The Influence of Gas Hydrate Morphology on Reservoir Permeability and Geophysical Shear Wave Remote Sensing
Sourav Kumar Sahoo, Angus I. Best
Abstract
Abstract 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.