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Understanding the initiation, propagation and evolution of water injection-induced fractures is essential for geo-energyapplications. Hydromechanical stimulation experiments were conducted in a deep borehole drilled into crystalline bedrockto gain insights into these processes, involving simultaneous in-situ measurements of three-dimensional fracture displace-ments, injection flow rates, and water pressure in 2.4-m isolated borehole sections at 500-m depth. Three distinct sectionswere tested in the COSC-1 borehole (Sweden): a section of intact rock, a section with a hydraulically conductive fractureand a section with nonconductive fractures. Acoustic televiewer measurements conducted before and after the experimentsconfirmed the generation of new fractures. Accurate positioning of measurement tools was ensured through gamma log pro-filing and an innovative FFEC-based method for detecting flowing fractures. The tests revealed several transitional pressurevalues associated with mechanical events, with intact rock requiring the highest pressure to induce fracturing, followed bythe nonconductive fracture section and the initially conductive fracture section. Following fluid injection, transient pressuredecays were observed that were associated with leakage from newly generated fractures, providing insights into fracturebehaviour under stimulation. Vertical displacements were predominant across the different tests, with measured displacementstypically ranging from 10 to 100 μm. Fracture activation modes primarily involved the normal opening of subhorizontalfractures that were parallel to the metamorphic foliation, with some irreversible slip at higher pressures. However, a morecomplex scenario was observed in the test interval with previously nonconductive fractures, involving competition betweenthe opening of subhorizontal fractures and reverse shearing of a steeply dipping fracture.

Discrete Fracture Network (DFN) models for evaluating flow and transport in low-permeability fractured rocks are important tools in safety assessments of nuclear waste repositories, and also important for other geoengineering and environmental applications. The well-known phenomena of flow channeling, arising from both intra-fracture and inter-fracture heterogeneities, is in general difficult to implement in these models. The present study uses the Channel Network Model (CNM) concept as a complementary approach to DFN models, with focus on channelized flow within fracture planes and in the fracture network. A method used to generate CNMs based on channels connecting centroids of fracture planes was implemented within a pychan3d library and applied to a 3D DFN model based on field data from Forsmark, Sweden. Three sets of realizations of the channel network are used to characterize the flow and transport system between deformation zones in the granitic host rock. The results indicate the significance of very low-conductivity fractures in providing critical flow connections in these rocks. It is shown that only a few (4 to 6 in our cases) key flow bridges within a network of 9000 or more fractures control its flow and transport. The use of CNMs together with DFN models enhances confidence in safety assessments for nuclear waste repositories and other applications, while providing valuable insights into complex flow and transport behavior in low-fracture-permeability rocks.

Performance Assessment (PA) is important in ensuring the isolation and long-term containment of spent nuclear fuel from the geosphere. It plays a crucial role in evaluating the long-term safety and effectiveness of underground nuclear waste storage, considering factors such as radionuclide release rates, transport mechanisms, and the performance of engineered barriers. This paper presents the findings of DECOVALEX 2023 Task F, which aimed to compare various models and conceptual approaches used in PA of a generic deep geologic repository in crystalline rock. The objective was to explore the contribution of modeling choices to uncertainty in PA model outputs. The study highlights the importance of characterizing the crystalline rock properties and the engineered barrier system in PA. The so-called reference case, a simplified version of a PA focused on the transport of two conservative tracers from the deposition hole to the surface, neglecting waste package performance was used as an example. Seven international teams (Canada, Czech Republic, Germany, Korea, Sweden, Taiwan, and United States) developed and simulated the generic reference case, tracking tracer releases from waste package locations to the near field and ground surface. Quantities of Interest (QOI) such as remaining tracer in the repository and fluxes across the domain were compared. Technical and time constraints led some teams to exclude parts of the engineered barrier system which resulted in faster release of tracers and radionuclides from the repository region. Comparing all models highlighted the importance of explicitly including drifts, buffer, and backfill in the reference case models. The results also emphasize the utility of a diverse set of modeling approaches in building confidence with performance assessment analysis.

This study presents newly developed benchmarks for modeling flow and transport within discrete fracture networks (DFNs) and useful methods for analyzing the results. The new benchmarks are designed to test modeling approaches for use in probabilistic performance assessment models of deep geologic repositories in fractured rock. The benchmarks simulate flow and transport through a 1 km3 block of fractured rock. The first simulates migration of a short pulse of tracer through a simple network of four intersecting fractures. The second adds 1089 stochastically generated fractures. The third changes the pulse to a continuous point source. Evaluation of model performance relies on moment analysis and comparison of the results of different models. The expected nondimensional first moment of the conservative tracer for each benchmark is 1. The benchmarks were simulated by teams from Canada, Czechia, Germany, Korea, Sweden, Taiwan, and the United States as part of a DECOVALEX-2023 study (decovalex.org). The teams used various approaches, including explicit DFN modeling, DFN upscaling to an equivalent continuous porous medium (ECPM), and a combination of both methods. Transport mechanisms are modeled using either the advection-dispersion equation or particle tracking. Results demonstrate strong agreement among the models in breakthrough behavior up to the 75th percentile. Significant deviations in first moments and well-clustered outputs led to the identification of inaccuracies in several models. Such findings exemplify the benefit of exercising these benchmarks and using the presented methods to test DFN flow and transport models.

