James William Carey

Pre-site characterization risk analysis for commercial-scale carbon sequestration.

This study develops a probability framework to evaluate subsurface risks associated with commercial-scale carbon sequestration in the Kevin Dome, Montana. Limited knowledge of the spatial distribution of physical attributes of the storage reservoir and the confining rocks in the area requires using regional data to estimate project risks during the pre-site characterization analysis. A set of integrated Monte Carlo simulations are used to assess four risk proxies: the CO2 injectivity, area of review (AoR), migration rate into confining rocks, and a monitoring strategy prior to detailed site characterization. Results show a reasonable likelihood of reaching the project goal of injecting 1 Mt in 4 years with a single injection well (>58%), increasing to >70% if the project is allowed to run for 5 years. The mean radius of the AoR, based on a 0.1 MPa pressure change, is around 4.8 km. No leakage of CO2 through the confining units is seen in any simulations. The computed CO2 detection probability suggests that the monitoring wells should be located at less than 1.2 km away from the injection well so that CO2 is likely to be detected within the time frame of the project. The scientific results of this study will be used to inform the detailed site characterization process and to provide more insight for understanding operational and technical risks before injecting CO2.

CO2/Brine transport into shallow aquifers along fault zones.

Unintended release of CO(2) from carbon sequestration reservoirs poses a well-recognized risk to groundwater quality. Research has largely focused on in situ CO(2)-induced pH depression and subsequent trace metal mobilization. In this paper we focus on a second mechanism: upward intrusion of displaced brine or brackish-water into a shallow aquifer as a result of CO(2) injection. Studies of two natural analog sites provide insights into physical and chemical mechanisms controlling both brackish water and CO(2) intrusion into shallow aquifers along fault zones. At the Chimayó, New Mexico site, shallow groundwater near the fault is enriched in CO(2) and, in some places, salinity is significantly elevated. In contrast, at the Springerville, Arizona site CO(2) is leaking upward through brine aquifers but does not appear to be increasing salinity in the shallow aquifer. Using multiphase transport simulations we show conditions under which significant CO(2) can be transported through deep brine aquifers into shallow layers. Only a subset of these conditions favor entrainment of salinity into the shallow aquifer: high aspect-ratio leakage pathways and viscous coupling between the fluid phases. Recognition of the conditions under which salinity is favored to be cotransported with CO(2) into shallow aquifers will be important in environmental risk assessments.

Understanding hydraulic fracturing: a multi-scale problem

Despite the impact that hydraulic fracturing has had on the energy sector, the physical mechanisms that control its efficiency and environmental impacts remain poorly understood in part because the length scales involved range from nanometres to kilometres. We characterize flow and transport in shale formations across and between these scales using integrated computational, theoretical and experimental efforts/methods. At the field scale, we use discrete fracture network modelling to simulate production of a hydraulically fractured well from a fracture network that is based on the site characterization of a shale gas reservoir. At the core scale, we use triaxial fracture experiments and a finite-discrete element model to study dynamic fracture/crack propagation in low permeability shale. We use lattice Boltzmann pore-scale simulations and microfluidic experiments in both synthetic and shale rock micromodels to study pore-scale flow and transport phenomena, including multi-phase flow and fluids mixing. A mechanistic description and integration of these multiple scales is required for accurate predictions of production and the eventual optimization of hydrocarbon extraction from unconventional reservoirs. Finally, we discuss the potential of CO2 as an alternative working fluid, both in fracturing and re-stimulating activities, beyond its environmental advantages. This article is part of the themed issue ‘Energy and the subsurface’.

A geostatistical modeling study of the effect of heterogeneity on radionuclide transport in the unsaturated zone, Yucca Mountain.

Retardation of certain radionuclides due to sorption to zeolitic minerals is considered one of the major barriers to contaminant transport in the unsaturated zone of Yucca Mountain. However, zeolitically altered areas are lower in permeability than unaltered regions, which raises the possibility that contaminants might bypass the sorptive zeolites. The relationship between hydrologic and chemical properties must be understood to predict the transport of radionuclides through zeolitically altered areas. In this study, we incorporate mineralogical information into an unsaturated zone transport model using geostatistical techniques to correlate zeolitic abundance to hydrologic and chemical properties. Geostatistical methods are used to develop variograms, kriging maps, and conditional simulations of zeolitic abundance. We then investigate, using flow and transport modeling on a heterogeneous field, the relationship between percent zeolitic alteration, permeability changes due to alteration, sorption due to alteration, and their overall effect on radionuclide transport. We compare these geostatistical simulations to a simplified threshold method in which each spatial location in the model is assigned either zeolitic or vitric properties based on the zeolitic abundance at that location. A key conclusion is that retardation due to sorption predicted by using the continuous distribution is larger than the retardation predicted by the threshold method. The reason for larger retardation when using the continuous distribution is a small but significant sorption at locations with low zeolitic abundance. If, for practical reasons, models with homogeneous properties within each layer are used, we recommend setting nonzero K(d)s in the vitric tuffs to mimic the more rigorous continuous distribution simulations. Regions with high zeolitic abundance may not be as effective in retarding radionuclides such as Neptunium since these rocks are lower in permeability and contaminants can only enter these regions through molecular diffusion.

