Micro to Macro scale geophysical monitoring: A case study for reducing uncertainty with geophysical gas hydrate quantification.
Gas hydrates are ice–like compounds found in marine sediments and permafrosts. A significant fraction of all known hydrocarbons in nature is in the form of hydrate. Gas hydrates are a potential energy resource, with possible roles in seafloor slope stability and climate change. As such, improved geophysical methods are needed to identify and quantify in situ natural hydrates to better study their potential impacts. Current estimates of the distribution and volume of gas hydrates vary widely, by orders of magnitude, largely because of uncertainties in our understanding of how gas hydrate affects geophysical properties of host sediments and hence inversion results.
We conducted multi frequency (ultrasonic, sonic, and seismic) geophysical (P and S wave velocity and attenuation) experiments; also used electrical resistivity tomography and high resolution synchrotron imaging to understand properties of gas hydrate. We found that not all the gas formed hydrate, even when the system was under hydrate stability conditions with excess water. The synchrotron CT results suggest that the dominant mechanism for co-existing gas is the formation of hydrate films on gas bubbles; these bubbles either rupture, releasing trapped gas, or remain trapped within an aggregate of hydrate grains. From a geophysical remote sensing perspective, such co-existing gas could cause errors in hydrate saturation estimates from electrical resistivity as both gas and hydrate are resistive compared to saline pore fluid. Hydrate starts forming in the pore-floating morphology (where hydrate grains are surrounded by brine) and evolves into the pore-bridging morphology (where hydrate connects mineral grains). Eventually, hydrate from adjacent pores joins and forms a pore hydrate framework, interlocking with the sand grain framework and separated by thin water films. We related these changes in morphology to our elastic wave measurements using the HBES (Hydrate Bearing Effective Sediment) rock physics model. 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 implemented changes in permeability with hydrate saturation into well-known Biot-type poro-elastic models. 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.
Such multi scale and multi property observations can help us in understanding several complicated fluid flow systems in the subsurface, like carbon seqruestration, gas chimney, etc.