[1] This paper reports the results of a series of resonant column tests on specimens where gas hydrate has been formed in sands using an ''excess water'' technique. In these specimens the amount of hydrate formed is restricted by the amount of gas in the specimen and with an excess of water being present in the pore space. Results of resonant column tests carried out to determine compressional and shear wave velocities suggest that gas hydrate formed in this way are frame supporting. In contrast, the behavior observed in sands where the hydrate is formed from finite water where the remaining pore space is saturated with methane gas, termed in this paper the ''excess gas'' method, exhibits a cementing behavior, while tetrahydrofuran-hydrate sands or where the hydrate is formed from dissolved methane within the pore water, exhibit a pore-filling behavior for hydrate saturations less than 40%. For sands where the hydrate is formed using the excess water method, much larger volumes of hydrate are required before a significant increase in shear wave velocity occurs, although increases in compressional wave velocity are seen at lower hydrate contents. These results suggest that hydrate interaction with the sediment is strongly dependent on morphology, and that natural hydrate may exhibit contrasting seismic signatures depending upon the geological environment in which it forms.

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A B S T R A C TAccurate estimation of CO 2 saturation in a saline aquifer is essential for the monitoring of supercritical CO 2 injected for geological sequestration. Because of strong contrasts in density and elastic properties between brine and CO 2 at reservoir conditions, seismic methods are among the most commonly employed techniques for this purpose. However the relationship between seismic (P-wave) velocity and CO 2 saturation is not unique because the velocity depends on both wave frequency and the CO 2 distribution in rock. In the laboratory, we conducted measurements of seismic properties of sandstones during supercritical CO 2 injection. Seismic responses of small sandstone cores were measured at frequencies near 1 kHz, using a modified resonant bar technique (Split Hopkinson Resonant Bar method). Concurrently, saturation and distribution of supercritical CO 2 in the rock cores were determined via x-ray CT scans. Changes in the determined velocities generally agreed with the Gassmann model. However, both the velocity and attenuation of the extension wave (Young's modulus or 'bar' wave) for the same CO 2 saturation exhibited differences between the CO 2 injection test and the subsequent brine re-injection test, which was consistent with the differences in the CO 2 distribution within the cores. Also, a comparison to ultrasonic velocity measurements on a bedded reservoir rock sample revealed that both compressional and shear velocities (and moduli) were strongly dispersive when the rock was saturated with brine. Further, large decreases in the velocities of saturated samples indicated strong sensitivity of the rock's frame stiffness to pore fluid.

Cementing of sediment occurs naturally in many soils and weak rocks, during both the early and late stages of diagenesis. This paper reports the results of a series of resonant column tests carried out on a range of sand-sized geomaterials to explore the effects of different hydrate cement morphologies on the very small strain stiffness of the materials. It is shown that the proportion of void space filled by hydrate cement, cement location, sand size and grain shape all have a significant effect on shear modulus. The change in stiffness between different host materials is affected by the density, specific surface and grading of the geomaterial.