Sharad Kelkar

FEHMN 1.0: Finite element heat and mass transfer code

A computer code is described which can simulate non-isothermal multiphase multicomponent flow in porous media. It is applicable to natural-state studies of geothermal systems and ground-water flow. The equations of heat and mass transfer for multiphase flow in porous and permeable media are solved using the finite element method. The permeability and porosity of the medium are allowed to depend on pressure and temperature. The code also has provisions for movable air and water phases and noncoupled tracers; that is, tracer solutions that do not affect the heat and mass transfer solutions. The tracers can be passive or reactive. The code can simulate two-dimensional, two-dimensional radial, or three-dimensional geometries. A summary of the equations in the model and the numerical solution procedure are provided in this report. A user`s guide and sample problems are also included. The main use of FEHMN will be to assist in the understanding of flow fields in the saturated zone below the proposed Yucca Mountain Repository. 33 refs., 27 figs., 12 tabs.

Exploration drilling and reservoir model of the Platanares geothermal system, Honduras, Central America

Abstract Results of drilling, logging, and testing of three exploration core holes, combined with results of geologic and hydrogeochemical investigations, have been used to present a reservoir model of the Platanares geothermal system, Honduras. Geothermal fluids circulate at depths ≥ 1.5 km in a region of active tectonism devoid of Quaternary volcanism. Large, artesian water entries of 160 to 165°C geothermal fluid in two core holes at 625 to 644 m and 460 to 635 m depth have maximum flow rates of roughly 355 and 560 l/min, respectively, which are equivalent to power outputs of about 3.1 and 5.1 MW(thermal). Dilute, alkali-chloride reservoir fluids (TDS ≤ 1200 mg/kg) are produced from fractured Miocene andesite and Cretaceous to Eocene redbeds that are hydrothermally altered. Fracture permeabillity in producing horizons is locally greater than 1500 and bulk porosity is ≤ 6%. A simple, fracture-dominated, volume-impedance model assuming turbulent flow indicates that the calculated reservoir storage capacity of each flowing hole is approximately 9.7 × 106 l/(kg cm−2), Tritium data indicate a mean residence time of 450 yr for water in the reservoir. Multiplying the natural fluid discharge rate by the mean residence time gives an estimated water volume of the Platanares system of ≥ 0.78 km3. Downward continuation of a 139°C/km “conductive” gradient at a depth of 400 m in a third core hole implies that the depth to a 225°C source reservoir (predicted from chemical geothermometers) is at least 1.5 km. Uranium-thorium disequilibrium ages on calcite veins at the surface and in the core holes indicate that the present Platanares hydrothermal system has been active for the last 0.25 m.y.

Modeling solute transport through saturated zone ground water at 10 km scale: Example from the Yucca Mountain license application

This paper presents a study of solute transport through ground water in the saturated zone and the resulting breakthrough curves (BTCs), using a field-scale numerical model that incorporates the processes of advection, dispersion, matrix diffusion in fractured volcanic formations, sorption, and colloid-facilitated transport. Such BTCs at compliance boundaries are often used as performance measures for a site. The example considered here is that of the saturated zone study prepared for the Yucca Mountain license application. The saturated zone at this site occurs partly in volcanic, fractured rock formations and partly in alluvial formations. This paper presents a description of the site and the ground water flow model, the development of the conceptual model of transport, model uncertainties, model validation, and the influence of uncertainty in input parameters on the downstream BTCs at the Yucca Mountain site.

A Model for Tracking Fronts of Stress-Induced Permeability Enhancement

Using an analogy to the classical Stefan problem, we construct evolution equations for the fluid pore pressure on both sides of a propagating stress-induced damage front. Closed form expressions are derived for the position of the damage front as a function of time for the cases of thermally-induced damage as well as damage induced by over-pressure. We derive expressions for the flow rate during constant pressure fluid injection from the surface corresponding to a spherically shaped subsurface damage front. Finally, our model results suggest an interpretation of field data obtained during constant pressure fluid injection over the course of 16 days at an injection site near Desert Peak, NV.

Benchmark Problems of the Geothermal Technologies Office Code Comparison Study

............................................................................................................................................. iii Summary ............................................................................................................................................. v Acknowledgments ............................................................................................................................. vii Acronyms and Abbreviations ............................................................................................................. ix 1.0 Introduction .............................................................................................................................. 1.1 1.1 Approach ......................................................................................................................... 1.3 1.1.1 Study Objectives .................................................................................................. 1.3 1.1.2 Study History and Structure ................................................................................. 1.3 1.2 Participants and Codes .................................................................................................... 1.5 1.3 Benchmark Problems ...................................................................................................... 1.9 1.3.1 Benchmark Problem 1: Poroelastic Response in a Fault Zone (PermeabilityPressure Feedback) ............................................................................................... 1.9 1.3.2 Benchmark Problem 2: Shear stimulation of randomly oriented fractures aby injection of cold water into a thermo-poro-elastic medium with stress-dependent permeability ........................................................................................................ 1.10 1.3.3 Benchmark Problem 3: Fracture opening and sliding in response to fluid injection .............................................................................................................. 1.11 1.3.4 Benchmark Problem 4: Planar EGS fracture of constant extension, pennyshaped or thermo-elastic aperture in impermeable hot rock .............................. 1.12 1.3.5 Benchmark Problem 5: Amorphous Silica dissolution/precipitation in a fracture zone .................................................................................................................... 1.13 1.3.6 Benchmark Problem 6: Injection into a fault/fracture in thermo-poroelastic rock1.14 1.3.7 Benchmark Problem 7: Surface deformation from a pressurized subsurface fracture ............................................................................................................... 1.15 1.4 Comparison Standard .................................................................................................... 1.16 2.0 Governing and Constitutive Equations .................................................................................... 2.1 2.1 Heat Transfer Modeling .................................................................................................. 2.1 2.2 Fluid Flow Modeling ....................................................................................................... 2.2 2.2.1 Fracture Transmissivity ........................................................................................ 2.2 2.3 Rock Mechanics Modeling .............................................................................................. 2.3 2.3.1 Continuum Geomechanics ................................................................................... 2.4 2.3.2 Discrete Fracture Geomechanics .......................................................................... 2.5 2.3.3 Joint Models ....................................................................................................... 2.10 2.4 Geochemical Reaction Modeling .................................................................................. 2.12 2.4.1 Aqueous Reaction Rates ..................................................................................... 2.14 3.0 Numerical Solution Schemes ................................................................................................... 3.1 3.1 Sequential Schemes ......................................................................................................... 3.1 3.2 Iterative Schemes ............................................................................................................ 3.1

