DISCLAIMER

This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsi- bility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Refer- ence herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recom- mendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

Long-Term Brine Migration Through an Engineered Shaft Seal System

D.G. Fryar', J.A. Beach', V.A. Kelley', and M.K. Knowles'

Abstract

The shaft seal system for the Waste Isolation Pilot Plant (WIPP) must provide a barrier to the migration of fluids within the shafts to prevent the release of contaminants to the accessible environment. To investigate the performance of the shaft seal system, a set of fluid flow performance models was developed based upon the physical characteristics of the WIPP shaft seal system and the surrounding geologic media. This paper describes the results of a numerical model used to investigate the long-term potential for brine migration through the shaft seal system. Modeling results demonstrate that the WIPP shaft seal system will effectively limit brine migration within the repository shafts.

I -

Introduction

The Waste Isolation Pilot Plant ONIpP) is a planned geologic repository for the permanent disposal of transuranic waste generated by the U.S. Department of Energy (DOE). The WIPP facility is located in southeastern New Mexico. Disposal regions are approximately 650 m below ground surface within the bedded evaporites of the Salado Formation. The repository horizon is serviced by four shafts which connect the repository complex with the surface. Prior to initiation of disposal activities, it must be demonstrated that WIPP will comply with applicable regulations which limit contaminant releases from the disposal region to the accessible environment.

The Salado Formation is overlain by a sedimentary sequence known as the Rustler Formation. The regulatory boundary for contaminant migration is the contact between the Rustler and Salado Formations. Site characterization activities have demonstrated that, in the vicinity of the repository horizon, the vertical hydraulic gradient between the Salado Formation and the overlying Rustler Formation is upward (Mercer, 1983). Overpressurization within the undisturbed Salado is not of concern due

'INTERA Inc., 6850 Austin Center Blvd., Suite 300, Austin, TX 7873 I 'Sandia National Laboratories, Repository Isolation S Albuquerque, NM 871 85-1 322

to its exceedingly low transmissivity. The shafts, however, represent potential conduits for upward migration of fluids and contaminants.

Recent efforts at the WIPP have included development of a multi-component shaft seal system. The WIPP shaft seal system is composed of four primary seal materials; compacted clay, compacted salt, salt-saturated concrete, and asphalt. The shaft seal system is designed to limit fluid flow such that the WIPP will remain in compliance with applicable regulations. Because the design will not be implemented for several years, and no field analogs exist fiom which to draw firm conclusions regarding the performance of the shaft seal system, a detailed numerical model of a representative shaft seal system was developed.

The modeling for this investigation was conducted using SWIFT I1 (Sandia - Waste Isolation, Flow, and Transport Code), Version 2F. SWIFT I1 was selected because it is versatile and has been extensively verified against analytical results.

Conceptual Model and Assumptions

A representative full-shaft, saturated-flow model was designed to examine the potential for and quantity of brine flow which could migrate upward through the shaft seal system in response to ambient pressure conditions present several hundred years after closure. The Air Intake Shaft (AIS) was chosen as a basis for the model geometry and stratigraphy, with the assumption that the AIS analysis is representative of the performance for the three other WIPP shafts. In addition, the model assumes that each shaft can be considered independently with no hydraulic interference between shafts. The model was initialized at non-hydrostatic conditions based on estimated undisturbed heads in the Rustler Formation and the maximum estimated formation pressure in the Salado Formation (Beauheim et al. (1 993), Brinster (1 99 I), LaVenue et al. (1 990)). Fluid flow due to temperature gradients was assumed to be negligible and was not considered in this model.

The full-shaft model was composed of the shaft seal components, a disturbed rock zone ( D E ) , and the intact host rock. The model was implemented using a cylindrical grid extending radially fiom the center of the shaft to an outer radius of 30.9 m. Vertically the model extended fiom the Shaft Station Monolith at an elevation of 387.4 m amsl to an elevation of 872.6 m amsl within the Rustler Formation. The grid was composed of 19 radial columns by 99 vertical layers. Grid spacing was selected so that the flow field within each seal component and each unit of the host formation was adequately represented without unduly compromising computational efficiency.

