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 (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.