Review ofTOUGH2 Numerical Modeling of the WCS Facility, Andrews County, Texas

The Bureau of Economic Geology (BEG) was tasked with reviewing the TOUGH2 numerical modeling submitted by WCS (two 3D models and one 2D model for the low-level application and one 2D model for the byproduct application). The preliminary step to the review consisted of the lengthy installation of the software and ensuring that the results agreed with those supplied by WCS and the software developers. The TOUGH2 2D transport modeling by the applicant addressed subsurface-parameter uncertainty in a relatively thorough fashion except for one variable: assumed top-boundary fluxes of ~0.01 inch/yr are too low. Simulations performed for and presented in this report with top flux increases to 0.1 and 1 inch/yr lead to much less conservative results, decreasing breakthrough time from ~14,000 years (applicant's base case) to less than 5,000 years (flux x 10) and less than 1,000 years (flux x 100) for Tc-99. Chloride's breakthrough time, used as a marker for the byproduct facility, also decreases from over 1,000 years (applicant's base case) to less than 200 years in other cases. Note that cases in which simulation results do not meet concentrations suggested by regulations do not necessarily invalidate the site. Because the 2D model used in the simulations is so conservative, a more realistic conceptual model consistent with site geology and hydrology would probably yield results that would be less extreme than some of those presented in this analysis. It is, however, the applicant's charge to develop such models, for example, by modeling the bottom liner (as applicable), by evaluating fracture extent and connectivity, and by better understanding the source (leachate chemical composition was obtained with no credit given to containers; in addition, high water flux is also likely to translate into a much lower radionuclide concentration). TOUGH2 3D models do provide insight into the behavior of the natural system but should be better calibrated and better constrained to provide arguments to the unproven applicant's contention that the system is and has been at steady state for tens of thousands of years.Bureau of Economic Geolog

Self Sealing Evaporation Ponds for Desalination Facilites in Texas

The State of Texas has taken a renewed interest in desalination of brackish water. Because the Texas population is expected to grow tremendously in coming decades, many municipalities and other water-supplying entities will need to supplement their current freshwater sources. Desalination of brackish water is high on the list of water-source alternatives for supplying some or all of the increased water needs in many communities. However, disposal of desalination concentrates may pose legal, technical, and economic barriers, especially for smaller communities with water supplies of less than one million gallons per day (MGD). In this report, we examine evaporation ponds and the possibility of incorporating a low-permeability layer (precipitant) into the pond-liner system as a liner component or possibly as the liner itself. One aspect of this analysis was to investigate the regulatory requirements and barriers of using self-sealing ponds, if this strategy proves to be a technically viable alternative to standard pond liners. Another part of the work consisted of understanding the favorable chemical conditions, natural or induced, for the precipitation of such a compound(s). The third and last facet of this work was to investigate the savings or extra costs of this approach. The following observations characterize the regulatory issues relating to self-sealing pond liners. (1) No significant regulatory barriers currently exist that would prevent the permitting of self-sealing evaporation pond-liner technologies at desalination facilities in Texas. (2) No Federal authorizations are required, but a Texas Land Application Permit (TLAP) must be obtained from the Texas Commission on Environmental Quality (TCEQ) Water Quality Division. (3) TCEQ has considerable latitude for approving alternative permit requirements for industrial permits. Rules for municipal wastewater treatment are used as guides for the evaluation of industrial evaporation ponds but do not impose strict regulatory requirements. Currently approved pond liners include a 3-foot-thick layer of in-situ clay or compacted clay (with a maximum hydraulic conductivity of 10-7 cm/s) or a geomembrane liner (polyvinyl chloride [PVC], high-density polyethylene [HDPE], butyl rubber, polypropylene, etc.) of 30 mils (0.76 mm) or more with leak detection monitoring. An alternative liner technology may be approved by TCEQ if it can be demonstrated to achieve and maintain equivalent containment capabilities to the pre-approved liners and that the resulting liner material(s) will not deteriorate because of reactivity with salinity or other compounds in the effluent stream or other ambient conditions. Supporting demonstration information may include previous research, pilot projects, and monitoring data from existing operational facilities currently utilizing the proposed technology. Regulatory processing for the permitting of an evaporation pond could be simplified if the self-sealing technology were recognized by the TCEQ as an accepted type of liner, equivalent to compacted clay or geomembrane liners. No statutory change or rulemaking would be required to revise the permit instructions to add self-sealing pond liners to the list of acceptable methods, although compelling scientific and engineering evidence would be necessary to justify such a modification.Bureau of Economic Geolog