No-flow boundary conditions were imposed at the bottom and top of the model and along the vertical boundary of radial symmetry at the center of the shaft. Infinite aquifer Carter-Tracy boundary conditions were set at the outer edge of the modeled region. Therefore, it was unnecessary to extend the radial grid to large distances. The radial boundary was fixed at 10 shaft radii (30.9 m) to allow sufficient area for capture

2 Fryar et al.

f - j

of any vertical flow components in the host rock.

The Rustler Formation is composed of five members which are, from oldest to youngest, the unnamed lower member, the Culebra dolomite, the Tamarisk, the Magenta dolomite, and the Forty-niner. The unnamed lower member, the Tamarisk, and the Forty-niner have been subdivided into informal lithologic units. The unnamed lower member is composed of two units, the transitionhioturbated clastics and anhydrite Vmudstone 1. The Tamarisk includes anhydrite 2, mudstone 3 and anhydrite 3. The Forty-niner is also divided into three Units; anhydrite 4, mudstone 4 and anhydrite 5.

The Salado Formation is composed primarily of halite interbedded with polyhalite, anhydrite, and mudstone. Several of the anhydrite and polyhalite beds are very thin with thicknesses less than 0.5 m in many instances. In order to reduce the total number of model grid cells to a manageable level, several stratigraphic units within the Salado Formation were merged into combined units. Units were merged together based upon proximity, thickness, and lithology. Elevations of the shaft seal components and the stratigraphic units incorporated in the model are shown in Figure 1.

Effective DRZ Permeability

A DRZ forms around excavations in the bedded halite of the Salado Formation

the formation creeps into excavations (Stormont, 1990). The DRZ extends radially out from the shaft wall into the host formation. The DRZ is expected to have the following characteristics: (1) increased volume resulting from micro- or macro-fracturing, (2) increased fluid (gas or liquid) permeability, (3) decreased brine saturation, (4) decreased load-bearing capacity, and (5) decreased lithostatic pressure (Stormont, 1990; Van Sambeek et al., 1993). Because of these properties, the DRZ could act as a vertical path for brine and gas flow around the shaft seal.

,--- I immediately upon passage of the mining tools and progressively develops over time as

Field testing in the AIS determined that the permeability of the Salado halite can vary orders of magnitude across the DRZ @ale and Hurtado, 1996). An effective DRZ permeability can be estimated through a functional relationship for the change in permeability as a function of radial distance in the DRZ. The field data support the assumption that the DRZ permeability is greatest near the excavation face and decreases radially outward away from the shaft wall. A log-linear model of permeability as a function of radial distance yields a reasonably good correlation with the field results (Repository Isolation Systems Department, 1996).

Figure 2 shows a schematic of a shaft with a DRZ of inner radius r, and outer radius r,. The permeability at the excavation face, k, , is assumed to be several orders of magnitude higher than the intact undisturbed permeability, k,, defined at the outer radius of the DRZ. The equation shown in Figure 2 was derived to calculate the effective DRZ permeability assuming that the change in permeability within the DRZ is log-linear. For a given r,, 4, G , and & , the calculated effective DRZ permeability

3 Fryar et al.

accounts for both the decrease in DRZ permeability and the increase in flow area as a function of radial distance away from the excavation.

Rock mechanics calculations (Repository Isolation Systems Department, 1996) were performed to predict the DRZ extent in both the Rustler and Salado Formations. The extent of the DRZ within the Rustler Formation is a function of rock type and depth. Mechanical calculations indicate that no DRZ develops in anhydrites and dolomites, and that the DRZ extent increases with depth for mudstones. However, the model incorporated a DRZ for the Magenta and Culebra dolomites, which are naturally fractured, to account for the effect of pre-existing fractures which were not considered in the mechanical calculations. The radial extent of the DRZ for dolomite was set equal to one shaft radius. The DRZ in the Rustler is assumed not to heal as a function of time.

The DRZ extent within the Salado halite was calculated as a function of depth, shaft seal material, and time (Repository isolation Systems Department, 1996). In the Salado, the halite DRZ is at a maximum at closure and heals as a function of time. The rate of DRZ healing increases with increased depth of burial and increased strength (bulk modulus) of the shaft seal material. Mechanical calculations predict that the DRZ in the Salado anhydrites varies as a function of thickness with no DRZ forming for anhydrite units over 3 m in thick. Because it is assumed that anhydrite and polyhalite DRZs do not heal, the values calculated for DRZ permeability do not change with time for these units. The DRZ extent for polyhalite and anhydrite units less than 3 m in thick was assumed to be equal to the halite DRZ extent for an open shaft condition. Because the purpose of this model was to investigate the long-term potential for brine migration through the shaft seal system, DRZ healing was assumed to be complete.

