Paul J. Cook

Field tests and model analyses of seepage into drift

Abstract This paper focuses on field test results and model analyses of the first set of five niche seepage tests conducted in the Exploratory Studies Facility at Yucca Mountain. The test results suggest that (1) a niche opening (short drift excavated for this study) acts as a capillary barrier; (2) a seepage threshold exists; and (3) the seepage is a fraction of the liquid released above the ceiling. Before seepage quantification, air injection and liquid release tests at two niche locations were conducted to characterize the fracture flow paths. Nearly two-order-of-magnitude changes in air permeability values were measured before and after niche excavation. The dyed liquid flow paths, together with a localized wet feature potentially associated with an ambient flow path, were mapped during dry excavation operations. After niche excavation, the seepage is quantified by the ratio of the water mass dripped into a niche to the mass released above the opening at selected borehole intervals. For the first set of five tests conducted at Niche 3650 site, the ratios range from 0% (no dripping for two tests) to 27.2%. Changes in flow path distributions and water accumulation near seepage threshold were observed on the niche ceiling. The seepage test results compare reasonably well with model results without parameter adjustments, using capillary barrier boundary condition in the niche and two-dimensional and three-dimensional conceptualizations to represent discrete fracture and fracture network for the flow paths. Model analyses of the niche tests indicate that the seepage is very sensitive to the niche boundary condition and is moderately sensitive to the heterogeneity of the fracture flow paths and to the strengths of matrix imbibition. Strong capillary strength and large storage capacity of the fracture flow paths limit the seepage. High permeability value also enhances diversion and reduces seepage for low liquid release rate.

Inverse and predictive modeling of seepage into underground openings

We discuss the development and calibration of a model for predicting seepage into underground openings. Seepage is a key factor affecting the performance of the potential nuclear-waste repository at Yucca Mountain, Nevada. Three-dimensional numerical models were developed to simulate field tests in which water was released from boreholes above excavated niches. Data from air-injection tests were geostatistically analyzed to infer the heterogeneous structure of the fracture permeability field. The heterogeneous continuum model was then calibrated against the measured amount of water that seeped into the opening. This approach resulted in the estimation of model-related, seepage-specific parameters on the scale of interest. The ability of the calibrated model to predict seepage was examined by comparing calculated with measured seepage rates from additional experiments conducted in different portions of the fracture network. We conclude that an effective capillary strength parameter is suitable to characterize seepage-related features and processes for use in a prediction model of average seepage into potential waste-emplacement drifts.

Modeling Coupled Evaporation and Seepage in Ventilated Cavities

Cavities excavated in unsaturated geological formations are important to activities such as nuclear waste disposal and mining. Such cavities provide a unique setting for simultaneous occurrence of seepage and evaporation. Previously, inverse numerical modeling of field liquid-release tests and associated seepage into cavities were used to provide seepage-related large-scale formation properties, ignoring the impact of evaporation. The applicability of such models was limited to the narrow range of ventilation conditions under which the models were calibrated. The objective of this study was to alleviate this limitation by incorporating evaporation into the seepage models. We modeled evaporation as an isothermal vapor diffusion process. The semiphysical model accounts for the relative humidity (RH), temperature, and ventilation conditions of the cavities. The evaporation boundary layer thickness (BLT) over which diffusion occurs was estimated by calibration against free-water evaporation data collected inside the experimental cavities. The estimated values of BLT were 5 to 7 mm for the open underground drifts and 20 mm for niches closed off by bulkheads. Compared with previous models that neglected the effect of evaporation, this new approach showed significant improvement in capturing seepage fluctuations into open cavities of low RH. At high relative-humidity values (>85%), the effect of evaporation on seepage was very small.

Field testing of modular borehole monitoring with simultaneous distributed acoustic sensing and geophone vertical seismic profiles at Citronelle, Alabama

A modular borehole monitoring concept has been implemented to provide a suite of well-based monitoring tools that can be deployed cost effectively in a flexible and robust package. The initial modular borehole monitoring system was deployed as part of a CO2 injection test operated by the Southeast Regional Carbon Sequestration Partnership near Citronelle, Alabama. The Citronelle modular monitoring system transmits electrical power and signals, fibre-optic light pulses, and fluids between the surface and a reservoir. Additionally, a separate multi-conductor tubing-encapsulated line was used for borehole geophones, including a specialized clamp for casing clamping with tubing deployment. The deployment of geophones and fibre-optic cables allowed comparison testing of distributed acoustic sensing. We designed a large source effort (>64 sweeps per source point) to test fibre-optic vertical seismic profile and acquired data in 2013. The native measurement in the specific distributed acoustic sensing unit used (an iDAS from Silixa Ltd) is described as a localized strain rate. Following a processing flow of adaptive noise reduction and rebalancing the signal to dimensionless strain, improvement from repeated stacking of the source was observed. Conversion of the rebalanced strain signal to equivalent velocity units, via a scaling by local apparent velocity, allows quantitative comparison of distributed acoustic sensing and geophone data in units of velocity. We see a very good match of uncorrelated time series in both amplitude and phase, demonstrating that velocityconverted distributed acoustic sensing data can be analyzed equivalent to vertical geophones. We show that distributed acoustic sensing data, when averaged over an interval comparable to typical geophone spacing, can obtain signal-to-noise ratios of 18 dB to 24 dB below clamped geophones, a result that is variable with noise spectral amplitude because the noise characteristics are not identical. With vertical seismic profile processing, we demonstrate the effectiveness of downgoing deconvolution from the large spatial sampling of distributed acoustic sensing data, along with improved upgoing reflection quality. We conclude that the extra source effort currently needed for tubing-deployed distributed acoustic sensing vertical seismic profile, as part of a modular monitoring system, is well compensated by the extra spatial sampling and lower deployment cost as compared with conventional borehole geophones.

