Publications

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A helium leakage detection system was modified to measure gas permeability on extracted cores of nearly impermeable rock. Here we use a Helium - Mass - Spectrometry - Permeameter (HMSP) to conduct a constant pressure, steady state flow test through a sample using helium gas. Under triaxial stress conditions, the HMSP can measure flow and estimate permeability of rocks and geomaterials down to the nanodarcy scale (10-21 m2). In this study, measurements of flow through eight shale samples under hydrostatic conditions were in the range of 10-7 to 10-9 Darcy. We extend this flow measurement technology by dynamically monitoring the release of helium from a helium saturated shale sample during a triaxial deformation experiment. The helium flow, initially extremely low, consistent with the low permeability of shale, is observed to increase in advance of volume strain increase during deformation of the shale. This is perhaps the result of microfracture development and flow path linkage through the microfractures within the shale. Once microfracturing coalescence initiates, there is a large increase in helium release and flow. This flow rate increase is likely the result of development of a macrofracture in the sample, a flow conduit, later confirmed by post-test observations of the deformed sample. The release rate (flow) peaks and then diminishes slightly during subsequent deformation; however the post deformation flow rate is considerably greater than that of undeformed shale.

The calculation of radionuclide concentrations in a receptor well of a generic reference aquifer is highly dependent on the mixing that occurs in the aquifer and well. This study tests two simple analytical approaches for estimating well concentrations: a Generic Aquifer Transport and Capture (GATC) approach and an adapted IAEA ERB1B model. These approaches are tested against a 3-D generic repository model simulated numerically using PFLOTRAN. Because the PFLOTRAN model in this study simulates a diffusive release to the aquifer and the ERB1B model is designed for advective releases, the ERB1B model had to be adapted to accept a diffusive release. The results show that for this PFLOTRAN simulation the GATC equations accurately predict the initial breakthrough and peak concentration at the well but somewhat overestimate concentrations between these points. The adapted ERB1B model only predicts the peak accurately. Similar studies with broader aquifer domains and different types of advective and diffusive releases to the aquifer are needed to fully test the ranges of these analytical approaches.

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directly, rather than through simplified abstractions. It also a llows for complex representations of the source term, e.g., the explicit representation of many individual waste packages (i.e., meter - scale detail of an entire waste emplacement drift). This report fulfills the Generic Disposal System Analysis Work Packa ge Level 3 Milestone - Performance Assessment Modeling and Sensitivity Analyses of Generic Disposal System Concepts (M 3 FT - 1 4 SN08080 3 2 ).

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In this study, we use PFLOTRAN, a highly scalable, parallel, flow and reactive transport code to simulate the concentrations of 3H, 3He, CFC-11, CFC-12, CFC-113, SF6, 39Ar, 81Kr, 4He and themean groundwater age in heterogeneous fields on grids with an excess of 10 million nodes. We utilize this computational platform to simulate the concentration of multiple tracers in high-resolution, heterogeneous 2-D and 3-D domains, and calculate tracer-derived ages. Tracer-derived ages show systematic biases toward younger ages when the groundwater age distribution contains water older than the maximum tracer age. The deviation of the tracer-derived age distribution from the true groundwater age distribution increases with increasing heterogeneity of the system. However, the effect of heterogeneity is diminished as the mean travel time gets closer the tracer age limit. Age distributions in 3-D domains differ significantly from 2-D domains. 3D simulations show decreased mean age, and less variance in age distribution for identical heterogeneity statistics. High-performance computing allows for investigation of tracer and groundwater age systematics in high-resolution domains, providing a platform for understanding and utilizing environmental tracer and groundwater age information in heterogeneous 3-D systems. Groundwater environmental tracers can provide important constraints for the calibration of groundwater flow models. Direct simulation of environmental tracer concentrations in models has the additional advantage of avoiding assumptions associated with using calculated groundwater age values. This study quantifies model uncertainty reduction resulting from the addition of environmental tracer concentration data. The analysis uses a synthetic heterogeneous aquifer and the calibration of a flow and transport model using the pilot point method. Results indicate a significant reduction in the uncertainty in permeability with the addition of environmental tracer data, relative to the use of hydraulic measurements alone. Anthropogenic tracers and their decay products, such as CFC11, 3H, and 3He, provide significant constraint oninput permeability values in the model. Tracer data for 39Ar provide even more complete information on the heterogeneity of permeability and variability in the flow system than the anthropogenic tracers, leading to greater parameter uncertainty reduction.

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The purpose of this study is to develop an engineering and operational understanding of CAES performance for a depleted natural gas reservoir by evaluation of relative permeability effects of air, water and natural gas in depleted natural gas reservoirs as a reservoir is initially depleted, an air bubble is created, and as air is initially cycled. The composition of produced gases will be evaluated as the three phase flow of methane, nitrogen and brine are modeled. The effects of a methane gas phase on the relative permeability of air in a formation are investigated and the composition of the produced fluid, which consists primarily of the amount of natural gas in the produced air are determined. Simulations of compressed air energy storage (CAES) in depleted natural gas reservoirs were carried out to assess the effect of formation permeability on the design of a simple CAES system. The injection of N2 (as a proxy to air), and the extraction of the resulting gas mixture in a depleted natural gas reservoir were modeled using the TOUGH2 reservoir simulator with the EOS7c equation of state. The optimal borehole spacing was determined as a function of the formation scale intrinsic permeability. Natural gas reservoir results are similar to those for an aquifer. Borehole spacing is dependent upon the intrinsic permeability of the formation. Higher permeability allows increased injection and extraction rates which is equivalent to more power per borehole for a given screen length. The number of boreholes per 100 MW for a given intrinsic permeability in a depleted natural gas reservoir is essentially identical to that determined for a simple aquifer of identical properties. During bubble formation methane is displaced and a sharp N2methane boundary is formed with an almost pure N2 gas phase in the bubble near the borehole. During cycling mixing of methane and air occurs along the boundary as the air bubble boundary moves. The extracted gas mixture changes as a function of time and proximity of the bubble boundary to the well. For all simulations reported here, with a formation radius above 50 m the maximum methane composition in the produced gas phase was less than 0.5%. This report provides an initial investigation of CAES in a depleted natural gas reservoir, and the results will provide useful guidance in CAES system investigation and design in the future.

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