Methane Hydrate Film Thickening in Porous Media

Abstract

Methane hydrate formation accompanied by upward migration of methane in sediments was studied. Methane hydrate film forms initially along the interface between the gas and surrounding pore fluid when methane reaches the hydrate stability zone. Subsequent film thickening and related mass transfer are critical but have not been studied previously. Experimental and theoretical studies were performed to investigate the film thickening and rate-limiting mechanisms. Initial gas density affects the stability of the growing hydrate film, and mass transfer through hydrate film is proposed as a rate-limiting step for hydrate growth.Plain Language Summary Gas hydrate films can quickly form when gas and water meet each other given proper pressure and temperature. Yet, little is known about what occurs afterward. We use both experimental and theoretical approaches to investigate slow transformation of methane bubbles to methane hydrate. Two experiments were conducted using gas densities lower or higher than methane density in methane hydrate. In the lower-density case, the initially formed hydrate film breaks, which lets water invade into the gas bubble. By contrast, the hydrate film maintains its integrity in the higher-density case, and the slowly formed hydrate takes over both the spaces initially occupied by gas and water. Observations indicate that both water transfer into the gas bubble and outward gas transfer from the bubble occur through hydrate films. A theoretical analysis explores these processes based on the slow mass migration via seemingly solid hydrate: (1) gas diffusion driven by gas concentration gradient in hydrate and (2) water flow via conduits within polycrystalline hydrate driven by pressure gradients. The mass transfer rates match experimental observations and agree with previous studies. Results suggest that natural hydrate formed from free gas at significant depth could have residual gas inclusions for millions of years.

Pore‐Scale Investigation Of Methane Hydrate‐Bearing Sediments Under Triaxial Condition

“…This study focuses on the influence of hydrate on the geomechanical behavior of hydrate‐bearing sediments. Detailed descriptions on the hydrate formation process and resultant hydrate morphology in pore spaces are covered in previous works (Lei & Seol, ; Lei, Seol, Choi, & Kneafsey, ; Lei, Seol, & Myshakin, ). CT scan configuration and CT image processing are in Lei et al ().…”

Timescales and Processes of Methane Hydrate Formation and Breakdown, With Application to Geologic Systems

“…In practice, gas hydrate growth rates even in passive systems tend to be faster than would be anticipated by assuming growth occurs only via diffusion of methane or water through the full thickness of the gas hydrate film. Several studies have shown that the gas hydrate interfaces tend to crack as they form (Jin et al, 2012; Lei et al, 2019; Tohidi, Østergaard, et al, 2001; Warzinski et al, 2014), creating pathways for more rapidly combining methane and water to form additional gas hydrate.…”

Hydrate is a Nonwetting Phase in Porous Media

“…Even though hydrate is solid, it can move through dissolution and reformation (e.g., Liu & Flemings, 2011). Regardless of how hydrate nucleates in a system, hydrate growth behaves similarly to a mobile, nonwetting phase that will rearrange itself to minimize energy through Ostwald ripening type behavior (e.g., Chen & Espinoza, 2018; Lei, Seol, & Myshakin, 2019). This movement is driven by minimizing free energy, and the hydrate/water interface minimizes its energy by deforming to a state of constant curvature.…”