Infiltration and Seepage Through Fractured Welded Tuff

The Nopal I mine in Pena Blanca, Chihuahua, Mexico, contains a uranium ore deposit within fractured tuff. Previous mining activities exposed a level ground surface 8 m above an excavated mining adit. In this paper, we report results of ongoing research to understand and model percolation through the fractured tuff and seepage into a mined adit both of which are important processes for the performance of the proposed nuclear waste repository at Yucca Mountain. Travel of water plumes was modeled using one-dimensional numerical and analytical approaches. Most of the hydrologic properly estimates were calculated from mean fracture apertures and fracture density. Based on the modeling results, we presented constraints for the arrival time and temporal pattern of seepage at the adit.

Hydrologic Imaging of Fractured Rock

Various geophysical and hydrologic tests were conducted in a cluster of nine wells to image the hydrologic connections of a fractured rock mass. Results of intra-borehole flow surveys and cross-hole radar and seismic tomography surveys correlated very well, and indicated that there is a major feature at a depth of 30m. Systematic injection tests were conducted in all nine wells. Three to four intervals in each well were isolated using pneumatic packers. Each interval was equipped with a high resolution pressure transducer. Some 130 injections tests were conducted, and more than 4,100 cross-hole transient pressure measurements were obtained. A computer algorithm was developed to analyze such massive interference data systematically. As a result of the analysis, an image of the fracture connections emerged which is consistent with the geophysical data.

Heterogeneous Seepage at the Nopal I Uranium Mine, Chihuahua, Mexico

HETEROGENEOUS SEEPAGE AT THE NOPAL I URANIUM MINE, CHIHUAHUA, MEXICO Patrick F. Dobson, Paul J. Cook, Teamrat Ghezzehei, J. Alfredo Rodriguez (Instituto de Ecologia, A.C.), and Rodrigo de la Garza (UACH) Contact: Patrick F. Dobson, 510/486-5373, pfdobson@lbl.gov RESEARCH OBJECTIVES The primary objective of this analogue study is to evaluate flow and transport processes of relevance to the proposed Yucca Mountain repository. Seepage data obtained from this study will be used to constrain flow and transport models being developed for the Nopal I system. APPROACH A water collection system, consisting of 240 separate 30 cm × 30 cm compartments that are each connected to a 125 mL bottle, was installed in April 2005 within the +00 m adit of the Nopal I mine, to collect water that had infiltrated from the +10 m level and seeped into the adit. This system was upgraded in November 2005, when instrumentation was added to six collector sites to measure seepage rates continuously. An automated weather station was installed at the site in March 2006 to permit correlation of local precipitation events with seepage. ACCOMPLISHMENTS Rainfall in central Chihuahua is seasonal, with most precipitation occurring during the summer monsoon period. Initial modeling of infiltration and seepage through a series of planar, vertical fractures was conducted to evaluate flow transit times and seepage rates (Ghezzehei et al., 2006). Using a range of fracture apertures and frequencies, and assuming no fracture-matrix interaction, infiltration through the 8 m high vertical fracture system and seepage into the adit was predicted to occur within 24 hours after a 6-hour rainfall event. Monitoring of seepage within the adit between April 2005 and December 2006 indicated that seepage is highly heterogeneous in both time and space. Within the back adit area, there were a few zones where large volumes of water have been collected. These large volume seepage events (Figure 1) were linked to fast flow path fractures ( 25 mm). In most locations, however, there was a significant (1–6 month) time lag between major precipitation events and seepage within the adit, with longer water residence times observed for the front adit area. SIGNIFICANCE OF FINDINGS The wide variability in the location, timing, and amount of seepage occurring within the Nopal I adit suggests that a number of fast-flow fracture pathways are active immediately after large rainfall events. Flow focusing along these pathways may explain the heterogeneous seepage distribution. However, delayed seepage observed in many locations within the adit indicate that even a relatively thin (8 m) rock mass can exert a noticeable damping effect on infiltration, and that flow and transport models must incorporate fracture flow heterogeneity. Processes such as evaporation and flow diversion resulting from flow focusing or capillary barrier effects may be responsible for the low seepage volumes relative to the total precipitation for most seepage locations. The capillary barrier effect may be dominant in the back adit region

Final report of the Peña Blanca natural analogue project

The Pena Blanca region, 50 km north of Chihuahua City, Chihuahua, Mexico, was a target of uranium exploration and mining by the Mexican government. After mining ceased in 1981, researchers became interested in this region as a study area for subsurface uranium migration with relevance to geologic disposal of nuclear waste. Many studies related to this concept were conducted at the Nopal I mine site located on a cuesta (hill) of the Sierra Pena Blanca. This site has geologic, tectonic, hydrologic, and geochemical similarities to Yucca Mountain, Nevada, a formerly proposed site for a high-level nuclear-waste repository in the unsaturated zone. The U.S. Department of Energy (U.S. DOE), Office of Civilian Radioactive Waste Management (OCRWM), sponsored studies at Nopal I in the 1990s and supported the drilling of three research wells – PB1, PB2, and PB3 – at the site in 2003. Beginning in 2004, the Pena Blanca Natural Analogue Project was undertaken by U.S. DOE, OCRWM to develop a three-dimensional conceptual model of the transport of uranium and its radiogenic daughter products at the Nopal I site.