Effect of Dilution and Contaminants on Strength and Hydraulic Conductivity of Sand Grouted With Colloidal Silica Gel

Colloidal silica (CS) is a low-viscosity liquid that can be made to gel by addition of brine. This property allows it to be injected into, or mixed with, soil, so that after gelling the colloidal silica blocks the pore space in the soil and forms a barrier to the flow of contaminated groundwater or non-aqueous liquids (NAPLs). Gelled-in-place CS was first studied for the petroleum industry and later for protecting groundwater quality. Noll investigated the use of colloidal silica diluted so that its solids content was reduced from 30% (a typical nominal value for material as delivered) to values as low as 5%. The more dilute colloids could still be made to gel, although more slowly, and the resulting gel was weaker. Because the proposed application of colloidal silica grout involves emplacing it in the subsurface by permeation, jet grouting, or soil mixing where its role as a barrier will be to resist flow of contaminants, the effects of these contaminants on the properties of the grouted soil is also of interest. This work comprised four tasks. In Task 1, samples of grouted sand were prepared with a range of CS dilutions, for measurement of hydraulic conductivity and unconfined-compressive strength. In Task 2, these properties were measured on samples of grouted sand that incorporated 5% volumetric saturation of NAPLs. In Task 3, samples, prepared without any contaminants, were immersed in contaminant liquids and tested after 30 and 90 days. Task 4 was added because NAPL contamination in the samples of Tasks 2 and 3 impelled modifications in the test methods, and comparison of the results of Task 2 and Task 1 suggested that these modifications had introduced errors. In Task 4, samples were tested both ways, to confirm that in Tasks 2 and 3 strength was underestimated and hydraulic conductivity was overestimated. Despite the existence of these known systematic errors, the inclusion of control samples in Tasks 2 and 3 permits conclusions to be drawn from these data

Sorption of cesium and strontium on Savannah River soils impregnated with colloidal silica

Colloidal silica (CS) is being considered as an injectable low viscosity fluid for creation of impermeable barrier containment of low level radioactive waste at the Savannah River Site (SRS), South Carolina. The sorption behavior of cesium and strontium on Savannah River Site Soils impregnated with Colloidal Silica was studied using a batch experimental method. The samples were prepared by addition of CS and an aqueous solution of CaCl{sub 2} to the soil materials. Sorption studies were conducted after the gelation of the CS samples had occurred. The variation of the sorption ratio, R, as a function of cesium or strontium concentration was examined. The Freundlich isotherm was used to fit the data and very good results were obtained

On the development of MP-TOUGH2

The authors are developing MP-TOUGH2 for exploiting massively parallel computers. The goals of this effort are to (1) create a data-parallel subsurface transport code for solving larger problems than currently practical on workstations, (2) write portable code that can take advantage of scalability to run on machines with more processors, and (3) minimize the necessity for additional validation and verification of the resulting code. The initial strategy they have followed is to focus on optimizing the generic and time-consuming task of linear equation solution while leaving the bulk of TOUGH2 unmodified. In so doing, they have implemented a massively parallel direct solver (MPDS) that takes advantage of the banded structure of TOUGH2 Jacobian matrices. The authors have compared timings of the iterative conjugate gradient solvers DSLUBC, DSLUCS, and DSLUGM written in Fortran77 for the front end with the MPDS which uses the data parallel unit. The MPDS shows good performance relative to the iterative conjugate gradient solvers on the free-convection test problem. The robust direct solution provided by MPDS can be used to (1) check on the veracity of a given iterative conjugate gradient solution, or (2) be used on certain problems where iterative solvers fail to converge. The test problem used in this study is the Elder pure thermal convection problem. For generality, the authors use the TOUGH2 equation of state module EOS3 for water, air, and heat, although this particular problem is a single-phase flow problem

Mathematical modeling of permeation grouting and subsurface barrier performance

The injection of solution grouts into the subsurface can be used to form underground barriers for the containment of contaminants. The technology requires identifying suitable grout materials, specifically fluids which exhibit a large increase in viscosity after injection and eventually solidify after a controllable period, thus sealing permeable zones. The authors have developed a new fluid property module for the reservoir simulator TOUGH2 to model grout injection, taking into account the increase of liquid viscosity as a function of time and gel concentration. They have also incorporated into the simulator a model which calculates soil hydraulic properties after solidification of the gel within the pore space. The new fluid property module has been used to design and analyze laboratory experiments and field pilot tests in saturated and unsaturated formations under a variety of subsurface conditions. These applications include modeling barrier emplacement in highly heterogeneous soils in the vadose zone, grout injection into the saturated zone in combination with extraction wells for flow control, the design of verification strategies, and the analysis of barrier performance. In this paper the authors discuss the modeling approach and present simulation results of multiple grout injections into a heterogeneous, unsaturated formation

A field test of a waste containment technology using a new generation of injectable barrier liquids

A first stage field injection of a new generation of barrier liquids was successfully completed. Two types of barrier liquids, colloidal silica (CS) and polysiloxane (PSX), were injected into heterogeneous unsaturated deposits of sand, silt, and gravel typical of many of the arid DOE cleanup sites and particularly analogous to the conditions of the Hanford Site. Successful injection by commercially available chemical grouting equipment and the tube-a-manchette technique was demonstrated. Excavation of the grout bulbs permitted visual evaluation of the soil permeation by the grout, as well as sample collection. Both grouts effectively permeated all of the formation. The PSX visually appeared to perform better, producing a more uniform and symmetric permeation regardless of heterogeneity, filling large as well as small pores and providing more structural strength than the CS. Numerical simulation of the injection tests incorporated a stochastic field to represent site heterogeneity and was able to replicate the general test behavior. Tiltmeters were used successfully to monitor surface displacements during grout injection

