Towards the generic conceptual and numerical framework for the simulation of CO2 sequestration in different types of georeservoirs

In this paper, conceptual and numerical modeling of coupled thermo-hydromechanical (THM) processes during CO2 injection and storage is presented. The commonly used averaging procedure combining the Theory of Mixtures and the Concept of Volume Fractions serves as background for the complex porous media approach presented here. Numerical models are based on a generalized formulation of the individual and overall balance equations for mass and momentum, as well as, in non-isothermal case, the energy balance equation. Within the framework of a standard Galerkin approach, the method of weighted residuals is applied to derive the weak forms of governing equations. After discretizing spatially these weak forms, a system of nonlinear algebraic equations can be obtained. For the required time discretization a generalized first order difference scheme is applied, linearization is performed using Picard or Newton-Raphson methods. The corresponding models are implemented within the scientific open source finite element code OpenGeoSys (OGS) developed by the authors, which is based on object oriented programming concepts. This assists the efficient treatment of different physical processes, whose mathematical models are of similar structure. Thus, the paper is mainly focused on a generic theoretical framework for the coupled processes under consideration. Within this context, CO2 sequestration in georeservoirs of different type can be simulated (e.g., saline aquifers, (nearly) depleted hydrocarbon reservoirs)

Numerical analysis of CO2 injection into deformable saline reservoirs: benchmarking and initial observations

A numerical scheme is presented for the solution of coupled multiphase hydromechanical problems in deformable porous media. Model verification is conducted against analytical solutions for multiphase flow with capillarity and coupled multiphase hydromechanical consolidation. A hybrid monolithic(flow)-staggered(mechanical) numerical solution scheme is verified to be stable for real materials, provided proper error control is placed on the hydraulic to mechanical iteration and the time-stepping scheme. Initial results of CO2 injection into an aquifer-caprock system do not show significant differences in CO2 migration rate between flow-only and hydro-mechanical simulations for conservative injection scenarios. However, the results highlight important regions in the reservoir with regard to potential mechanical failure and caprock integrity and suggest the need for further analysis

Numerical simulation of thermal-hydrologic-mechanical-chemical processes in deformable, fractured porous media

Joshua Taron, Derek Elsworth and Ki-Bok Mi

Simulating thermal-hydrologic-mechanical-chemical evolution surrounding fluid injection in a fractured porous geothermal reservoir

© 2006: American Geophysical UnionComputational analysis is conducted on the coupled thermal-hydrologic-mechanical-chemical (THMC) behavior of a stimulated EGS geothermal reservoir. Numerical analyses utilize a newly developed simulator capable of examining THMC processes in fractured porous geologic media. The simulator links the thermal-hydrologic- chemical (THC) computational code TOUGHREACT with the mechanical (M) capability of FLAC3D, where the response of pore fluid pressure to mechanical disturbance is treated as an undrained system and mineral precipitation/dissolution generates porosity and permeability change within each dual-permeability continuum. Non-linear permeability response to thermal-hydrologic-mechanical (THM) mechanisms is accommodated via embryonic mechanical and transport constitutive laws, and is considered to act in union with permeability changes associated with the removal or addition of minerals within the system. This construct is applied to the geometry of an injector-withdrawal doublet within the Coso Geothermal field, where in situ stress conditions, thermal state, and mineralogical composition at 3000m depth are extracted from recorded field data. Initial results for feasible parametric settings show that permeability reduction in the vicinity of a cool (80°C) injection well may be significant, within an order of magnitude, and accompanied by large (MPa) changes in the stress field throughout the reservoir for imposed boundary conditions of constant stress.Taron, J., Min, K. and Elsworth, D

Mechanisms for rainfall-concurrent lava dome collapses at Soufriere Hills Volcano, 2000-2002

The evolution of rainfall-concurrent dome collapses at Soufrière Hills volcano is followed using a limit equilibrium model for rain infiltration into a hot lava carapace. Magma infusing into the dome both supplies heat and builds the slopes. The dome rocks are cooled by episodic rain infiltration and climatic cooling. Rainfall infiltrates fractures that develop in the hot dome carapace, occludes the void space, and staunches effusive gas flow. Gases may originate from juvenile de-gassing of the dome interior, or result from the vaporization of infiltrating water. Gas pressures build in cracks blocked-off by rain, and may destabilize the dome. The effects of dome growth, heating by magma infusion, and cooling by rain infiltration and climatic influences, are combined to follow the growth of the dome towards ultimate collapse. For a fixed suite of strength and transport parameters, and for measured magma influx rates, the evolution of instability may be followed. The evolving factor of safety tracks the observed March 2000 and July 2001 rainfall-concurrent collapse events, which evolve over months. However, the resolution of the hindcast is unable to discriminate between the effects of closely-timed rainfall events (order of hours). The heightening of the dome is shown to exert the principal influence on average slopes and in the evolution of instability. Collapse removes the over-heightened dome, and temporarily restores stability

Implications of magma transfer between multiple reservoirs on eruption cycling

Volcanic eruptions are episodic despite being supplied by melt at a nearly constant rate. We used histories of magma efflux and surface deformation to geodetically image magma transfer within the deep crustal plumbing of the Soufrière Hills volcano on Montserrat, West Indies. For three cycles of effusion followed by discrete pauses, supply of the system from the deep crust and mantle was continuous. During periods of reinitiated high surface efflux, magma rose quickly and synchronously from a deflating mid-crustal reservoir (at about 12 kilometers) augmented from depth. During repose, the lower reservoir refilled from the deep supply, with only minor discharge transiting the upper chamber to surface. These observations are consistent with a model involving the continuous supply of magma from the deep crust and mantle into a voluminous and compliant mid-crustal reservoir, episodically valved below a shallow reservoir (at about 6 kilometers)

Stress- and Chemistry-Mediated Permeability Enhancement/Degradation in Stimulated Critically-Stressed Fractures

This work has investigated the interactions between stress and chemistry in controlling the evolution of permeability in stimulated fractured reservoirs through an integrated program of experimentation and modeling. Flow-through experiments on natural and artificial fractures in Coso diorite have examined the evolution of permeability under paths of mean and deviatoric stresses, including the role of dissolution and precipitation. Models accommodating these behaviors have examined the importance of incorporating the complex couplings between stress and chemistry in examining the evolution of permeability in EGS reservoirs. This document reports the findings of experiment [1,2] and analysis [3,4], in four sequential chapters

Rural communities experience higher radon exposure versus urban areas, potentially due to drilled groundwater well annuli acting as unintended radon gas migration conduits

Abstract Repetitive, long-term inhalation of radioactive radon gas is one of the leading causes of lung cancer, with exposure differences being a function of geographic location, built environment, personal demographics, activity patterns, and decision-making. Here, we examine radon exposure disparities across the urban-to-rural landscape, based on 42,051 Canadian residential properties in 2034 distinct communities. People living in rural, lower population density communities experience as much as 31.2% greater average residential radon levels relative to urban equivalents, equating to an additional 26.7 Bq/m3 excess in geometric mean indoor air radon, and an additional 1 mSv/year in excess alpha radiation exposure dose rate to the lungs for occupants. Pairwise and multivariate analyses indicate that community-based radon exposure disparities are, in part, explained by increased prevalence of larger floorplan bungalows in rural areas, but that a majority of the effect is attributed to proximity to, but not water use from, drilled groundwater wells. We propose that unintended radon gas migration in the annulus of drilled groundwater wells provides radon migration pathways from the deeper subsurface into near-surface materials. Our findings highlight a previously under-appreciated determinant of radon-induced lung cancer risk, and support a need for targeted radon testing and reduction in rural communities