Clays and clay-rich rocks play often an important role in nuclear waste disposal due to their low permeability and high sorption capacity, acting as natural barriers to fluid movement and contaminant migration. Understanding the transport and sorption behaviours of hazardous elements in clay-rich environments is therefore essential for long-term simulations with validated models of experimental data. This study investigates the reactive transport of 17 ionic compounds in the Woodford claystone using both experimental and modelling approaches. The experiment was conducted by injecting a multi-tracer solution into a column filled with crushed claystone, employing a flow-interruption method for examining kinetic behaviour during diffusion-dominated mass transfer. TOUGHREACT V4.0 OMP reactive transport code was applied to replicate the tests, using an advective-diffusive single porosity flow model that considers mineral dissolution/precipitation and cation exchange. The modelling results demonstrated that cation exchange and diffusion, along with advection, were the primary processes influencing ionic concentrations in the experiment. The primary mineral dissolution reactions were pyrite oxidation and silicate weathering, releasing Si, Al, and Fe that reprecipitated or contributed to cation exchange. The findings indicated that the claystone sample effectively sorbs Cs, Pb, and Eu through cation exchange. While the model showed good agreement with the experimental data, an excessive diffusion effect was simulated using the single-porosity model, which would likely be less if employing a dual-porosity model and accounting for immobile water.

The generalized radial flow (GRF) model in well-test analysis employs noninteger flow dimensions to represent the variation in flow area with respect to radial distance from a borehole. However, the flow dimension is influenced not only by changes in flow area, but also by permeability variations in the flow medium. In this report, the flow dimension from the combined effect of flow dimensionality and permeability/conductance variation is interpreted and referred to as apparent flow dimension (AFD). AFD is determined using the second derivative of the drawdown-time plot from pressure transient testing, which may have varied noninteger values with time. A systematic set of investigations is presented, starting from idealized channel networks in one, two and three dimensions (1D, 2D and 3D, respectively), and proceeding to a case study with a complex fracture network based on actual field data. Interestingly, a general relation between the AFD upsurge/dip and the conductance contrast between adjacent flow channels is established. The relation is derived from calculations for 1D networks but is shown to be useful even for data interpretation for more complex 2D and 3D cases. In an application to fracture network data at a real site, the presence of flow channel clusters is identified using the AFD plot. Overall, the AFD analysis is shown to be a useful tool in detecting the conductance/dimensionality changes in the flow system, and may serve as one of the different data types that can be jointly analysed for characterizing a heterogeneous flow system.

Performance assessment of nuclear waste disposal in deep crystalline bedrock demands a thorough understanding of the related flow and transport processes. Uncertainties may arise both from the selection of the conceptual model as well as the estimation of the related model parameters. Discrete fracture network (DFN) models are widely used for such modeling while channel network models (CNM) provide an alternative representation, the latter focusing on the fact that flow and transport in deep fractured media often are dominated by a small number of long preferential flow paths. This study applies the principle of channel networks, implemented in the Pychan3d simulator, to analyze the hydraulic and tracer transport behavior in a 450-m-deep fractured granite system at the Äspö Hard Rock Laboratory in Sweden, where extensive site characterization data, including hydraulic and tracer test data are available. Semi-automated calibration of channel conductances to field characterization data (flow rates, drawdowns, and tracer recoveries) is performed using PEST algorithm. It was observed that an optimal CNM connectivity map for channel conductance calibration can only be developed by jointly fitting flow rates, drawdowns and tracer mass recovery values. Results from data-calibrated CNM when compared to a corresponding calibrated DFN model shows that the CNM calibrates and adapts better than a DFN model with uniform fracture surfaces. This comparative study shows the differences and uncertainties between two models as well as examines the implications of using them for long term model predictions.

A decrease in reservoir pressure can lead to remobilization of residually trapped CO₂. In this study, the pore-scale processes related to trapped CO₂ remobilization under pressure depletion were investigated with the use of high-resolution 3D X-ray microtomography. The distribution of CO₂ in the pore space of Bentheimer sandstone was measured after waterflooding at a fluid pressure of 10 MPa, and then at pressures of 8, 6 and 5 MPa. At each stage CO₂ was produced, implying that swelling of the gas phase and exsolution allowed the gas to reconnect and flow. After production, the gas reached a new position of equilibrium where it may be trapped again. At the end of the experiment, we imaged the sample again after 30 hours. Firstly, the results showed that an increase in saturation beyond the residual value was required to remobilize the gas, which is consistent with earlier field-scale results. Additionally, Ostwald ripening and continuing exsolution lead to a significant change in fluid saturation: transport of dissolved gas in the aqueous phase to equilibriate capillary pressure led to reconnection of the gas and its flow upwards under gravity. The implications for CO₂ storage are discussed: an increase in saturation beyond the residual value is required to mobilize the gas, but Ostwald ripening can allow local reconnection of hitherto trapped gas, thus enhancing migration and may reduce the amount of CO₂ that can be capillary trapped in storage operations.

Remobilization of residually trapped CO₂ can occur under pressure depletion, caused by any sort of leakage, brine extraction for pressure maintenance purposes, or simply by near wellbore pressure dissipation once CO₂ injection has ceased. This phenomenon affects the long-term stability of CO₂ residual trapping and should therefore be considered for an accurate assessment of CO₂ storage security. In this study, pore-network modeling is performed to understand the relevant physics of remobilization. Gas remobilization occurs at a higher gas saturation than the residual saturation, the so-called critical saturation; the difference is called the mobilization saturation, a parameter that is a function of the network properties and the mechanisms involved. Regardless of the network type and properties, Ostwald ripening tends to slightly increase the mobilization saturation, thereby enhancing the security of residual trapping. Moreover, significant hysteresis and reduction in gas relative permeability is observed, implying slow reconnection of the trapped gas clusters. These observations are safety enhancing features, due to which the remobilization of residual CO₂ is delayed. The results, consistent with our previous analysis of the field-scale Heletz experiments, have important implications for underground gas and CO₂ storage. In the context of CO₂ storage, they provide important insights into the fate of residual trapping in both the short and long term.