The set of equations is solved using a segregated approach with the PIMPLE algorithm in the OpenFoam framework. Abstract. Mud volcanism is a worldwide ...
Multiphase modelling of mud-volcanoes Simone Colucci1*, Mattia de' Michieli Vitturi1, Amanda Clarke2
1 INGV, sezione di Pisa; 2 School of Earth and Space Exploration, Arizona State University
2. Model equations where
z
OUTLET 3m
Momentum equation for phase 1 (liquid+dissolved gas) are, respectively, the mass fraction per unit volume per time of component i moving from phase 2 (exsolution of component i), and from phase 1 (condensation of component i). Total mass fraction per unit volume per time moving from phase 2 (exsolution)
Momentum equation for phase 2
Energy equation for phase 1 Total mass fraction per unit volume per time moving from phase 1 (condensation)
SIM A
Raoult law
The activity of LUSI mud-volcano has been well documented (Vanderkluysen et al., 2014), in particular in terms of mass flow rate of mud and gas species (water vapour, carbon dioxide and methane). This gives us the opportunity to compare the measured rates at LUSI with the numerical outputs in terms of mass fractions of each single component in the gas phase and mass fraction of each phase. component water vapour
0.07291 [Kg/s]
0.0028
methane gas
0.951 [Kg/s]
0.036
carbon dioxide gas
SIM B nx =16 ny =1000
1. Introduction
mass fractions in the gas phase 0.9612
z=0
250m
Energy equation for phase 2
mass flow rate 25.36 [Kg/s]
OUTLET 3m
1500m
Continuity equation for phase 1 (exsolved gas)
1500m
Mud volcanism is a worldwide phenomenon, classically considered as the surface expression of piercement structures rooted in deep-seated over-pressured sediments in compressional tectonic settings. The release of fluids at mud volcanoes during repeated explosive episodes has been documented at numerous sites and the outflows resemble the eruption of basaltic magma. As magma, the material erupted from a mud volcano becomes more fluid and degasses while rising and decompressing. The release of those gases from mud volcanism is estimated to be a significant contributor both to fluid flux from the lithosphere to the hydrosphere, and to the atmospheric budget of some greenhouse gases, particularly methane. For these reasons, we simulated the fluid dynamics of mud volcanoes using a newly-developed compressible multiphase and multidimensional transient solver in the OpenFOAM framework, taking into account the multicomponent nature (CH4, CO2 , H2 O) of the fluid mixture, the gas exsolution during the ascent and the associated changes in the constitutive properties of the phases. The numerical model has been tested with conditions representative of the LUSI, a mud volcano that has been erupting since May 2006 in the densely populated Sidoarjo regency (East Java, Indonesia), forcing the evacuation of 40,000 people and destroying industry, farmland, and over 10,000 homes. The activity of LUSI mud-volcano has been well documented (Vanderkluysen et al., 2014) and here we present a comparison of observed gas fluxes and mud extrusion rates with the outcomes of numerical simulations.
3. Computational domain
40m
Abstract
mass flow rate
mass fractions
1736 [Kg/s]
0.985
26.3839 [Kg/s]
0.015
Henry law
Initial conditions
phase
conduit (z-1m) liquid volume fraction: 10-5 temperature: 300K air: 0.99999 wt% of gas phase
x
Boundary conditions inlet gas volume fraction: 10-5 temperature: 400K velocity: 4.55x10-2m/s outlet pressure: 101300Pa
4. Simulation results Left panel: figures show the box of simulation A with the colour bar indicating the density; red colours indicate higher density (i.e., mud), blue colours lower density (i.e., gas)). It is worth noting the layering with the denser part on the bottom and the escaping gas. Central panel: plots of component mass fractions in the gas phase along the conduit of simulation B. Note the different evaporation/exsolution level of water, carbon dioxide and methane: methane starts to exsolve at the bottom, carbon dioxide in the middle and water evaporates at the top of the conduit. This is in accordance with the conclusions by Vanderkluysen et al. (2014) that gas bubble nucleation depths are >4000 m for methane and approximately 600 m for carbon dioxide and that the primary driver of the cyclic bubble-bursting activity is decompressional boiling of water, which initiates a few tens of meters below the surface, setting up slug flow in the upper conduit. Right panel: mass fractions of gas components in the gas phase as a function of time. The simulations outputs fit with measurements (see table in Introduction). In the subplot the mass of the mixture.
mass
SIM B
time (s)
horizontal scale 10X
Conclusions and future improvements Multiphase modelling represents a good tool to understand the complex dynamics of mud-volcanoes and relate intra-conduit dynamics to the gas measurements and the geochemical data collected. Future improvements will extend the simulation times in order to study periodic events. Furthermore we will test the simulations with different drag models and tau parameters.
time (s)
Aknowledgments The research leading to these results has received funding from Progetto INGV/UNIVOL-MIUR Premiali 2012, EU/FP7/VUELCO, the BAKRIE INITIATIVE FOR GEOLOGICAL HAZARDS at ARIZONA STATE UNIVERSITY.
References Vanderkluysen, L.; Burton, M.R.; Clarke, A.B.; Hartnett, H. & Smekens, J.-F. Composition and flux of explosive gas release at LUSI mud volcano (East Java, Indonesia) Geochem. Geophys. Geosyst., 2014, 15, 2932-2946.
