Flow and transport simulations using T2CG1, a package of conjugate gradient solvers for the TOUGH2 family of codes Page: 65 of 131
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4.7. Test Problem 7: Vertical Section of WIPP
(2-D, Two-Phase, Isothermal with High Permeability Contrasts
and Phase Appearance and Disappearance)
4.7.1. Problem description
The WIPP (Waste Isolation Pilot Plant) is a U.S. Department of Energy (DOE)
research and development facility for the underground disposal of transuranic waste from
U.S. defense-related activities. The WIPP repository is located in a bedded salt formation
near Carlsbad, New Mexico, and is 655 m underground within the Salado formation,
which is brine saturated and consists of a large number of beds of relatively pure halite and
impure halite containing interspersed clays and polyhalite. Thin interbeds of anhydrite,
with associated underlying clay seams, are present in laterally extensive areas. Several of
the more prominent anhydrite interbeds have been designated as Marker Beds. The
repository horizon is separated by a few meters of halite from the overlying Marker Bed
138 and the underlying Marker Bed 139. A stratigraphic section on the Salado formation in
the vicinity of the repository is shown in Figure 22. The layer permeabilities vary by four
to five orders of magnitude.
The present simulation problem as designed by Webb and Phelan [1995] includes a
preliminary model of the repository and the surrounding detailed stratigraphy with explicit
representation of the various layers of pure halite, argillaceous halite, polyhalitic halite, and
anhydrite. The purpose of the model is to evaluate effects of gas generation and two-phase
flow on repository performance within a complex stratigraphy, and to compare with other
models that use a simplified representation of the stratigraphy.
At the WIPP, waste that is emplaced in the repository will generate gas due to
microbial degradation and anoxic corrosion. This gas generation may increase the pressure
sufficiently to drive gas and brine into the surrounding Salado Formation, which is brine-
saturated with a pore pressure of approximately 12 Mpa. The present simulation models
the gas generation rate by specification of gas sources. At the end of the simulation (=1000
years), gas generation has increased the room pressure sufficiently to drive gas out of the
room and into the Salado Formation through the more permeable layers indicated in Figure
23. These results are based on a preliminary model which has evolved considerably since
this problem was specified. The final version of this study is discussed briefly by Webb et
al. [1995].
The simulated domain consists of 1200 elements in a two-dimensional vertical
section grid. This is an isothermal two-phase flow problem, and is run with the EOS3
(water, air) module using NK = NEQ = 2 in block MULTI. This results in a total of N =
2400 equations. The file "PROB7" on the accompanying diskette contains the data inputs
for this run. Permeabilities in the problem were stratified, generally very low (ranging
between 10-23 and 10-15 m2), with extremely high permeability contrast in the vicinity of
the more permeable repository. Porosities of the various strata were very low and all equal
to 4 = 0.01. The initial pressures varied in the different strata. Capillary pressures and
relative permeabilities followed the Brooks and Corey [1966] relationships. These are not
available in the original TOUGH2 code, but coding them requires a minimum of effort.
For the user's convenience, we include the FORTRAN code for the Pc and kr relationships
(in the subroutines "PCAP" and "RELP" respectively) in the accompanying disk. The
added functions are clearly marked for easy idendification.
This type of problem is among the most challenging for iterative solvers because
elements of the Jacobian matrix along the same row may differ by many orders of
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Moridis, G. & Pruess, K. Flow and transport simulations using T2CG1, a package of conjugate gradient solvers for the TOUGH2 family of codes, report, April 1, 1995; California. (https://digital.library.unt.edu/ark:/67531/metadc624956/m1/65/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.