This paper describes preliminary interpretation of in-situ pressure and flow measurements of the Salado Formation at the Waste Isolation Pilot Plant (WIPP). The WIPP facility is located 660 m underground in the Salado, a bedded salt deposit. Shut-in pressure tests were conducted prior to, and subsequent to, the mining of a circular drift in order to evaluate excavation effects on pore pressure, permeability, and host rock heterogeneity. Borehole deformation was measured during these tests and used to correct for changes in the test region volume due to salt creep effects. Preliminary pre-excavation results indicate that the flow properties of this layered host rock are heterogeneous. Resulting pore pressures range from 1 to 14 MPa and permeabilities range from below measurable to about 1 nanodarcy. Normalized borehole diameter change rates were between -4 and 63 microstrains/day. Shut-in pressures and borehole diameters in all test boreholes were affected by the excavation of Room Q coincident with the advances of the boring machine. Preliminary results from post-excavation test results show decreased pore pressures compared to pre-excavation values.
Evaluation of the performance of the WIPP repository involves modeling of brine and gas flow in the host rocks of the Salado Formation, which consist of halite and higher permeability anhydrite interbeds. Numerous physical, chemical, and structural processes, some of them coupled, must be understood to perform this modeling. Gas generation within the repository, for example, is strongly coupled to the amount of brine inflow to the repository because brine aids in the corrosion of metals and associated generation of hydrogen gas. Increasing gas pressure in the repository, in turn, decreases the rate of brine inflow. Ultimately, the gas pressure may exceed the brine pressure and gas may flow out of the repository. The initial models used by WIPP Performance Assessment (PA) were simplified because of a lack of WiPP-specific data on important processes and parameters. Relative-permeability curves and a correlation between threshold pressure and permeability taken from studies reported in the literature were used in PA models prior to being experimentally verified as appropriate for WIPP. In addition, interbed permeabilities were treated as constant and independent of effective stress in early models. Subsequently, the process of interbed fracturing (or fracture dilation) was recognized to limit gas pressures in the repository to values below lithostatic, and assumed (and unverified) relationships between porosity, permeability, and pore pressure were employed. Parametersensitivity studies performed using the simplified models identified important parameters for which sitespecific data were needed. Unrealistic modeling results, such as room pressures substantially above lithostatic, showed the need to include additional processes in the models. Field and laboratory experimental programs have been initiated in conjunction with continued model development to provide information on important processes and parameters. Current field experiments are aimed at determining the permeability of anhydrite and halite beds under undisturbed conditions, the pressuredependence of anhydrite fracture permeability, and the threshold pressure of anhydrite fractures. Laboratory experiments are being performed on anhydrite core samples to determine capillary-pressure curves, relative-permeability curves, and permeability and porosity as a function of applied stress. The current PA models built upon these experimental data are considerably more realistic and credible than the initial models used and, in some cases, predict significantly different consequences. 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 responsibility 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. Reference herein t...
FileSalado Two-Phase Flow Laboratory Program was established to address concerns regarding two-phase flow properties and to provide WlPP-specific, geologically consistent experimental data to develop more appropriate correlations li)r Salado rock to replace those currently used in Performance Assessment models. Researchers in Sandia's Fluid t:low and Transport Department originally identified and emphasized the need Ibr laboratory measurements of Salado threshold pressure and relative permeability.The program expanded to include the measurement of capillary pressure, rock compressibility, porosity, and intrinsic permeability and the assessment of core damage. Sensitivity analyses identified the anhydrite interbed layers as the most likely path for the dissipation of waste-generated gas from waste-storage rooms because of their relatively high permeability.Dt_c to this the program will initially focus on the anhydrite irlterbed material. The program may expand to include similar rock and flow measurements on other WIPP materials including impure halite, pure halite, and backfill and seal materials.This conceptual plan presents the scope, ot!jectives, and historical documentation of the development of the Salado Two-Phase Flow F'rogram through January 1993. Potential laboratory techniques for assessing core damage and measuring porosity, rock compressibility, capillary and threshold pressure, permeability as a function of stress, and relative permeability are discussed. Details of actual test designs, test procedures, and data analysis are not included in this report, but will be included in the Salado Two-Phase Flow l.aboratory Program Test Plan pending the restllis of experimerltal arid other scoping activities irl I:Y93. MASTER L-, ACKNOWLEDGMENTS
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