Risk-based approach to assess CO2 storage capacity

Bureau of Economic Geolog

Historical and 2006–2025 estimation of ground water use for gas production in the Barnett Shale, North Texas

The Barnett Shale play, currently the most prolific onshore gas play in the country, has seen a quick growth in the past decade with the development of new "frac" (a.k.a. fracture stimulation) technologies needed to create pathways to produce gas in the very low permeability mudstones. This technology uses large amounts of water in a short period of time to develop a gas well. There are currently over 5,600 wells producing gas from the Barnett Shale, with thousands more likely to be drilled in the next couple of decades as the play expands out of its core area. A typical vertical completion consumes approximately 1.2 million gallons, and a typical horizontal well completion 3.0 to 3.5 million gallons of fresh water. Almost 8,000 acre-feet of water (from all sources) was used in 2005, mostly in an area equivalent to a Texas county. This usage has raised some concerns among local communities and other groundwater stakeholders, especially in the footprint of the Trinity aquifer. In this study, we present projections of groundwater use by the oil and gas industry through 2025. Total water use is highly uncertain, being dependent on the price of gas above all. We approach this uncertainty by developing high, medium, and low scenarios that can be somewhat understood as cases with decreasing gas prices. Other important factors include geologic risk factors in the Barnett (maturity of the shale, thickness of the formation, presence of features limiting or hampering well completion), technological factors (horizontal vs. vertical wells, water recycling), operational factors (number of well completions that can be done in a year, proximity of a fresh-water source), and regulatory factors. The high scenario cumulates most of the high-end water use of the previous parameters, whereas the low scenario uses the low values of their range. The low scenario utilizes 29,000 AF of groundwater to the 2025 horizon (1,500 AF/yr on average), a clear retreat from the current annual rate of water use by the industry, corresponding to a large drop in gas price. The high scenario calls for a total water use between 2007 and 2025 of 417,000 AF of groundwater (~22,000 AF/yr on average). It corresponds to sustained high gas prices allowing operators to expand to all economically viable areas and produce most of the accessible resource but also includes the assumption that water use is not limiting. All scenarios assume that operators continue using water at a per-well rate similar to that of today and that no technological breakthrough will bring it down. The medium scenario assumes a groundwater use of 183,000 AF (~10,000 AF/yr on average). In the high scenario, groundwater use steadily climbs from ~5,000 AF/yr in 2005 to 20,000 AF/yr in 2010 and then slowly increases to a maximum of ~25,000 AF/yr in 2025. The medium scenario follows a similar path, climbing to a maximum of ~13,000 AF/yr in 2010 and then slowly decreasing to ~7,500 AF/yr in 2025. The medium case is not necessarily the most likely. Because the Barnett Shale play is dependent on gas prices, a more accurate statement would be to formulate that the medium case is the most likely under the condition that gas prices stay at their current level.Bureau of Economic Geolog

