|Controls On Methane Expulsion During Melting Of Natural Gas Hydrate Systems
||Last Reviewed 6/24/2014
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 due to atmospheric warming by anthropogenic forcing (decadal time scale).
University of Texas at Austin, Austin, TX 73713-7726
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. Gas venting through the regional hydrate stability zone is accomplished by alteration of the regional equilibrium conditions (creation of three-phase 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 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.
Equilibrium thermodynamics will be coupled with conservation of mass (for methane, water, salt) and energy (accounting for the latent heat of formation of hydrate) with multiphase transport models in geologically heterogeneous sediments to simulate macro-scale behavior. Based on previous efforts, these models will illustrate important behaviors that have important first-order controls on how degassing occurs during warming. This project includes laboratory experiments explicitly designed to (in)validate the models, which will illustrate how much confidence is warranted in the model predictions, greatly increasing their impact. Thus, the technology to be developed in this project could provide an essential component of the portfolio of technologies and knowledge needed to understand the impact of climate change on hydrate degassing.
- A fully coupled one-dimensional model that simulates hydrate 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 oceanic examples of ocean warming and develop 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 (black line ‘SH’, Fig. 1) and high in-situ salinities (red dots, Fig. 1) between 30 and160 mbsf. The salinity necessary for three-phase equilibrium across the gas hydrate stability zone (dashed line, Fig. 1) was estimated using a thermodynamic model. These results suggest that the in-situ conditions needed to maintain three-phase equilibrium are in Zones 2 and 4.
Figure 1. Estimated hydrate saturation and in-situ salinity in Well NGHP Site 01-10A (Krishna-Godavari Basin, offshore India)
Current Status (June 2014)
Researchers will apply the fully coupled 1-D dynamic hydrate model to (1) arctic examples of climate-induced warming to understand when and how gas is venting to the atmosphere and (2) laboratory scale problems to design laboratory experiments and better understand results.
The effect of warming, induced by thermal perturbation from above and below, on the formation of methane hydrate will be studied In a much larger laboratory apparatus (1 m vs. 0.1 m length).
Project Start: October 1, 2012
Project End: September 30, 2015
Project Cost Information:
DOE Contribution: $1,170,806
Performer Contribution: $311,000
NETL – John Terneus (John.Terneus@netl.doe.gov or 304-285-4254)
University of Texas at Austin – Peter Flemings (firstname.lastname@example.org or 512-475-9520)
Quarterly Research Performance Progress Report [PDF-271KB] - Period ending 12-31-2012