CO2 Phase Separation During Flow in Fractures in Long Valley Caldera: Geological Evidence and Numerical Model
Abstract
The analysis of microstructures and fluid inclusions in quartz fillings of fractures cored in the Long Valley Exploratory Well (LVEW) show evidence of episodic CO2 phase separations. These fractures transect the basement underneath Long Valley Caldera and record part of its hydrothermal history. Quartz microstructures in sealed fractures exhibit alternating brittle and plastic deformation consistent with rapid loading by seismic events followed by creep during stress relaxation. The composition of fluid inclusions found in the quartz show that the fluids percolating the fractures were of low salinity. The variable composition of individual inclusions and the narrow range of bulk compositions suggests trapping from an effervescent system with a bulk CO2 molar fraction of 0.03 to 0.07. Homogenization temperatures are typically 300 to 350° C. Decompression of a saturated pore fluid induced by a seismic event could cause phase separation. The resulting effervescence diminishes the density of the fluid column further enhancing flow. Numerical models of phase separations in the H2O-CO2 system is addressed with TOUGH2. The purpose of the numerical simulations is to investigate the effects of phase separation (length and time scales) on the flow field. The model geometry consists of Long Valley basement rocks with fractured zones defined by vertical porous zones with enhanced permeability. Thermal conductivity and unfractured permeability and porosity are taken from experimental data on LVEW cores. The initial temperature conditions and basal heat input into the model was obtained from previous numerical models of the effect of shallow intrusions on temperature and flow field. The fluid is injected at the bottom of the model and the evolution of phase change with decreasing pressure as the fluid moves up is observed. The same model with pure water is used for comparison.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2002
- Bibcode:
- 2002AGUFM.U21A0008H
- Keywords:
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- 3210 Modeling;
- 8130 Heat generation and transport;
- 8135 Hydrothermal systems (8424);
- 8424 Hydrothermal systems (8135);
- 8499 General or miscellaneous