A new application of a finite element heat and mass transfer numerical modeling code (FEHM) to heat and fluid circulation in lava domes
Abstract
Lava domes have been well-characterized in terms of their surface structure and activity, but there is much to be learned about their internal structure and geothermal systems. Even when a lava dome is no longer actively erupting, subsurface studies are often difficult to conduct; lava domes are highly complex structures, but their rugged nature often precludes systematic drilling and/or geophysical surveys. Because of this, we know little about the internal geothermal activity that may still contribute to both hazards and opportunities for exploitation of mineral deposits and hot groundwater. Despite the difficulty of studying the interior of lava domes directly, numerical modeling can still provide insights into the behavior of their geothermal systems. Lava domes have the potential to be highly transmissive structures, and the presence of hot springs in the vicinity of lava domes (Santiaguito in Guatemala, La Soufriere on Guadeloupe) suggests that water circulation may be an important process in post-eruptive dome evolution. FEHM, a heat and mass transfer modeling code developed at Los Alamos National Laboratory (fehm.lanl.gov) is an ideal tool to study fluid and gas circulation in geologic structures. FEHM was developed for subsurface reservoir modeling (originally for the Hot Dry Rock geothermal project) and is capable of dealing with both high- (magmatic) and low-temperature fluids. In this study, FEHM has been used in combination with a LANL-developed grid-generating utility (LaGriT) to create an idealized model of water circulation in a saturated lava dome. Multiple material regions are used to represent the dome core, outer talus layer, conduit, and volcanic substrate. Material properties (such as permeability, porosity, density, etc.) were chosen from a combination of literature review and sensitivity testing using a simplified dome geometry and a continuum modeling approach that accounts for fractures (Equivalent Porous Medium) was used when applying those properties to dome material regions. Further modifications to the dome geometry and material properties were made to better simulate specific lava domes, including Santiaguito in Guatemala and Unzen in Japan. Preliminary results suggest that FEHM produces a reasonable simulation of fluid movement in a saturated, post-eruptive (cooling) lava dome, with recharge from meteoric water, over periods of years to decades. This model setup is applicable to the older lava domes at Santiaguito and the dome at Unzen. The simulations presented were restricted at this point to lower temperature materials (< 250°C) to avoid computationally more intensive phase change. A grid based on the geometry of the older Santiaguito lava domes has produced temperature gradients and fluid flow paths in a similar location to hot springs at the real dome complex.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.V13A2813B
- Keywords:
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- 1849 HYDROLOGY / Numerical approximations and analysis;
- 8424 VOLCANOLOGY / Hydrothermal systems