THERMO-HYDRO-MECHANICAL MODELING OF WORKING FLUID INJECTION AND THERMAL ENERGY EXTRACTION IN EGS FRACTURES AND ROCK MATRIX Page: 3 of 11
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IAPWS, 2008), improvements in the ability
to represent heterogeneity and reservoir
geometry (Chacon and Lapenta, 2006; Kirk
et al., 2006), and more robust computational
schemes (Heroux et al., 2008; Knoll and
Keyes, 2004; McHugh and Knoll, 1994). In
recent years, engineered (or enhanced)
geothermal systems (EGS) have been
proposed as a way to bring additional
geothermal resources online; however,
simulating EGS reservoir creation and
operation poses additional, and very
significant, computational challenges, as
EGS is based on dynamic changes in
fracture permeability that the current
generation of continuum or dual-continuum
hydrothermal models are ill-equipped to
Porous media deformation
and fracturing %
Water and steam
flow Energy transport
Figure 1. Strong nonlinear couplings
related to geothermal systems,
after Ingebritsen et al. (2008).
Multiphase fluid flow, energy transport,
geomechanical deformation (and potential
fracturing), and geochemically reactive
transport in geothermal reservoirs are indeed
a multiphysics problem in which controlling
physics are all tightly coupled together
(Figure 1). The feedbacks between flow-
transport, equations of state, and changes of
permeability and porosity due to
geomechanical deformation and
geochemical reactions render the system of
governing partial differential equations
strongly nonlinear. The ability to accurately
model these tightly-coupled physics that
control geothermal reservoir creation and
operation are paramount for large scale
implementation of EGS.
Examining coupled physics for fluid flow,
energy transport, and geomechanical
deformation is a relatively new area for the
geothermal community; however, simulating
coupled problems has been an important
topic of study in the reactive transport
community for decades. Yeh and Tripathi
(1989) and Steefel and MacQuarrie (1996),
cite three major approaches that differ in the
way coupling transport and reaction have
been considered for reactive transport
modeling: (1) Globally Implicit Approach
(GIA) that solves all governing nonlinear
equations simultaneously at each time step
using various forms of Newton's method,
(2) sequential iteration approach (SIA) that
subdivides the reactive transport problem
into transport and reaction sub-problems,
solves them sequentially, and then iterates,
and (3) sequential non-iteration approach
(SNIA) that solves the transport and reaction
problems sequentially without iteration,
which is often referred as operator-splitting.
The operator-splitting approach is perhaps
the simplest to implement and requires the
least computational resources in terms of the
memory and CPU time; thus, it became the
method of choice for subsurface reactive
transport modeling during the past three
decades. Examples of reactive transport
simulators that utilize the operator-splitting
approach include the widely used STOMP
(White and McGrail, 2005) and
TOUGHREACT (Xu et al., 2004) codes.
One recent geothermal example of the SNIA
approach is presented by Rutquist et al.
(2002), in which the widely used flow and
heat transport simulator TOUGH2 (Pruess
et al., 1999) is coupled to the commercial
rock mechanics simulator FLAC (Itasca,
1997) via input files.
One potential drawback of operator-splitting
approaches is the splitting error, when the
physics (either reactions-transport or flow-
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Podgorney, Robert; Lu, Chuan & Huang, Hai. THERMO-HYDRO-MECHANICAL MODELING OF WORKING FLUID INJECTION AND THERMAL ENERGY EXTRACTION IN EGS FRACTURES AND ROCK MATRIX, article, January 1, 2012; Idaho Falls, Idaho. (digital.library.unt.edu/ark:/67531/metadc835377/m1/3/: accessed January 16, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.