Magnetic Resonance Imaging of Methane - Carbon Dioxide Hydrate Reactions in Sandstone Pores

Abstract

Abstract Formation and growth of methane hydrates in porous sandstone was monitored using Magnetic Resonance Imaging (MRI). A series of 3-D MRI images collected during these experiments illustrated patterns of hydrate growth. Calibrated MRI intensity changes that occured during the hydrate growth correlated with methane gas consumption and gave dynamic and quantitative in-situ information on hydrate formation rate and spatial distribution of the hydrate formed. Gas permeability was measured at various hydrate saturations and during hydrate growth. Experimentally it was verified that methane hydrate in porous sandstone spontaneously converted to CO2 hydrate when exposed to liquid CO2at high pressure and low temperature. It has experimentally been determined that without heating, an exchange process between CO2 and methane occured allowing the injected CO2 to be stored as hydrate resulting in spontaneous production of methane, with no associated water production. The MRI images provided quantitative information on the methane production rates and amounts of methane released during the CH4-CO2 hydrate exchange reaction. Thermodynamic simulations based on Phase Field theory supported the measured results and predicted similar methane production rates observed in several reproduced experiments. Introduction Light hydrocarbon gases and water at high pressures and low temperatures may form hydrate structures, i.e. clathrates where water molecules encapsulate hydrocarbon molecules in rigid lattices. This solid occurs naturally at deep ocean floors, below sub marine bottoms and in permafrost regions. Hydrates have recently produced a lot of interest for different reasons; the net flux of methane released from underground hydrate accumulations reaching the atmosphere represents an environmental concern since methane is a more aggressive greenhouse gas (∼ 25 times) than CO2, and serious concern is related to the stability of these hydrate formations. Changes in local conditions of temperature, pressure or surrounding fluids or minerals change the dynamics of the system and might eventually lead to massive dissociation. The environmental impacts related to large amounts of methane released to the atmosphere may have dramatic effects on the greenhouse scenario. Another aspect of hydrate concern is the formation and dissociation of hydrates in relation to petroleum production activities; representing challenges for safe drilling operations and petroleum production and efficient gas transport. Increased use of sub-sea installations and ocean floor constructions raises concern to safety issues as discussed by Yakushev & Collett 1, 2. Finally, due to the condensed presence of natural gas in hydrate layers, 1 cu. ft. of hydrate corresponds to approximately 163 cu. ft at atmospheric conditions, hydrate accumulations are viewed as potentially large energy resources. The abundance and locations of the natural gas hydrate reserves covers all continents with the total energy corresponding to natural gas entrapped in hydrate reservoirs estimated to be more than twice the energy of all known energy sources of coal, oil and gas 3. Conventional production of natural gas from hydrate accumulations by pressure depletion in an underlying gas zone generates a large amount of associated water production, representing a significant environmental problem and limits the economic potential. Thermodynamically CO2 hydrate formation is more favorable compared to methane hydrate formation and this fact has initiated a study to determine if methane may be released from methane hydrates by exposing methane hydrate to liquid CO24. In the present laboratory study Magnetic Resonance Imaging (MRI) has been used to follow the dynamics of hydrate formation and exchange in porous sandstone. The paper emphasizes first the experimental procedures developed to form methane hydrate in porous sandstone while monitoring the dynamic process with 3D imaging at a millimetre scale and then determine the production of methane from methane hydrate, when exposed to liquid CO2; without external heating.