Mathematical Formulation Requirements and Specifications for the Process Models

The Advanced Simulation Capability for Environmental Management (ASCEM) is intended to be a state-of-the-art scientific tool and approach for understanding and predicting contaminant fate and transport in natural and engineered systems. The ASCEM program is aimed at addressing critical EM program needs to better understand and quantify flow and contaminant transport behavior in complex geological systems. It will also address the long-term performance of engineered components including cementitious materials in nuclear waste disposal facilities, in order to reduce uncertainties and risks associated with DOE EM's environmental cleanup and closure activities. Building upon national capabilities developed from decades of Research and Development in subsurface geosciences, computational and computer science, modeling and applied mathematics, and environmental remediation, the ASCEM initiative will develop an integrated, open-source, high-performance computer modeling system for multiphase, multicomponent, multiscale subsurface flow and contaminant transport. This integrated modeling system will incorporate capabilities for predicting releases from various waste forms, identifying exposure pathways and performing dose calculations, and conducting systematic uncertainty quantification. The ASCEM approach will be demonstrated on selected sites, and then applied to support the next generation of performance assessments of nuclear waste disposal and facility decommissioning across the EM complex. The Multi-Process High Performance Computing (HPC) Simulator is one of three thrust areas in ASCEM. The other two are the Platform and Integrated Toolsets (dubbed the Platform) and Site Applications. The primary objective of the HPC Simulator is to provide a flexible and extensible computational engine to simulate the coupled processes and flow scenarios described by the conceptual models developed using the ASCEM Platform. The graded and iterative approach to assessments naturally generates a suite of conceptual models that span a range of process complexity, potentially coupling hydrological, biogeochemical, geomechanical, and thermal processes. The Platform will use ensembles of these simulations to quantify the associated uncertainty, sensitivity, and risk. The Process Models task within the HPC Simulator focuses on the mathematical descriptions of the relevant physical processes

Scaling of Transport Coefficients of Porous Media under Compaction

Porous sediments in geological systems are exposed to stress by the above-laying mass and consequent compaction, which may be significantly nonuniform across the massif. We derive scaling laws for the compaction of sediments of similar geological origin. With these laws, we evaluate the dependence of the transport properties of a fluid-saturated porous medium (permeability, effective molecular diffusivity, hydrodynamic dispersion, electrical and thermal conductivities) on its porosity. In particular, we demonstrate that the assumption of a uniform geothermal gradient is not adequate for systems with nonuniform compaction and show the importance of the derived scaling laws for mathematical modelling of methane hydrate deposits; these deposits are believed to have potential for impact on global climate change and Glacial-Interglacial cycles.Comment: 6 pages, 2 figure

An international code comparison study on coupled thermal, hydrologic and geomechanical processes of natural gas hydrate-bearing sediments

Highlights • Code comparisons build confidence in simulators to model interdependent processes. • International hydrate reservoir simulators are compared over five complex problems. • Geomechanical processes significantly impact response of gas hydrate reservoirs. • Simulators yielded comparable results, however many differences are noted. • Equivalent constitutive models are required to achieve agreement across simulators. Geologic reservoirs containing gas hydrate occur beneath permafrost environments and within marine continental slope sediments, representing a potentially vast natural gas source. Numerical simulators provide scientists and engineers with tools for understanding how production efficiency depends on the numerous, interdependent (coupled) processes associated with potential production strategies for these gas hydrate reservoirs. Confidence in the modeling and forecasting abilities of these gas hydrate reservoir simulators (GHRSs) grows with successful comparisons against laboratory and field test results, but such results are rare, particularly in natural settings. The hydrate community recognized another approach to building confidence in the GHRS: comparing simulation results between independently developed and executed computer codes on structured problems specifically tailored to the interdependent processes relevant for gas hydrate-bearing systems. The United States Department of Energy, National Energy Technology Laboratory, (DOE/NETL), sponsored the first international gas hydrate code comparison study, IGHCCS1, in the early 2000s. IGHCCS1 focused on coupled thermal and hydrologic processes associated with producing gas hydrates from geologic reservoirs via depressurization and thermal stimulation. Subsequently, GHRSs have advanced to model more complex production technologies and incorporate geomechanical processes into the existing framework of coupled thermal and hydrologic modeling. This paper contributes to the validation of these recent GHRS developments by providing results from a second GHRS code comparison study, IGHCCS2, also sponsored by DOE/NETL. IGHCCS2 includes participants from an international collection of universities, research institutes, industry, national laboratories, and national geologic surveys. Study participants developed a series of five benchmark problems principally involving gas hydrate processes with geomechanical components. The five problems range from simple geometries with analytical solutions to a representation of the world's first offshore production test of methane hydrates, which was conducted with the depressurization method off the coast of Japan. To identify strengths and limitations in the various GHRSs, study participants submitted solutions for the benchmark problems and discussed differing results via teleconferences. The GHRSs evolved over the course of IGHCCS2 as researchers modified their simulators to reflect new insights, lessons learned, and suggested performance enhancements. The five benchmark problems, final sample solutions, and lessons learned that are presented here document the study outcomes and serve as a reference guide for developing and testing gas hydrate reservoir simulators