Permeability and Flow Field Evolution Due to Dissolution of Calcite in a 3-D Porous Rock Under Geothermal Gradient and Through-Flow

Flow of undersaturated water in limestone aquifer can cause continuous permeability growth due to dissolution. We have simulated the evolution of permeability field of a 3-D porous limestone aquifer subjected to geothermal temperature gradient and vertical through-flow. The upward flow through porous limestone results in dissolution since calcite is a retrograde soluble mineral. In addition to permeability growth by promoting more dissolution, through-flow also inhibits Rayleigh Benard convection. To understand the temporal evolution of permeability and flow fields, we have performed several simulations with various combinations of initial permeability and through-flow magnitude. Since our computational domain is different in size and boundary conditions from past studies related to buoyant convection in porous medium, we have carried out simulations without reactive alteration to distinguish the hydrothermal systems as stable or unstable. The permeability growth is insignificant in the central part of the reservoir as the temperature gradient vanishes due to forced convection. Permeability growth is more near the edges, where temperature gradients are significant due to conductive heat transfer from the boundaries. For small magnitudes of through-flow, convection rolls are formed near the corners. However, the growth is very localized and rolls never form when magnitude of through-flow is large.

Surface complexation modeling of americium sorption onto volcanic tuff.

Results of a surface complexation model (SCM) for americium sorption on volcanic rocks (devitrified and zeolitic tuff) are presented. The model was developed using PHREEQC and based on laboratory data for americium sorption on quartz. Available data for sorption of americium on quartz as a function of pH in dilute groundwater can be modeled with two surface reactions involving an americium sulfate and an americium carbonate complex. It was assumed in applying the model to volcanic rocks from Yucca Mountain, that the surface properties of volcanic rocks can be represented by a quartz surface. Using groundwaters compositionally representative of Yucca Mountain, americium sorption distribution coefficient (Kd, L/Kg) values were calculated as function of pH. These Kd values are close to the experimentally determined Kd values for americium sorption on volcanic rocks, decreasing with increasing pH in the pH range from 7 to 9. The surface complexation constants, derived in this study, allow prediction of sorption of americium in a natural complex system, taking into account the inherent uncertainty associated with geochemical conditions that occur along transport pathways.

Mechanical Behavior of the Near-field Host Rock Surrounding Excavations

This report is being prepared under the FY14 activity FT-14LA0818069, Mechanical and Hydrological Behavior of the Near-Field Host Rock Surrounding Excavations, and fulfills the Los Alamos National Laboratory deliverable M4FT-14LA08180610, which in PICS:NE is titled “Draft report, Test Plan for Mechanical and Hydrological Behavior of the Near-field Host Rock Surrounding Excavations.” Since the report is an intermediate deliverable intended as input to the eventual test plan for this test, rather than being an actual test plan, the activity title is used as the title of this document to avoid confusion as to the contents in the report. This report summarizes efforts to simulate mechanical processes occurring within a hypothetical high-level waste (HLW) repository in bedded salt. The report summarizes work completed since the last project deliverable, “Coupled model for heat and water transport in a high level waste repository in salt “, a Level 2 milestone submitted to DOE in September 2013 (Stauffer et al., 2013).

Extension of two-layer fracture-systems created in hot dry rock - simulation of exp. 2059 and exp. 2062

At Los Alamos National Laboratory, more than ten hydraulic fracturing experiments have been conducted to stimulate a hot dry rock reservoir. Two hydraulic fracturing attempts, Exp. 2059 and Exp. 2062, successfully established a large fracture system connecting an injection well, EE-3A, and a production well, EE-2. The initial closed loop flow test (ICFT) is planned (from May through June, 1986) to get information on volume impedance and temperature of the reservoir. The fracture systems created by the two experiments show characteristics that are different from each other.

Experiment 2062: Packer Stimulation of the Intermediate Portion of EE-3A (Below 12,000 FT)

The deepest EE-3A interval stimulated during Expt. 2061 did not connect to EE-2, but rather extended a fractured region downwards some 300 m below the bottom of EE-3A. Therefore, the decision has been made to abandon for now the bottom 600 feet of EE-3A, and to stimulate the three flowing fractures between 12,050 ft and 12,200 ft identified by temperature logging prior to Expt. 2059 (our successful Memorial Day reservoir connection experiment).