Investigation of water displacement following large CO2 sequestration operations

The scale of CO2 injection into the subsurface required to address CO2 atmospheric concentrations is unprecedented. Multiple injection sites injecting into multiple formations will create a large excess pressure zone extending far beyond the limited volume where CO2 is present. In a closed system, additional mass is accommodated by the compressibility of system components, an increase in fluid pressure, and possibly an uplift of the land surface. In an open system, as assumed in this analysis, another coping mechanism involves fluid flux out of the boundaries of the system, in which case the fresh-water-bearing outcrop areas, corresponding to the up-dip sections of the down-dip formations into which CO2 is injected, could be impacted. A preliminary study using a MODFLOW groundwater model extending far down-dip shows that injecting a large amount of fluid does have an impact some distance away from the injection area but most likely only in localized areas. A major assumption of this preliminary work was that multiphase processes do not matter some distance away from the injection zones. In a second step, presented in this paper, to demonstrate that a simplified model can yield results as useful as those of a more sophisticated multiphase-flow compositional model, we model the same system using CMG-GEM software. Because the chosen software lacks the ability to deal easily with unconfined water flow, we compare fluxes through time, as given by MODFLOW and CMG-GEM models at the confined/unconfined interface.Bureau of Economic Geolog

Pressure perturbations from geologic carbon sequestration: Area-of-review boundaries and borehole leakage driving forces

We investigate the possibility that brine could be displaced upward into potable water through wells. Because of the large volumes of CO2 to be injected, the influence of the zone of elevated pressure on potential conduits such as well boreholes could extend many kilometers from the injection site—farther than the CO2 plume itself. The traditional approach to address potential brine leakage related to fluid injection is to set an area of fixed radius around the injection well/zone and to examine wells and other potentially open pathways located in the “Area-of-Review” (AoR). This suggests that the AoR needs to be defined in terms of the potential for a given pressure perturbation to drive upward fluid flow in any given system rather than on some arbitrary pressure rise. We present an analysis that focuses on the changes in density/salinity of the fluids in the potentially leaking wellbore.Bureau of Economic Geolog

Divergence and flutter instabilities of some constrained two-degree-of-freedom systems

International audienceIt is now well ascertained that a variety of instability modes can appear before the conventional plastic limit condition is met. In this paper, both flutter and divergence instability modes are investigated. First, the mechanical meaning of these instability modes is reviewed, and the criterion for detecting their occurrence is established. Based on an illustration example, the competition between the occurrences of each of these instability modes is analyzed, showing that the prevalence of a given mode is strongly related to both the loading conditions and the stiffness properties of the material system in hand

Stability of non-conservative elastic structures under additional kinematics constraints

International audienceIn this paper, the specific effect of additional constraints on the stability of undamped non-conservative elastic systems is studied. The stability of constrained elastic system is compared to the stability of the unconstrained system, through the incorporation of Lagrange multipliers. It is theoretically shown that the second-order work criterion, dealing with the symmetric part of the stiffness matrix corresponds to an optimization criterion with respect to the kinematics constraints. More specifically, the vanishing of the second-order work criterion corresponds to the critical kinematics constraint, which can be interpreted as an instability direction when the material stability analysis is considered (typically in the field of soil mechanics). The approach is illustrated for a two-degrees-of-freedom generalised Ziegler's column subjected to different constraints. We show that a particular kinematics constraint can stabilize or destabilize a non-conservative system. However, for all kinematics constraints, there necessarily exists a constraint which destabilizes the non-conservative system. The constraint associated to the lowest critical load is associated with the second-order criterion. Excluding flutter instabilities, the second-order work criterion is not only a lower bound of the stability boundary of the free system, but also the boundary of the stability domain, for all mixed perturbations based on proportional kinematics conditions

