Abstract
AbstractWe show that direct estimates of the permeability of hydrate‐bearing geological formations are possible from remote measurements of shear wave velocity (Vs) and attenuation (Qs−1). We measured Vs, Qs−1 and electrical resistivity at time intervals during methane hydrate formation in Berea sandstone using a laboratory ultrasonic pulse‐echo system. We observed that Vs and Qs−1 both increase with hydrate saturation Sh, with two peaks in Qs−1 at hydrate saturations of around 6% and 20% that correspond to changes in gradient of Vs. We implemented changes in permeability with hydrate saturation into well‐known Biot‐type poro‐elastic models for two‐ and three‐phases for low (Sh < 12%) and high (Sh > 12%) hydrate saturations respectively. By accounting for changes in permeability linked to hydrate morphology, the models were able to describe the Vs and Qs−1 observations. We found that the first Qs−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 Qs−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 Vs and Qs−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.
Highlights
Vast amounts of methane, a potent greenhouse gas, are trapped in natural methane hydrate deposits that are highly sensitive to global warming
We show that direct estimates of the permeability of hydrate-bearing geological formations are possible from remote measurements of shear wave velocity (Vs) and attenuation (Qs−1)
We found that the first Qs−1 peak is caused by a reduction of permeability during hydrate formation associated with a transition from pore-floating to pore-bridging hydrate morphology; the second Qs−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
Summary
A potent greenhouse gas, are trapped in natural methane hydrate deposits that are highly sensitive to global warming. The inter-pore hydrate framework behaves as a single solid phase which interlocks with the sediment solid phase, and results in a significant increase in shear coupling between the two solid phases, and an increase in shear wave velocity (Sahoo, Madhusudhan, et al, 2018) Another aspect of gas hydrate which affects the physical properties is homogenous or heterogeneous distribution of hydrates in sediments at larger scale within the area of interest (e.g., Dai et al, 2012). We aim to combine these models and discover whether the permeability of hydrate bearing sediments can be inferred from geophysical properties To this end, we analyzed experiment data to see if the permeability of gas hydrate bearing sandstone can be inferred from shear wave measurements. The resulting models provide a way to interpret the permeability of hydrate deposits from S-wave seismic survey and well log data
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