Sumit Mukhopadhyay

Modeling seepage into heated waste emplacement tunnels in unsaturated fractured rock

Predicting the amount of water that may seep into waste emplacement tunnels (drifts) is important for assessing the performance of the proposed geologic repository for high-level radioactive waste at Yucca Mountain, Nevada. The repository will be located in thick, partially saturated fractured tuff that-for the first several hundred years after emplacement-will be heated to above-boiling temperatures as a result of heat generation from the decay of radioactive waste. Heating of rock water to above-boiling conditions induces water saturation changes and perturbs water fluxes that affect the potential for water seepage into drifts. In this paper, we describe numerical analyses of the coupled thermal-hydrological (TH) processes in the vicinity of waste emplacement drifts, evaluate the potential of seepage during the heating phase of the repository, and discuss the implications for the performance of the site. In addition to the capillary barrier at the rock-drift interface-independent of the thermal conditions-a second barrier exists to downward percolation at above-boiling conditions. This barrier is caused by vaporization of water in the fractured rock overlying the repository. A TOUGH2 dual-permeability simulation model was developed to analyze the combined effect of these two barriers; it accounts for all relevant TH processes in response to heating, while incorporating the capillary barrier condition at the drift wall. Model results are presented for a variety of simulation cases that cover the expected variability and uncertainty of relevant rock properties and boundary conditions.

Modeling of thermally driven hydrological processes in partially saturated fractured rock

Modeling of Thermally Driven Hydrological Processes in Partially Saturated Fractured Rock Y. W. Tsang, J. T. Birkholzer, and S. Mukhopadhyay Earth Sciences Division Lawrence Berkeley National Laboratory University of California Berkeley, California, U. S. A.

A model comparison initiative for a CO2 injection field test: an introduction to Sim-SEQ

Because of the complex nature of subsurface flow and transport processes at geologic carbon storage (GCS) sites, modelers often need to implement a number of simplifying choices while building their conceptual models. Such simplifications may lead to a wide range in the predictions made by different modeling teams, even when they are modeling the same injection scenario at the same GCS site. Sim-SEQ is a new model comparison initiative with the objective to understand and quantify uncertainties arising from conceptual model choices. While code verification and benchmarking efforts have been undertaken in the past with regards to GCS, Sim-SEQ is different, in that it engages in model comparison in a broader and comprehensive sense, allowing modelers the choice of interpretation of site characterization data, boundary conditions, rock and fluid properties, etc., in addition to their choice of simulator. In Sim-SEQ, 15 different modeling teams, nine of which are from outside the USA, are engaged in building their own models for one specific CO2 injection field test site located in the southwestern part of Mississippi. The complex geology of the site, its location in the water leg of a CO2-EOR field with a strong water drive, and the presence of methane in the reservoir brine make this a challenging task, requiring the modelers to make a large number of choices about how to model various processes and properties of the system. Each model team starts with the same characterization data provided to them but uses its own conceptual models and simulators to come up with model predictions, which can be iteratively refined with the observation data provided to them at later stages. Model predictions will be compared with one another and with the observation data, allowing us to understand and quantify the model uncertainties.

Pressure Buildup During Supercritical Carbon Dioxide Injection From a Partially Penetrating Borehole into Gas Reservoirs

Injecting CO2 into a subsurface formation causes a buildup of pressure in the vicinity of the injection well. While a large injection rate can reduce the cost associated with injection, an indefinitely large injection rate can result in excessive formation damage. To obtain an optimal injection rate without exceeding the safe pressure limits, one will like to have some knowledge of the transient pressure buildup characteristics resulting from a particular injection rate. While elaborate numerical simulations can provide reliable pressure buildup predictions, they require extensive knowledge about the formation, which is normally not available at the start of an injection process. To alleviate this problem, using some simplifying assumptions, we have developed a solution to predict the transient buildup of pressure resulting from injection of supercritical carbon dioxide from a partially penetrating well into a gas reservoir. The solution in space and time is first obtained in the Fourier–Laplace transform space, and then inverted back into real space (in cylindrical coordinates) and time. We use the solution to study pressure transient characteristics for different formation permeabilities and anisotropy ratios. Results obtained using the solution compared well with those from numerical simulations.

The Sim-SEQ Project: Comparison of Selected Flow Models for the S-3 Site

Sim-SEQ is an international initiative on model comparison for geologic carbon sequestration, with an objective to understand and, if possible, quantify model uncertainties. Model comparison efforts in Sim-SEQ are at present focusing on one specific field test site, hereafter referred to as the Sim-SEQ Study site (or S-3 site). Within Sim-SEQ, different modeling teams are developing conceptual models of

The Impact of Preferential Flow on the Vaporization Barrier Above Waste Emplacement Drifts at Yucca Mountain, Nevada

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Estimation of field‐scale thermal conductivities of unsaturated rocks from in situ temperature data

CO2 injection at the S-3 site. In this paper, we select five flow models of the S-3 site and provide a qualitative comparison of their attributes and predictions. These models are based on five different simulators or modeling approaches: TOUGH2/EOS7C, STOMP-CO2e, MoReS, TOUGH2-MP/ECO2N, and VESA. In addition to model-to-model comparison, we perform a limited model-to-data comparison, and illustrate how model choices impact model predictions. We conclude the paper by making recommendations for model refinement that are likely to result in less uncertainty in model predictions.

Parameter estimation from flowing fluid temperature logging data in unsaturated fractured rock using multiphase inverse modeling

Abstract We describe a joint inversion approach that combines geophysical and thermal-hydrological data for the estimation of (a) thermal-hydrological parameters (such as permeability, porosity, thermal conductivity, and parameters of the capillary pressure and relative permeability functions) that are necessary for predicting the flow of fluids and heat in fractured porous media and (b) parameters of the petrophysical function that relates water saturation, porosity, and temperature to the dielectric constant. The approach incorporates the coupled simulation of nonisothermal multiphase fluid flow and ground-penetrating radar (GPR) travel times within an optimization framework. We discuss application of the approach to a large-scale in situ heater test that was conducted at Yucca Mountain, Nevada, to better understand the coupled thermal, hydrological, mechanical, and chemical processes that may occur in the fractured rock mass around a geologic repository for high-level radioactive waste. We provide a description of the time-lapse geophysical data (i.e., cross-borehole GPR) and thermal-hydrological data (i.e., temperature and water content data) collected before and during the 4-yr heating phase of the test and analyze the sensitivity of the most relevant thermal-hydrological and petrophysical parameters to the available data. To demonstrate feasibility of the approach, and as a first step toward comprehensive inversion of the heater test data, we apply the approach to estimate a single parameter: the permeability of the rock matrix.