In the shaft model, the extent of the DRZ in all geologic units was specified as 12 percent of the shaft radius. This yields DRZ areas which differ from those determined by the rock mechanics calculations. The calculated effective DRZ permeabilities were adjusted to account for these differences in area.

Model Parameters

Parameters necessary for model simulations were derived from field and laboratory data. Table 1 lists the permeability, porosity, and compressibility assigned to each shaft seal material and lithologic unit. The disturbed permeability listed in Table 1 is the estimated permeability at the shaft/DRZ interface.

Under base-case assumptions, the anhydrites in the Rustler Formation and anhydrites greater than 3 m thick in the Salado Formation had no DRZ. This assumption had negligible effects within the Salado Formation where the long-term halite DRZ healing causes a discontinuous DRZ regardless of the anhydrite DRZ. It does, however, create a discontinuous DRZ through the Rustler Formation which would otherwise be continuous. In order to assess the impact of a discontinuous DRZ, a second, more conservative case with a continuous DRZ through the Rustler Formation

4 Fryar et al.

was also modeled. Runs 1 and 2 incorporated discontinuous DRZs while Runs 3 and 4 had continuous DRZs through the Rustler Formation.

To determine the effect of the asphalt layer in the concrete/asphalt waterstops, two of the simulations (Runs 1 and 3) included the asphalt layer within the concrete plugs and the other two simulations (Runs 2 and 4) replaced the asphalt permeability with that of degraded concrete. Comparison of these simulations allows an assessment of the long-term impact of the asphalt layers within the concrete/asphalt waterstops.

Model Results

Table 2 summarizes the results of the four simulations and highlights the principle differences between them. Results are presented in terms of steady-state brine flow rates (m3/yr) through the DRZ and the shaft seal components. Because the vertical gradient is directed upward, the reported flow rates are also upward. The steady-state upward flow rates measured at the Rustler/Salado contact, the top of the compacted salt column, and just above the base of the compacted salt column are listed in Table 2 and shown graphically in Figure 3. Almost all flow past the Rustler/Salado contact passes through the DRZ since the shaft seal at the level of the contact is composed of asphalt which has extremely low permeability. At the base and top of the salt column, almost all of the flow passes through the shaft seal because of the much lower permeability of the healed DRZ. -*.

Flow rates at the top and base of the salt column are not strongly effected by either the discontinuous DRZ or degradation of the asphalt layers in the concretelasphalt waterstops. As expected, flow rates at the Rustler/Salado contact show a marked increase for both the continuous DRZ and the degraded asphalt layers. All simulations demonstrated that the seal system design successfully limited brine migration to the regulatory boundary with the highest flow rate slightly over one cubic meter per thousand years.

ADpendix A: References

Beauheim, R.L., R.M. Roberts, T.F. Dale, M.D. Fort, and W.A. Stensrud. 1993. Hydraulic Testing of Salad0 Formation Evaporites at the Waste Isolation Pilot Plant Site: Second Interpretive Report. SAND92-0533. Albuquerque, NM: Sandia National Laboratories.

Brinster, K.F. 1991. Preliminary Geohydrologic Conceptual Model of Los Medanos Region Near zhe Waste Isolation Pilot Plant for the Purpose of Performance Assessment. SAND89-7 147. Albuquerque, NM: Sandia National Laboratories.

Dale, T. and L.D. Hurtado, 1996. WIPP Air Intake Shaft Disturbed Rock Zone Study. Proceedings, 4th Conference on the Mechanical Behavior of Salt, Montreal, Quebec, Canada, June 17-1 8.

LaVenue, A M , T.L. Cauffman, and J.F. Pickens. Ground-Wazer Flow Modeling of the Culebra Dolomite Volume I: Model Calibration. SAND89-706811. Albuquerque, NM: Sandia National Laboratories.

