The influence of gas hydrate morphology on reservoir permeability and geophysical shear wave remote sensing

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 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.