Controls On Methane Expulsion During Melting Of Natural Gas Hydrate Systems
Project Number
DE-FE0010406
Last Reviewed Dated
Goal
The project goal is to predict, given characteristic climate-induced temperature change, the conditions under which gas will be expelled from existing accumulations of gas hydrate into the shallow ocean or directly to the atmosphere. When those conditions are met, the fraction of the gas accumulation that escapes and the rate of escape shall be quantified. The predictions shall be applicable in Arctic regions and in gas hydrate systems at the updip limit of the stability zone on continental margins. The behavior shall be explored in response to both longer term changes in sea level rise (e.g., twenty-thousand years) and shorter term changes due to atmospheric warming by anthropogenic forcing (decadal time scale).
Performer(s)
University of Texas at Austin, Austin, TX 73713-7726
Background
The central hypothesis proposed is that hydrate melting (dissociation) due to climate change generates free gas that can, under certain conditions, propagate through the gas hydrate stability zone and vent at the seafloor or into the atmosphere. Gas venting through the regional hydrate stability zone is accomplished by alteration of the regional equilibrium conditions by increased salinity and heat due to hydrate formation. This research will explore the controls on whether methane reaches the seafloor (or atmosphere) as the original hydrate deposit dissociates and how to determine the magnitude of these fluxes. Previous efforts to simulate hydrate formation and disassociation have shown that coupling all physical processes is critical to understanding the macro-scale process of hydrate melting.
Impact
Equilibrium thermodynamics will be coupled with conservation of mass and energy with multiphase transport models in geologically heterogeneous sediments to simulate macro-scale behavior. Conservation of mass will involve methane, water, and salt; conservation of energy will use the latent heat of formation of hydrate. Based on previous efforts, these models will illustrate distinctive behaviors that have important first-order controls on how degassing occurs during warming. This research includes laboratory experiments explicitly designed to validate the models, which will illustrate how much confidence is warranted in the model predictions, and thus, greatly increasing their impact. The technology to be developed in this project could provide an essential component to the portfolio of technologies and knowledge needed to understand the impact of climate change on hydrate degassing.
Accomplishments (most recent listed first)
A fully coupled one-dimensional model that simulates hydrate formation and dissociation, water ice formation and melting, heat and mass transport, and assumes equilibrium thermodynamics at the laboratory and field scale was developed. Researchers have applied this model to study ocean warming on continental margins and atmospheric warming in Arctic permafrost regions. They have also applied the model to simulate laboratory experiments. They have successfully simulated the formation and dissociation of hydrate and measured resistivity and density under three-phase conditions.
Researchers formed and dissociated hydrate in saline pore water at a range of temperature conditions in the laboratory.They mixed fine sand with pore water of seawater salinity at a 51 percent water saturation and 35 percent porosity. Gas flows into or out of the sample and pressure is held constant at 1,000 pounds per square inch. The temperature is reduced in a stepwise fashion to 0.5°C and then increased. Hydrate forms on Day 8, which is illustrated by an increase in density. The measured hydrate saturation is slightly less than the model prediction. During warming, the hydrate dissociates immediately with an increase in temperature. This was interpreted such that at each temperature increment enough hydrate forms or dissociates to keep the system close to equilibrium with each phase present. With greater cooling, more hydrate forms and salinity is elevated; thus, the three-phase condition occurs at a lower temperature. An increase in density associated with hydrate formation was also observed.
Researchers documented three hydrate-bearing sites that have elevated salinities and three-phase equilibrium zones. By combining Archie’s equation with core-derived salinity data, researchers found that well NGHP Site 01-10A (Krishna-Godavari Basin, offshore India) has high hydrate saturations and high in-situ salinities between 30 and160 mbsf. The salinity necessary for three-phase equilibrium across the gas hydrate stability zone was estimated using a thermodynamic model.
Researchers have developed an analytical solution describing the multiphase multicomponent flow and transport in a hydrate system based on method of characteristics. It is the first analytical solution quantifying the hydrate system development in the literature. This solution provides a convenient way to understand the gas-rich hydrate system in the field low dip angle sand layers and in laboratory experiments.
Researchers successfully formed hydrate through the intrusion of gas into a coarse-grained sample and interpreted that hydrate formed with front velocity as predicted by the MOC model (0.0299cm/hr). The bulk saturation of the hydrate behind the front is 0.74 and it has an in-situ salinity of 0.27wt% NaBr (3.86 times the initial salinity). In this experiment the in-situ salinity was elevated to three-phase equilibrium conditions through salt exclusion.
Researchers have developed a fully-coupled multiphase multicomponent fluid flow and heat transport model that includes the effect of hydrate and ice formation. The thermodynamic equilibrium-based model emphasizes the role of salinity in both ice and hydrate dynamics. They investigated the intertwined ice and hydrate formation and dissociation in the Arctic region during climate change. This model is used to explore the possible formation mechanisms of intra- and sub-permafrost hydrate reservoirs in the Arctic region. The model is applied to investigate the stability of the sub-permafrost hydrate deposits at Mallik during future global warming with different ice saturations within the permafrost.
Researchers have extended their one dimensional model to two dimensions and are now simulating geologically realistic scenarios that include dipping permeable layers that extend across the hydrate stability zone. The results illustrate how methane can be focused along permeable layers into the hydrate stability zone.
Current Status
(November 2015)
The project has been completed. The final report is available below under "Additional Information".