Mercer, J.W. 1983. Geohydrology of the Proposed Waste Isolation Pilot Plant Site. Los Medanos Area, Southeastern New Mexico. Water-Resources Investigations Report 83-4016. Albuquerque, NM: U.S. Geological Survey.

Repository Isolation Systems Department. 1996. Waste Isolation Pilot Plant Shaft Sealing Syszem

1990.

5 Fryar et al.

Compliance Submittal Design Report. SAND96-132611. Albuquerque, NM: Sandia National Laboratories.

Summary of I988 WIPP Facility Horizon Gas Flow Measurements. SAND89-2497. Albuquerque, NM: Sandia National Laboratories.

1993. Seal Design Alternatives Study. SAND92-7340. Albuquerque, NM: Sandia National Laboratories.

Stormont, J.C.

Van Sambeek, L.L., D.D. Luo. M.S. Lm, W. Ostrowski, and D. Oyenuga.

1990.

Sandia is a multiprogram laboratory oeprated by Sandia Corporation, a Lockheed Martin Company, f o r the United S ta t e s Department of Energy under Contract DE-AC04-94AL85000.

Table 1. Summary of Best Estimate Permeability, Porosity, and Compressibility for the Base-Case Simulation.

Culebra 2.09 x 2.09 x I0l3 0.16 1.1 10-9 6.9 x 10'~

Anhydrite 1/ 1.00 x 1019 1.00 x lO'I9 0.05 2.25 x lo-'' 4.5 x Mudstone 1 Transitionl 2.24 x 10" 2.24 x I O " 0.20 7.9 x lo-" 3.9 x Bioturbated Clastics

Salado Anhydrite 1.00 x i o f 9 LOO x i o i 9 0.01 2.23 x lo-'' 2.23 x Thickness > 3 m

Anhydrite 1 .oo x 10-i9 1.00 x 10-15 0.01 2.23 x 10" 2.23 x Thickness < 3 m

Halite 1.00 x IOZ' 1.00 x lo-[' 0.01 8.05 x IO-" 8.05 x Polyhalite 3.00 x IO2' 1.00 x IO-" 0.01 2.23 x lo-" 2.23 x 10'~ Mudstone 1.49 x 1019 I .49 x 10-1~ 0.20 6.6 x lo-'' 3.3 10-9

(If Anhydrite 5, Anhydrite 4, Anhydrite 3, and Anhydrite 2

Table 2. Summary of Model Simulations.

I I Model Variations I Flow Rate Up the Shaft and DRZ (m3/yr) I Base of I Concrete Plugs Column

Top of Salado Rustler/Salado Run Continuous Asphalt Degraded in Salado Salt Salt Column Contact DRZ

1 1 1 No I No I 4.76~ 10" I 1.68~ IO4 1 8.27~ lo4 I 2 No Yes 4.80 x 10' 1.87 x lo4 9.30 x IO4 3 Yes No 4.76 x i o 5 1.71 x lo4 9.68 x lo4 4 Yes Yes 4.81 10-5 1.90 x IO4 I .09 i 0~

6 Fryar et al.

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--. I . . I

800

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750

700

ki E

0 D ([I - $

600

550

500

450

Shaft Seal Stratigraphy Components

Rustlerhhydrite Ruder Dolorrite I

Salado Marker Beds & Brine Seepage Zones

400 -

Asphalt Column

Clay Column

Salado salt

Column

Figure 1. Stratigaphy and Shaft Seal Components.

7

Eafthen Fill

Concrete

ConcretefAsphalt waterstop

Con cretdAsphait Waterstop

Concrete/AsphaH Waterstop

Shaff Station Monolith

Fryar et al.

G r L I Shaft DRZ I I

I ri > I ro > I I

I I I

I I

I

I

I

I

I

1

I

I

Intact Rock

Figure 2. Log-Linear Model for the Calculation of an Effective Permeability of the DRZ.

1.2E-3

I .OE-3

8.OE4

-- 6.0E-4 k E

4.0E-4

2.0E-4

O.OE+O Run 1 2 3 4 Continuous DRZ No No Yes Yes Asphalt Degraded No Yes No Yes

Figure 3. Flow Rates for Model Simulations.

8 Fryar et al.