Date: June 7, 2012

Operated for the U.S. Department of Energy by the

Sandia Corporation

Defense Waste Management Programs

4100 National Parks Highway

Carlsbad, New Mexico 88220

To: Paul Shoemaker

From: R. Chris Camphouse (6211)

Technical Review: Dan Clayton

QA Review: Shelly Nielsen

Management Review: Janis Trone

Subject: An overview of the BRAGFLO two-phase flow parameters used to model the run-ofmine salt panel closures implemented in the PCS-2012 PA

Introduction

BRAGFLO Version 6.0 is the WIPP PA code used in the PABC-2009 to calculate brine and gas flows in and 

around the repository. Version 6.0 is the most recent qualified version of BRAGFLO, and is the code 

version that will be used for the PCS-2012 PA. BRAGFLO assumes an immiscible water/gas system 

(Nemer 2007). Immiscible fluids are not capable of mixing and have interfaces across which pressure 

discontinuities exist. This interfacial tension produces a capillary pressure between the water and gas 

phases. Capillary pressure modeling is a component of the overall flow modeling implemented in 

BRAGFLO.

This memo describes the two-phase flow model and the parameters associated with the run-of-mine 

(ROM) salt panel closures to be implemented in the PCS-2012 (Camphouse et al 2012). Generally, the 

two-phase flow flags used in BRAGFLO specify the relative permeability model and whether the capillary 

pressure model is to be used or not for a given material. Capillary pressure, which is a function of 

saturation and threshold pressure, is used to constrain the brine and gas pressures. Parameters 

associated with these models must be chosen carefully as they can cause BRAGFLO numerical 

difficulties. In particular, capillary pressure modeling in BRAGFLO typically causes numerical difficulties 

when materials undergo instantaneous changes in permeability. Capillary pressure modeling in 

BRAGFLO is often disabled for these kinds of materials. 

BRAGFLO Capillary Pressure Modeling

Several PA parameters are used as part of the capillary pressure modeling implemented in BRAGFLO. 

The threshold capillary pressure, PT, characterizes the point at which gas, the non wetting phase, can

start to flow in the porous media saturated with brine, and thus defines the capillary pressure at 100% 
brine saturation. The capillary pressure must exceed the threshold capillary pressure before the gas 

phase can start to drain the porous medium and flow.

There are two BRAGFLO material parameters that are used to calculate the threshold capillary pressure 

for a given material. These two parameters are named PCT_A and PCT_EXP. In BRAGFLO, the practice 

has been to correlate the threshold capillary pressure to permeability according to (Vaughn 1996)

PT = PCT_A * kPCT_EXP, (Equation 1)

where k is the brine permeability (m2) in the x direction. A capability was included in BRAGFLO during 

the preparation of the CCA that provided a dependence of threshold capillary pressure on dynamic 

changes in permeability. The database parameter KPT is used as a flag that provides an instruction to 

BRAGFLO to dynamically update the threshold capillary pressure as k changes temporally. Setting KPT = 

1 allows threshold capillary pressure to be updated if permeability changes in the material. The quantity 

PT is not updated when KPT is set to 0. KPT has been set to 0 for all BRAGFLO materials in the CCA and 

every PA performed since, so threshold capillary pressure has never been dynamically updated in WIPP 

PA. 

BRAGFLO solves a system of two mass balance equations for brine pressure and gas saturation as well as

two constraint equations for saturation and capillary pressure in the calculation of two-phase flow

(Nemer 2007). Instantaneous changes in permeability, due to material changes in a grid region, cause

difficulty in numerically satisfying the convergence criterion for the capillary pressure constraint 

equation. When the convergence criterion cannot be satisfied, the capillary pressure model is disabled 

for the responsible materials so that a convergent solution can be obtained from BRAGFLO. The flag for 

this is to set PCT_A = 0, which results in the capillary pressure component being disabled. When this 

occurs, the convention is to also set PCT_EXP = 0. 

BRAGFLO grid regions used to represent the ROM salt panel closures in the PCS-2012 will undergo 

instantaneous changes in permeability. Specifically, the permeability prescribed to these BRAGFLO grid 

regions will change instantaneously at 100 and 200 years as closure materials change from PCS_T1 to 

PCS_T2 and from PCS_T2 to PCS_T3 (Camphouse et al 2012). It is currently planned that capillary 

pressure modeling will be enabled for these materials. However if numerical convergence difficulties 

are encountered in BRAGFLO, then capillary pressure modeling will be disabled for them.

In addition to properties PCT_A and PCT_EXP, each material in BRAGFLO has a flag named CAP_MOD 

associated with it. A value of 1 or 2 is prescribed to CAP_MOD in WIPP PA for all BRAGFLO materials. As 

illustrated in the BRAGFLO Version 6.0 User’s Manual (see e.g. Figure 18 and Figure 19), capillary 

pressure can be unbounded as a function of brine saturation, depending on the relative permeability 

model employed. A value of CAP_MOD = 2 results in the capillary pressure being bounded above by a 

maximum value which is assigned to material property PC_MAX. PC_MAX is 1 x 108Pa by default for all 

materials in BRAGFLO. The convention used in WIPP PA is to set CAP_MOD = 1 for materials that have 

capillary pressure disabled, and to set CAP_MOD = 2 otherwise. That is, CAP_MOD = 1 for materials that 

also have PCT_A = 0. Thus, all BRAGFLO materials in which capillary pressure is used have a default 

value of 1 x 108Pa specified for PC_MAX (and have since the CCA). 
Property CAP_MOD is denoted as KPC inside the actual BRAGFLO Fortran code. There is a capillary 

pressure model number 3 (KPC = 3) included in the BRAGFLO Fortran code in which a minimum brine 

pressure is defined, denoted by P0_MIN. However, while the capillary pressure model 3 (CAP_MOD = 3) 

was developed and coded in BRAGFLO in preparation for the original certification of WIPP, it has never 

actually been used in any PA, including the original certification PA. Even though CAP_MOD = 3 has 

never been used in PA, BRAGFLO still expects a value to be specified for P0_MIN for all materials, so all 