On the stability of nonconservative elastic systems under mixed perturbations

International audienceThis paper shows that the loading mode strongly influences the stability of discrete non-conservative elastic systems. The stability of the constrained system is compared to the stability of the unconstrained system, through the incorporation of Lagrange multipliers. Initially, the approach is illustrated for a two-degrees-of-freedom generalized Ziegler's column. Then, it is applied to a two-degrees-of-freedom model representing a soil constrained with isochoric loading. The isochoric instability load is not necessarily greater than the instability load of the free problem. Excluding flutter instabilities, it is shown that the second-order work criterion is not only a lower bound of the stability boundary of the free system, but also the boundary of the stability domain, in presence of mixed perturbations based on proportional kinematic conditions.Cet article étudie l'influence du mode de chargement sur la stabilité de systèmes élastiques discrets non conservatifs. La stabilité du système contraint est comparée à celle du système libre, par l'introduction de multiplicateurs de Lagrange. L'approche est illustrée avec le pendule généralisé de Ziegler. Elle est ensuite appliquée à un modèle à deux degrés de liberté représentant un sol contraint par un chargement isochore. On montre que le chargement isochore affecte sensiblement la frontière de stabilité pour le problème conservatif et pour le problème non conservatif. En dehors des instabilités par flottement, le critère de travail du second-ordre constitue une borne inférieure de la frontière de stabilité du système libre ainsi que la frontière du domaine de stabilité du système sous chargements mixtes proportionnels en déplacement

Geographical, Geological, and Hydrogeological Attributes of Formations in the Footprint of the Eagle Ford Shale

This document provides an overview of the geological characteristics of formations within the footprint of the South Texas Eagle Ford (EF) Shale play, with a particular focus on water. The EF play, spanning approximately 25 counties, has experienced significant development in recent years, with expansion into additional counties to the north. Despite its recent growth, the EF area has a long history of oil and gas exploration and production, with over 110,000 wells drilled in the past century, excluding the approximately 5,000 EF wells as of March 2013. The EF shale serves as a source rock, supplying oil and gas to reservoirs such as the Big Well, Pearsall fields, and the Giddings field. While predominantly rural, the EF area encompasses several large cities such as San Antonio and Laredo, which border its edges. This document focuses on two key aspects of hydraulic fracturing (HF) in the EF play: water use and water disposal. The South Texas location of the play, coupled with its limited surface water resources, intensifies perceived conflicts with other water users. The significant depth of the folded Paleozoic basement beneath the EF (exceeding 15,000 ft) allows for a thick sediment sequence of Jurassic and younger age. Positioned in the middle of this sequence (approximately 4,000 to 11,000 ft deep), the EF shale is separated from the ground surface by numerous formations, including the Midway Clay. This geological setup provides multiple horizons for fluid disposal. The thickness of the EF varies from approximately 100 ft east of Austin to over 500 ft at the Mexican border. The sedimentary sequence above the basement initially comprises carbonate-rich formations such as the Edwards, Glenrose, and Austin Chalk formations, with the EF itself being a carbonate mudrock. Towards the end of the Cretaceous period, the succession transitions to siliciclastic formations characterized by alternating sandstones and claystones deposited in fluvial and/or deltaic environments. Some sand-rich intervals within this succession form freshwater aquifers in the EF footprint, including the Carrizo aquifer, as well as other aquifers of lesser water quality such as the Wilcox and Yegua-Jackson aquifers. Shallow subsurface water tends to be brackish outside of the aquifer outcrop areas. In 2011, water use in the EF play amounted to approximately 24 thousand acre-feet (AF). The top HF users in the EF during that year were Webb (4.6 kAF), Karnes (3.9 kAF), Dimmit (3.7 kAF), and La Salle (2.9 kAF) counties. Although overall water use has increased, water use per well has decreased due to operational changes, including a shift from gas to oil and condensate production and the use of gelled HF treatments instead of slick-water treatments. Currently, operators recycle minimal amounts of flowback/produced water, with brackish water accounting for approximately 20% of total water use. Recycling remains limited due to insufficient flowback volumes for subsequent HF operations, particularly in the early stages. Flowback/produced water is primarily disposed of in injection wells, with approximately 2,500 Class II injection wells active between 2008 and 2012, many of which are associated with waterflood operations rather than disposal. Preferred disposal horizons include formations of the Navarro-Taylor Groups in the Maverick Basin and the Wilcox and Edwards formations.Bureau of Economic Geolog