geo-scientific data collection and integration for the waste isolation pilot plant

Site characterisation for the Waste Isolation Pilot Plant (WIPP) (Figure 1) began with the drilling of the first-borehole in 1974. Over the next two decades, site-characterisation activities included drilling, logging, and testing of over 100 boreholes; surface-based and borehole geophysical surveys, shaft and repository excavation and mapping, hydraulic testing in wells of formations above and below the repository host formation, hydraulic testing of the host formation in boreholes drilled from within the repository, hydrogeochemical sampling, geomechanical testing, and a host of geologic studies (sedimentology, mineralogy, petrology, deformation, dissolution, etc.). Integration of the information provided by these activities has yielded a robust understanding of the geologic setting of WIPP, but uncertainties remain with respect to the cause and distribution of fracturing in formations above and below the host formation. This paper describes recent efforts to identify relationships among various geo-scientific properties, processes, and parameters to allow extrapolation of observed/measured conditions to less-characterised regions. Numerous types of geologic data from boreholes, well logs, and core have been combined with hydrologic measurements and observations to develop conceptual and numerical models of the distribution of transmissivity in the Culebra. Transmissivity extremes are found east and west of the WIPP site, and have well-understood geologic explanations. But no testable explanations exist for the localised occurrence of fracturing in areas where no dissolution of the upper Salado has occurred. Large-scale pumping tests have been performed to provide both information on the distribution of fractures and transient response data that can be used for model calibration. Uncertainty is addressed probabilistically by generating multiple realizations of the Culebra T field. Sensitivity studies performed on the calibrated T fields allow identification of the uncertain areas having the greatest effect on travel time, guiding the future installation and testing of wells. A new drilling and testing programme has been initiated to test the hypotheses and correlations developed thus far, and to provide additional data for future modelling. A new generation of T fields will be developed for the next WIPP re-certification application based on the improved conceptual model and recently collected data. A new evaluation of variations in Culebra brine compositions is also planned to allow us to eliminate T fields that predict flow directions that are inconsistent with observed water quality variations. (authors)