Fluid Composition and Kinetics of the in Situ Replacement in CH₄–CO₂ Hydrate System

Abstract

The exchange process between CO2 and methane hydrate has been observed in numerous laboratory experiments, computer simulations, and recently confirmed in a field test. Yet, to date there is no kinetic model capable of accurately predicting the swapping process at given fluid composition and p-T conditions. Major obstacles on the way to an adequate mathematical description are caused by the insufficient characterization of experimental environments and a nearly complete lack of information on the time-resolved composition of the two-phase fluid at the gas hydrate interface. Here we show that all necessary data can be provided by a combination of cryo-SEM, Raman, and neutron diffraction measurements that deliver accurate space-averaged, time-resolved in situ data on the CH4–CO2 exchange reactions at conditions relevant to sedimentary matrixes of continental margins. Results from diffraction are cross-correlated with ex situ Raman spectroscopy to provide reliable information on the preferential sites for CO2 and CH4 in the (partially) exchanged hydrate. We also show a novel approach based on scattering of neutrons to probe the fluid composition during the in situ replacement in a time-resolved, noninvasive manner. The replacement is seen as a two-step process including (1) a fast surface reaction parallel to a fast enrichment of the surrounding fluid phase with CH4 followed by (2) a much slower permeation-limited gas swapping between the gas hydrate and mixed ambient CH4–CO2 fluid. The main part of the replacement reaction takes place in the second stage. Based on our earlier experimental studies and existing literature we work toward a quantitative gas exchange model which elaborates the hole-in-cage-wall diffusion mechanism to describe the two-component gas replacement.

Experimental Study of Mixed Gas Hydrates from Gas Feed Containing CH4, CO2 and N2: Phase Equilibrium in the Presence of Excess Water and Gas Exchange

“…In the second phase the pressure in the cell is slowly increasing again, complemented by a parallel further, gentle CH4 increase in the vapor phase, indicating that a coupled gas exchange of CH4 by CO2 in the hydrate phase becomes the dominant process. This direct gas hydrate conversion has been described previously by the shrinking-core process [57]. Visual observation of the evolution of the different phases during gas hydrate crystallization (from left to right: directly after water injection; after stirrer has been set to 400 rpm; 1 min after the gas hydrate formation incipient; 48 min after the gas hydrate formation incipient).…”

“…In the second phase the pressure in the cell is slowly increasing again, complemented by a parallel further, gentle CH 4 increase in the vapor phase, indicating that a coupled gas exchange of CH 4 by CO 2 in the hydrate phase becomes the dominant process. This direct gas hydrate conversion has been described previously by the shrinking-core process [57].…”

From Microscale (400 μl) to Macroscale (425 L): Experimental Investigations of the CO2/N2‐CH4 Exchange in Gas Hydrates Simulating the Iġnik Sikumi Field Trial

“…According to literature, the interactions between a CO 2 phase and an initial CH 4 hydrate phase are described as a replacement of the encased CH 4 molecules with CO 2 molecules: The process is divided into two steps. The first step is a fast reaction at the interface between the hydrate grain and the CO 2 fluid, where a partial destruction of the hydrate cavities and/or rearrangements of water molecules results in a fast release of CH 4 molecules (Falenty et al, ; Ota et al, ). According to Falenty et al (), this destruction is limited to the surface before new, mixed hydrate forms.…”

“…Depending on the experimental conditions, the amount of CH 4 recovered from a hydrate phase due to CO 2 replacement varied from 15% in 800 hr (Hirohama et al, ) to 50% within 5 hr (Lee et al, ) to 60% in 300 hr (Kvamme et al, ), to give just few examples. It could be shown that the exchange of CH 4 with CO 2 initially takes place at the surface of the hydrate grain and goes along with a partial opening or rearrangement of the cavities (Falenty et al, ; Ota et al, ; Schicks et al, ). Laboratory experiments usually avoid free pore water that may transform into CO 2 hydrate when CO 2 is injected resulting in a decrease of permeability and thus reduced injectivity or clogging.…”

Introduction to Special Issue on Gas Hydrate in Porous Media: Linking Laboratory and Field‐Scale Phenomena

“…The Ignik Sukumi trial in 2012 (e.g., Boswell et al, ) involved such an apparent exchange of CO 2 for methane within the gas hydrates in a permafrost‐associated reservoir near Prudhoe Bay, Alaska. Both CO 2 and CH 4 form Structure I gas hydrates, and some studies have suggested that replacement of methane by CO 2 in the reservoir's gas hydrates may occur without dissociation of the original methane hydrate (Falenty et al, ; Ota et al, ). The results of Schicks et al () indicate that methane hydrate underwent conventional depressurization‐driven dissociation during injection of CO 2 and N 2 in the Ignik Sikumi test and imply that a mixed hydrate containing all three gases may have formed in the reservoir.…”