Confining layer

Fig. 4. Multilayered aquifer flow where t represents thickness of confining layers between units 1, 2, and 3.

Determine geometry of aquifers and confining layers, aquifer recharge and discharge... 

Clearly additional layers may be used to accomplish other benefits, tailoring the energy profiles and mobilities across the entire organic stack. Splitting the transport layer(s) into two separate layers permits the optimization of injection into the layer nearest the electrode (sometimes called the injection layer), and transport in the farther layer [101]. Layers of insulator (charge confinement layers) have also been used in an attempt to control the motion of the charges and ensure recombination in the desired region [102]. 

If osmotic effects are possible, several other effects would need to be considered in a geochemical-fate assessment, depending on whether the solute concentration is increased or decreased. If solute concentrations are increased, pressures associated with injection would increase beyond those predicted without osmotic effects. Also, the movement of ions to the injection zone from the aquifer with lower salinity (above the clay confining layer) would increase the salinity above those levels predicted by simple mixing of the reservoir fluid and the injected wastes. This action could affect the results of any geochemical modeling. 

If solute concentrations are decreased, the remote possibility exists that wastes would migrate through the confining layer. For this to occur, solute concentrations above the confining layer would have to be higher than those in the injection zone, and movement, in any event, would be very slow. As USDWs have salinities less than 10,000 mg/L, compared with typical salinities in injection zones of 20,000 to 70,000 mg/L, even if this process were to occur it would cause migration only to overlying aquifers that are not USDWs. 

This section provides information on the range of environmental conditions that occur in deep-well-injection zones in different geologic regions of the U.S. The section on lithology discusses the types of sedimentary formations that are suitable for deep-well injection and confining layers and provides some information on geologic formations that are used for deep-well injection of wastes. The section on brine chemistry discusses the typical range of chemical characteristics of formation waters found in injection zones. 

Sedimentary rocks that are most likely to meet the first three criteria are unfractured shale, clay, siltstone, anhydrite, gypsum, and salt formations. Massive limestones and dolomites (i.e., carbonates with no continuous fracturing and solution channels) can also serve as confining layers. Then-suitability must be determined on a case by case basis. The fourth criterion has no relationship to lithology. 

In extreme situations, incompatibility between injection fluids and reservoir components can be so great that deep-well disposal will not be the most cost-effective approach to waste disposal. In other situations, such remedial measures as pretreatment or controlling fluid concentrations or temperatures can permit injection even when incompatibilities exist. In addition to operational problems, waste-reservoir incompatibility can cause wastes to migrate out of the injection zone (casing/confining-layer failure) and even cause surface-water contamination (well blowout). 

Interactions between corrosive wastes and casing and packing can threaten the integrity of a well if proper materials have not been used in construction. Of equal concern is the potential for failure of the confining zone due to physical or chemical effects. For example, dissolution of an overlying carbonate confining layer may allow upward migration of wastes. This process was observed when hot acidic wastes were injected in a Florida well. 

The Belle Glade site, located southeast of Lake Okeechobee in south-central Florida, illustrates some of the problems that can develop with acidic-waste injection when carbonate rock is the confining layer. Contributing factors to the contamination of the aquifer above the confining zone were the dissolution of the carbonate rock and the difference in density between the injected wastes and the formation fluids. The injected waste was less dense than the groundwater because of its lower salinity and higher temperature.172... 

At the time injection began, a shallow monitoring well was placed 23 m (75 ft) south of the injection well in the upper part of the Floridan aquifer above the confining layer. A downgradient, deep monitoring well was placed in the injection zone 300 m (1000 ft) southeast of the injection well. Another shallow well, located 3.2 km (2 miles) southeast of the injection site at the University of Florida s Everglades Experiment Station, has also been monitored for near-surface effects. 

The wastes are injected into the lower part of the carbonate Floridan aquifer, which is extremely permeable and cavernous. The natural direction of groundwater flow is to the southeast. The confining layer is 45 m (150 ft) of dense carbonate rocks. The chloride concentration in the upper part of the injection zone is 1650 mg/L, increasing to 15,800 mg/L near the bottom of the formation.172 The sources used for this case study did not provide any data on the current injection zone. The native fluid was basically a sodium-chloride solution but also included significant quantities of sulfate (1500 mg/L), magnesium (625 mg/L), and calcium (477 mg/L). 

The injection zone consisted of multiple Upper Cretaceous strata of sand, silty sand, clay, and some thin beds of limestone (see Figure 20.14). The clay confining layer was about 30 m (100 ft) thick. 

The injection zone was a cavernous dolomite, and the native groundwater was very saline, with TDS levels ranging from 21,000 to 26,000 mg/L. No information was provided on the confining layer, but it is discussed in the work by Brower and colleagues183 in detail. 

Warner, D.L., Davis, S.N., and Syed, T., Evaluation of confining layers for containment of injected wastewater, in Proc. Int. Symp. Subsurface Injection of Liquid Wastes, New Orleans, National Water Well Association, Dublin, OH, 1986, pp. 417-446. 

Some injected wastes are persistent health hazards that need to be isolated from the biosphere indefinitely. For this reason, and because of the environmental and operational problems posed by loss of permeability or formation caving, well operators seek to avoid deterioration of the formation accepting the wastes and its confining layers. When wastes are injected, they are commonly far from chemical equilibrium with the minerals in the formation and, therefore, can be expected to react extensively with them (Boulding, 1990). The potential for subsurface damage by chemical reaction, nonetheless, has seldom been considered in the design of injection wells. 

Y. Ohmori, M. Uchida, K. Muro, and K. Yoshino, Effects of alkyl chain lengths and carrier confinement layer on characteristics of poly(3-alkylthiophene) electroluminescent diodes, Solid State Commun., 80 605-608, 1991. 

Figure 5 Typical velocity relationship of kinetic friction for a sliding contact in which friction is from adsorbed layers confined between two incommensurate walls. The kinetic friction F is normalized by the static friction Fs. At extremely small velocities v, the confined layer is close to thermal equilibrium and, consequently, F is linear in v, as to be expected from linear response theory. In an intermediate velocity regime, the velocity dependence of F is logarithmic. Instabilities or pops of the atoms can be thermally activated. At large velocities, the surface moves too quickly for thermal effects to play a role. Time-temperature superposition could be applied. All data were scaled to one reference temperature. Reprinted with permission from Ref. 25. 

Potential problems can result if monitoring well construction is such that the gravel pack and screened interval are not discretely terminated at the confining layer and its grouted annulus. 

When the subsurface materials are uniform and isotropic (no gradational changes or confining layers) the airflow pathways are also uniform. The airflow paths developed for an open system and a covered system are shown in Figure 10.5. Selection of covered or uncovered is determined by the air paths necessary to contact the contamination. At some sites, inlet vent wells are installed to ensure air entry at specific locations. 

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