Process & System Modeling Projects
Gas Threshold Pressure Testing
INTERA has conducted and interpreted hydrogeologic tests in support of the French site characterization program for deep disposal of radioactive wastes. As part of the investigations at Bure, INTERA designed and analyzed the Gas Threshold Pressure Test (GTPT) to quantify two-phase flow properties of the clay formation at this site. To accomplish this, we used numerical modeling to determine the hydraulic characteristics of the low-permeability clay formation based on the analysis of previously conducted hydrotests. Using the preliminary results of the hydrotests, design modeling of the GTPT was performed using the code TOUGH2 to optimize the rate and duration of the gas injection and recovery sequences. Apparent non-standard two-phase flow behavior was observed during the GTPT, and was analyzed with the TOUGH2 model to allow adjustments to the test procedure. The apparent fracturing during the GTPT was implemented in the model with pressure-dependent permeability functions that were different for the recovery and injection sequences due to the non-linear behavior between injection and recovery. A subsequent geomechanical assessment of the test conditions, that indicated a composite well-aquifer model, supported the inferred fracturing of the formation. The gas pressure exceeded the effective minimum stress in the formation, which was lowered by pore-water displacement from the inner zone into the outer zone, causing fracturing of the formation. Overall, the approach used for the analysis of the complex gas injection test sequence accurately reproduced the observed pressure response.
Hazardous Waste Deep-Injection Wells
INTERA conducted modeling of deep-well injection of hazardous waste for a large petrochemical refinery located in Ohio to support obtaining an exemption from the land-disposal restriction for hazardous waste. This facility operates three Class-I wells that inject at depths ranging from 2,800 to 3,200 feet below ground surface into a sandstone formation. The demonstration for no-migration exemption requires that, to a reasonable degree of certainty, there will be no migration of hazardous constituents from the permitted injection zone for as long as the injected waste remains hazardous. This was accomplished by demonstrating, through the use of a three-dimensional flow and transport model, that the injected waste will not migrate vertically out of a defined injection zone or migrate laterally to a point of discharge or interface with a source of drinking water over a period of 10,000 years.Our efforts included determining defensible parameter values for the numerical model based on spinner surveys and caliper logs to define the effective injection intervals; core measurements of porosity, bulk compressibility, horizontal and vertical permeability; horizontal permeability and anisotropy determined through analysis of drill-stem tests and interference tests; injectate specific gravity and viscosity and native formation brine specific gravity from laboratory analyses. Additional model inputs included longitudinal and transverse dispersivity and viscosity of the native formation brine. Site specific injection rates were used to model the historical injection period and conservative injection rates were used to model the future injection period.
The three-dimensional numerical model was calibrated to site specific data and used to simulate historical injection and to predict both lateral and vertical pressure responses and constituent migration due to future injection and pressure recovery after injection stops. Since the specific gravity of the injectate and native formation brine differed, the model included variable density effects. In addition to this site, INTERA has completed similar no-migration petitions for deep-well waste injection at 14 other sites across the U.S. In 1990, one of our clients became the first in the nation to win an exemption from the land-disposal restrictions for hazardous waste.
Two-Phase Flow Modeling-Switzerland
INTERA has played an integral role in supporting the two-phase flow characterization of fracture zones at the Swiss underground rock laboratory known as the Grimsel Test Site (GTS) through the analysis of gas injection tests. The two-phase flow analysis involved inverse simulation of complex gas injection sequences with subsequent recovery for estimating flow parameters. In addition, statistical analyses of the inverse simulation results were used to quantify the uncertainty of the parameter estimates and to evaluate different conceptual models of two-phase fracture flow. The two-phase flow characterization of fracture zones at the GTS was extended to gas tracer tests that were performed in the fracture rock investigation zone. Two gas tracers (He and Xe) with significantly different solubilities were injected into a stationary two-phase flow field between two boreholes that intersect the same fracture zone. For this test, two-phase flow and transport simulations were performed using the two-phase flow code TOUGH2 to evaluate the transport of gas tracers in the gas phase and the interaction of the gas tracers with the liquid phase.
The Gas Migration Test (GMT) experiment at the GTS is designed to study migration from waste generated gas through an engineered barrier system (EBS) into the geosphere under realistic conditions. The EBS consists of a concrete cylinder surrounded by backfill material emplaced in a cylindrical excavation in an underground drift at the GTS. INTERA performed design modeling of the GMT experiment, evaluated the feasibility of the experiment, refined the experimental procedure given updated information on the EBS and on the surrounding geosphere, and analyzed the measured response during the experiment with a numerical model to verify prediction and improve the understanding of the system response. The three-dimensional design model was implemented with the TOUGH2 code to simulate saturation of the EBS by natural water inflow from the geosphere and by water injection at specific locations inside the EBS. Inverse simulations with ITOUGH2 were performed to estimate hydraulic and two-phase flow parameters based on the measurements taken during the saturation experiment. The calibrated design model was then used to refine the test procedure for the subsequent gas migration experiment.
Two-Phase Flow Modeling-Japan
INTERA conducted two-phase flow simulations associated with gas migration from deep underground radioactive waste repositories. Our efforts included investigation of two-phase flow processes associated with the construction and operation of the repository and with migration of different waste-generated gases after repository closure. Numerical simulations were performed considering multiple gases, including hydrogen (H2), methane (CH4), CO2, and air. To investigate the operational period of the underground repository, a three-dimensional model was developed incorporating the complex folded structure of different sedimentary layers and associated faults. Saturated and unsaturated flow simulations were performed to investigate the effects of shaft excavation on the pressure and development of an unsaturated zone around the ventilated shafts, taking into account potential effects of dissolved CH4 and CO2 in groundwater. During depressurization near the shaft, degassing of the dissolved gases occur, which accumulate in the open shaft.
For the simulation of post-closure gas migration from an underground repository, both H2 and CH4 were considered, using the computer code TMVOC. This code is a further development of the Equation-of-State (EOS) modules of the TOUGH2 code, which considers three-phase non-isothermal flows of multi-component hydrocarbon mixtures in saturated-unsaturated heterogeneous media. The existing TMVOC version handles up to 10 different non-condensible gases (i.e., N2, O2, air, CO2, CH4, and several other hydrocarbons), and was modified to account for H2. Gas migration from the repository was simulated by injecting both H2 and CH4 at time-varying rates corresponding to the gas generation associated with corrosion and degradation of waste. Sensitivity simulations were performed to examine migration of waste-generated gas (H2 and CH4) from the repository through different host rocks to the land surface, taking into account potential effects of naturally occurring dissolved gases in groundwater.
