scanning were performed 115 h (∼5 days, Stage 2) and 213 h (∼9 days, Stage 3) after the onset of hydrate formation. These hydrate‐forming conditions are applicable to the excess gas method, which has been widely employed to synthesize gas hydrate samples in laboratory settings [e.g., Ebinuma et al ., ; Masui et al ., ; Waite et al ., ; Winters et al ., ; Priest et al ., ; Jung and Santamarina , ; Kim et al ., ].…”

“…Controlled laboratory experiments on synthesized hydrate‐bearing sediments have been performed in the past by some researchers to relate the pore saturation of gas hydrates to the physical properties of sediments. A number of routes to synthesize gas hydrate‐bearing sediment samples in the laboratory have been explored, including the ice seed method [e.g., Stern et al ., ; Waite et al ., ; Priest et al ., ; Hyodo et al ., ], excess gas method [e.g., Waite et al ., ; Winters et al ., ; Kim et al ., ], excess water method [ Priest et al ., ], and a method using dissolved gas or THF [e.g., Spangenberg et al ., ; Yun et al ., ; Lee et al ., ; Kwon and Cho , ; Lee et al ., , b]. Among these methods, forming gas hydrates from partially water‐saturated sediments, referred to as the excess gas method, has been widely adopted because of its simplicity and the ability to control the hydrate saturation.…”

“…Specification of the X-ray CMT Imaging Conditions a settings [e.g.,Ebinuma et al, 2005;Masui et al, 2005;Waite et al, 2004;Priest et al, 2009; Santamarina, 2011, 2012;Kim et al, 2013].…”

“…Geochemistry, Geophysics, Geosystems PUBLICATIONS Waite et al, 2002;Priest et al, 2005;Hyodo et al, 2013], excess gas method [e.g., Waite et al, 2004;Kim et al, 2013], excess water method [Priest et al, 2009], and a method using dissolved gas or THF [e.g., Spangenberg et al, 2005Spangenberg et al, , 2008Yun et al, 2005Yun et al, , 2007Lee et al, 2007;Kwon and Cho, 2009;Lee et al, 2010a, b]. Among these methods, forming gas hydrates from partially water-saturated sediments, referred to as the excess gas method, has been widely adopted because of its simplicity and the ability to control the hydrate saturation.…”