(638g) Carbon Dioxide-Induced Liberation of Methane from Laboratory-Formed Methane Hydrates: A Pathway to Economical Carbon Sequestration

The presence of vast methane (CH₄) hydrate reserves around the globe are known for decades. Gas production by external stimulation techniques such as steam injection, gas depressurization or hydrate-inhibitor injection is well-established but an economical production method remains a challenge. In parallel, there is a sense of urgency to minimize carbon dioxide (CO₂) levels in the atmosphere that have exceeded 400 ppm. Both CH₄ and CO₂ gases form structure sI hydrates and at temperatures below 10ºC, CO₂ hydrates are more thermodynamically stable at lower pressures than CH₄ hydrates. Considering that burying CO₂ in deep formations is one of the carbon sequestration options being considered, the CH₄-CO₂ exchange in natural methane hydrate reservoirs may be a way to address both issues. The CH₄-CO₂ exchange option was attractive enough to warrant a commercial test by ConocoPhilips in 2012 in which over 24,210 m³ of CH₄ was released when a 23 mol% CO₂ in N₂ mixture was injected in a hydrate reservoir in Alaska. Carbon dioxide was preferred over N₂ in the hydrate phase, as about 70% of the N₂ gas injected was recovered, while only 40% of the 1376 m³ of CO₂ injected was retrieved. Thus, the CH₄-CO₂ exchange reaction merits further research.

In this talk, we will describe several laboratory-scale experiments to understand the CH₄-CO₂ hydrate exchange kinetics. Five hydrate formation-decomposition runs focused on CH₄-CO₂ exchange, two baselines and three with host sediments, were performed in a 200 mL high-pressure Jerguson cell fitted with two glass windows that allowed visualization of the time-resolved hydrate phenomenon. The data show that the induction time for hydrate appearance was the largest at 96 h with CH₄ while with CO₂, the time shortened by a factor of four. However, when the secondary gas (CO₂ or CH₄) was injected into the system containing preformed hydrates, the entering gas formed the hydrate phase instantly (within minutes) and no lag was observed. In a system containing host Ottawa sand (104 g) and artificial seawater (38 mL), the induction period reduced to 24 h. The CO₂ hydrate formation in a system that already contained CH₄ hydrates was facile and they remained stable, whereas CH₄ hydrate formation in a system consisting of CO₂ hydrates as hosts were initially stable, but CH₄ gas in hydrates quickly exchanged with free CO₂ gas to form more stable CO₂ hydrates. In all five runs, even though the system was depressurized, left for over a week at room temperature, and flushed with nitrogen gas in between runs, hydrates formed instantaneously, irrespective of the gas used. The â??memory effectâ? was exhibited with either gas, a result in contradiction with that reported previously in the literature. The facile CH₄-CO₂ exchange observed under temperature-pressure conditions that mimic naturally-occurring CH₄ hydrates show promise to develop a commercial carbon sequestration system.