BRAGFLO materials have a prescribed value of 1.01325 x 105Pa for property PO_MIN,

A summary of the parameters used in BRAGFLO capillary pressure modeling is provided in the following 

bulleted list and Table 1:

In WIPP PA, threshold capillary pressure is defined by Equation (1). The threshold capillary 

pressure is constant, based on the initial permeability of each material, and has never been 

dynamically updated as a function of temporal permeability changes. Material property flag 

KPT has been set to 0 for all BRAGFLO materials since the CCA.

Capillary pressure modeling is disabled by setting PCT_A = 0. The convention used in PA is to 

set PCT_EXP = 0 when PCT_A = 0.

The convention used in PA is to set CAP_MOD = 1 for materials that have the capillary pressure 

model disabled. CAP_MOD = 2 corresponds to capillary pressure being bounded above by 

PC_MAX. All BRAGFLO materials have a value of PC_MAX = 1 x 108Pa prescribed to them. 

CAP_MOD = 3 corresponds to a capillary pressure model that depends on the minimum brine 

pressure P0_MIN. This model has never been used in any PA, including the original 

certification PA. BRAGFLO still expects a value to be specified for P0_MIN for all materials, 

however. The value used for P0_MIN is 1.01325 x 105Pa.

Table 1: Parameters Used for BRAGFLO Capillary Pressure Modeling

Property Description

Value

Capillary Pressure Model 

Enabled

Value

Capillary Pressure Model 

Disabled

PCT_A Threshold Capillary Pressure 

Linear Parameter

Material Dependent 

Nonzero Value 0

PCT_EXP Threshold Capillary Pressure 

Exponential Parameter

Material Dependent 

Nonzero Value 0

CAP_MOD Capillary Pressure Model Number 2 1

PC_MAX

(Pa)

Maximum Allowable Capillary 

Pressure 1 x 1081 x 108

KPT Flag for Permeability Determined 

Threshold Capillary Pressure 0 0

P0_MIN

(Pa)

Minimum Brine Pressure for 

Capillary Pressure Model 3 

(CAP_MOD = 3 has never been 

used in PA)

1.01325 x 1051.01325 x 105
BRAGFLO Relative Permeability Modeling

The relative permeability model associated with a given material in BRAGFLO is specified by property 

RELP_MOD. BRAGFLO panel closure materials have been given a value of RELP_MOD = 4 in WIPP PA. 

RELP_MOD = 4 corresponds to the Second Modified Brooks-Corey Model illustrated in Figure 20 of the 

BRAGFLO Version 6.0 User’s Manual (Nemer 2007) and the discussion pertaining to that figure. In this 

model, capillary pressure is a function of the effective saturation which depends on the current brine 

saturation and both the residual brine and gas saturations. The model also depends on a pore size 

distribution parameter, denoted as property PORE_DIS in WIPP PA. The run-of-mine (ROM) salt panel 

closures implemented in the PCS-2012 PA, specifically materials PCS_T1, PCS_T2, and PCS_T3, will be 

assigned a value of RELP_MOD = 4.

The DRZ has two components for the PCS-2012 PA: DRZ_1 and DRZ_PCS. The DRZ region overall (i.e. 

material DRZ_1) used a value of RELP_MOD = 4 in the PABC-2009. DRZ_PCS models the healed DRZ 

above and below panel closures, and also uses RELP_MOD = 4. The material properties assigned to 

material DRZ_PCS are exactly the same as those used for material DRZ_1, with the exception of 

permeability. The permeabilities are defined by their logarithm in the x, y, and z directions, and denoted 

by PRMX_LOG, PRMY_LOG, and PRMZ_LOG, respectively (Stein 2002). Materials DRZ_1 and DRZ_PCS 

will also be used in the PCS-2012 PA, and will use the same relative permeability model (RELP_MOD = 4)

and permeability parameters as used in the PABC-2009. 

References

Camphouse, R., Gross, M., Herrick, C., Kicker, D., and Thompson, B. 2012. Recommendations and 

Justifications of Parameter Values for the Run-of-Mine Salt Panel Closure System Design Modeled in the 

PCS-2012 PA. Memo to WIPP Records Center dated May 3, 2012. Sandia National Laboratories.

Carlsbad, NM. ERMS 557396.

Nemer, M. 2007. WIPP PA User’s Manual for BRAGFLO, Version 6.0. Sandia National Laboratories. 

Carlsbad, NM. ERMS 545016. 

Stein, J. 2002. Parameter Values for new materials CONC_PCS and DRZ_PCS. Memo to Kathryn Knowles 

dated February 13, 2002. Sandia National Laboratories. Carlsbad, NM. ERMS 520524.

Vaughn, P. 1996. KPT Modeling Feature. Memo to Martin Tierney dated February 19, 1996. Sandia 

National Laboratories. Carlsbad, NM. ERMS 235271.