A 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.

The majority of presently exploitable marine methane hydrate reservoirs are likely to host hydrate in disseminated form in coarse grain sediments. For hydrate concentrations below 25–40%, disseminated or pore‐filling hydrate does not increase elastic frame moduli, thus making impotent traditional seismic velocity‐based methods. Here, we present a theoretical model to calculate frequency‐dependent P and S wave velocity and attenuation of an effective porous medium composed of solid mineral grains, methane hydrate, methane gas, and water. The model considers elastic wave energy losses caused by local viscous flow both (i) between fluid inclusions in hydrate and pores and (ii) between different aspect ratio pores (created when hydrate grows); the inertial motion of the frame with respect to the pore fluid (Biot's type fluid flow); and gas bubble damping. The sole presence of pore‐filling hydrate in the sediment reduces the available porosity and intrinsic permeability of the sediment affecting Biot's type attenuation at high frequencies. Our model shows that attenuation maxima due to fluid inclusions in hydrate are possible over the entire frequency range of interest to exploration seismology (1–10⁶ Hz), depending on the aspect ratio of the inclusions, whereas maxima due to different aspect ratio pores occur only at sonic to ultrasound frequencies (10⁴–10⁶ Hz). This frequency response imposes further constraints on possible hydrate saturations able to reproduce broadband elastic measurements of velocity and attenuation. Our results provide a physical basis for detecting the presence and amount of pore‐filling hydrate in seafloor sediments using conventional seismic surveys.

Methane hydrate saturation estimates from remote geophysical data and borehole logs are needed to assess the role of hydrates in climate change, continental slope stability, and energy resource potential. Here we present laboratory hydrate formation/dissociation experiments in which we determined the methane hydrate content independently from pore pressure and temperature and from electrical resistivity. Using these laboratory experiments, we demonstrate that hydrate formation does not take up all the methane gas or water even if the system is under two phase water‐hydrate stability conditions and gas is well distributed in the sample. The experiment started with methane gas and water saturations of 16.5% and 83.5%, respectively; during the experiment, hydrate saturation proceeded up to 26% along with 12% gas and 62% water remaining in the system. The coexistence of hydrate and gas is one possible explanation for discrepancies between estimates of hydrate saturation from electrical and acoustic methods. We suggest that an important mechanism for this coexistence is the formation of a hydrate film enveloping methane gas bubbles, trapping the remaining gas inside.

SUMMARY: Knowledge of the effect of methane hydrate saturation and morphology on elastic wave attenuation could help reduce ambiguity in seafloor hydrate content estimates. These are needed for seafloor resource and geohazard assessment, as well as to improve predictions of greenhouse gas fluxes into the water column. At low hydrate saturations, measuring attenuation can be particularly useful as the seismic velocity of hydrate-bearing sediments is relatively insensitive to hydrate content. Here, we present laboratory ultrasonic (448–782 kHz) measurements of P-wave velocity and attenuation for successive cycles of methane hydrate formation (maximum hydrate saturation of 26 per cent) in Berea sandstone. We observed systematic and repeatable changes in the velocity and attenuation frequency spectra with hydrate saturation. Attenuation generally increases with hydrate saturation, and with measurement frequency at hydrate saturations below 6 per cent. For hydrate saturations greater than 6 per cent, attenuation decreases with frequency. The results support earlier experimental observations of frequency-dependent attenuation peaks at specific hydrate saturations. We used an effective medium rock-physics model which considers attenuation from gas bubble resonance, inertial fluid flow and squirt flow from both fluid inclusions in hydrate and different aspect ratio pores created during hydrate formation. Using this model, we linked the measured attenuation spectral changes to a decrease in coexisting methane gas bubble radius, and creation of different aspect ratio pores during hydrate formation.

We show that direct estimates of the permeability of hydrate‐bearing geological formations are possible from remote measurements of shear wave velocity (V_s) and attenuation (Q_s⁻¹). We measured V_s, Q_s⁻¹ and electrical resistivity at time intervals during methane hydrate formation in Berea sandstone using a laboratory ultrasonic pulse‐echo system. We observed that V_s and Q_s⁻¹ both increase with hydrate saturation S_h, with two peaks in Q_s⁻¹ at hydrate saturations of around 6% and 20% that correspond to changes in gradient of V_s. We implemented changes in permeability with hydrate saturation into well‐known Biot‐type poro‐elastic models for two‐ and three‐phases for low (S_h < 12%) and high (S_h > 12%) hydrate saturations respectively. By accounting for changes in permeability linked to hydrate morphology, the models were able to describe the V_s and Q_s⁻¹ observations. We found that the first Q_s⁻¹ peak is caused by a reduction of permeability during hydrate formation associated with a transition from pore‐floating to pore‐bridging hydrate morphology; similarly, the second Q_s⁻¹ peak is caused by a permeability reduction associated with a transition from pore‐bridging hydrate morphology to an interlocking network of hydrate in the pores. We inverted for permeability using our poro‐elastic models from V_s and Q_s⁻¹. This inverted permeability agrees with permeability obtained independently from electrical resistivity. We demonstrate a good match of our models to shear wave data at 200 Hz and 2 kHz frequencies from the literature, indicating the general applicability of the models.