A non-locking composite tetrahedron element for the combined finite discrete element method

Purpose In order to avoid the problem of volumetric locking often encountered when using constant strain tetrahedral finite elements, the purpose of this paper is to present a new composite tetrahedron element which is especially designed for the combined finite-discrete element method (FDEM). Design/methodology/approach A ten-noded composite tetrahedral (COMPTet) finite element, composed of eight four-noded low order tetrahedrons, has been implemented based on Munjiza’s multiplicative decomposition approach. This approach naturally decomposes deformation into translation, rotation, plastic stretches, elastic stretches, volumetric stretches, shear stretches, etc. The problem of volumetric locking is avoided via a selective integration approach that allows for different constitutive components to be evaluated at different integration points. Findings A number of validation cases considering different loading and boundary conditions and different materials for the proposed element are presented. A practical application of the use of the COMPTet finite element is presented by quantitative comparison of numerical model results against simple theoretical estimates and results from acrylic fracturing experiments. All of these examples clearly show the capability of the composite element in eliminating volumetric locking. Originality/value For this tetrahedral element, the combination of “composite” and “low order sub-element” properties are good choices for FDEM dynamic fracture propagation simulations: in order to eliminate the volumetric locking, only the information from the sub-elements of the composite element are needed which is especially convenient for cases where re-meshing is necessary, and the low order sub-elements will enable robust contact interaction algorithms, which maintains both relatively high computational efficiency and accuracy.

GEOCHEMICAL METHOD FOR IDENTIFYING ALKALI-SILICA-REACTION GEL

A geochemical method for staining various products of the alkali-silica reaction is presented. The method is based on both the composition of alkali-silica-reaction (ASR) gel and one of its properties (the ability to exchange cations with a fluid). Specifically, one stain (sodium cobalt-initrite) reacts with exchangeable potassium in the gel to form a bright-yellow precipitate on the gel surface. The other class of stains (a variety of rhodamine compounds) reacts with calcium-rich portions of the gel (and, for some compounds, with other calcium-rich components of the concrete) to form a pink-stained gel. The significance of the pink staining is twofold. First, it can provide a high contrast to the yellow-stained gels, making them easier to observe. Second, some rhodamine compounds react predominantly with Ca-rich ASR gels. Some aspects of staining by rhodamine remain the subject of continued study and may be useful for a more detailed understanding of ASR progression and other deterioration mechanisms. However, a positive diagnosis of ASR is indicated by the presence of yellow-stained gel within aggregate, at the aggregate-paste interface, along fractures, or in air voids. The technique can be used as a rapid field screening method or as a useful aid for detailed petrographic examinations.

Pore-Network Approach for Calculating the Interfacial Area of Wetting/Non-Wetting Phases in Porous Media

In order to better understand and quantify many flow and transport processes in porous media (e.g. soil remediation strategies, reactive transport, CO2 sequestration, etc.) it is important to acquire a detailed knowledge of fluid-fluid and fluid-solid interfacial areas since they play a key role in the dynamics of multiphase flow and transport in porous media. Fluid-fluid interfacial areas (e.g., between wetting and non-wetting phases) control many mass transfer processes in porous media such as phase partitioning (adsorption) and volatilization as well as colloidal and microbial transport since it has been shown that they can serve as sorption sites. Fluid-solid interfacial areas (e.g., in partially saturated media) controls mineral reaction rates and adsorption phenomena. During CO2 sequestration in a saline aquifer, the brine/CO2 interface controls the amount of CO2 dissolved in the brine phase and the amount of H2O dissolved in the CO2 plume, while the fluid/solid interface controls the dissolution of the porous media and precipitation of reaction products (thus affecting mineral sequestration of CO2). Due to the complicated nature of experimentally measuring the fluid-fluid interfaces it is useful to have a predictive approach. In this study, a 3-D pore-network approach is introduced to calculate the interface between wetting and non-wetting phases in porous media. We consider a regular, cubic lattice of pores and throats of different geometries and sizes. The pores have either spherical or cubic geometry and the throats have cylindrical, rectangular, or triangular geometries. The constructed network can contain one or multiple types of pores/throats and can have variable saturation of wetting and/or non-wetting phases, resulting in different interfacial areas. In a first step, different network saturations are obtained by randomly distributing wetting/non-wetting phases in the network based on concepts borrowed from Ordinary Percolation Theory. In a further step, we study the effect of the history of the fluid displacement (drainage, imbibition, etc.) on the resulting interfacial areas. These simulations are based on principals from Invasion Percolation Theory. We further report results on how the calculated interfacial areas are affected by parameters such as the range and distribution of pore/throat characteristic lengths, the wetting saturation, the pore-size distribution, and the extent of overlapping of pore/throat distributions.