Showing papers by "George J. Moridis published in 1995"

Injectable barriers for waste isolation

Abstract: In this paper the authors report laboratory work and numerical simulation done in support of development and demonstration of injectable barriers formed from either of two fluids: colloidal silica or polysiloxane. Two principal problems addressed here are control of gel time and control of plume emplacement in the vadose zone. Gel time must be controlled so that the viscosity of the barrier fluid remains low long enough to inject the barrier, but increases soon enough to gel the barrier in place. During injection, the viscosity must be low enough to avoid high injection pressures which could uplift or fracture the formation. To test the grout gel time in the soil, the injection pressure was monitored as grouts were injected into sandpacks. When grout is injected into the vadose zone, it slumps under the influence of gravity, and redistributes due to capillary forces as it gels. The authors have developed a new module for the reservoir simulator TOUGH2 to model grout injection into the vadose zone, taking into account the increase of liquid viscosity as a function of gel concentration and time. They have also developed a model to calculate soil properties after complete solidification of the grout. The numerical model has been used to design and analyze laboratory experiments and field pilot tests. The authors present the results of computer simulations of grout injection, redistribution, and solidification.

On the Development of MP-TOUGH2

Abstract: 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.

A new set of direct and iterative solvers for the TOUGH2 family of codes

Abstract: Two new solvers are discussed. LUBAND, the first routine is a direct solver for banded systems and is based on a LU decomposition with partial pivoting and row interchange. BCGSTB, the second routine, is a Preconditioned Conjugate Gradient (PCG) solver with improved speed and convergence characteristics. Bandwidth minimization and gridblock ordering schemes are also introduced into TOUGH2 to improve speed and accuracy. TOUGH2 simulates fluid and heat flows in permeable media and is used for the evaluation of WIPP and TEVES (Thermal Enhanced Vapor Extraction System) that will be used to extract solvents from the Chemical Waste Landfill at Sandia National Laboratories.

Air barriers for waste containment in the subsurface

Abstract: The increase of air saturation in a soil alters significantly its hydraulic characteristics by reducing its the relative permeability to liquids. This realization led to the concept that air injection could be used in the context of remedial strategies to create low permeability barriers to contaminated water and NAPL migration. Air offers a number of significant advantages as a barrier fluid: it is not a contaminant, already exists in the vadose zone, is abundant, easily available, free of charge, and has well-known thermodynamic properties. This report provides a brief summary of air barrier modeling results to date.