Residual saturation

We next determine the residual saturation, m , by first calculating the dimensionless capillary number ... 

The pore geometry described in the above section plays a dominant role in the fluid transport through the media. For example, Katz and Thompson [64] reported a strong correlation between permeability and the size of the pore throat determined from Hg intrusion experiments. This is often understood in terms of a capillary model for porous media in which the main contribution to the single phase flow is the smallest restriction in the pore network, i.e., the pore throat. On the other hand, understanding multiphase flow in porous media requires a more complete picture of the pore network, including pore body and pore throat. For example, in a capillary model, complete displacement of both phases can be achieved. However, in real porous media, one finds that displacement of one or both phases can be hindered, giving rise to the concept of residue saturation. In the production of crude oil, this often dictates the fraction of oil that will not flow. 

Even after a large supply of water has migrated downward through a soil zone, under conditions where gravity is the dominant force, some water will be retained on and between the soil particles as residual saturation. The relationship between several soil horizons in the unsaturated zone is shown in Figure 3.29. Water in each of these zones is held according to the local conditions of soil suction. It is important to note that water in liquid form cannot be held by soil if the soil suction is >0.7 atm (10 psi). The zones of unsaturated soil are listed below ... 

Residual saturation (hygroscopic water or pendular saturation) Water is held under tension of 31 to 10,000 atm force. 

Funicular saturation Similar to residual saturation, except that each grain is surrounded by a water film and has a large air content. 

Residual Saturation of-i DNAPL in Soil from Spill ... 

Typical Residual Saturation Data for Various LNAPL and DNAPL Types... 

Migration of free-phase NAPLs in the subsurface is governed by numerous properties including density, viscosity, surface tension, interfacial tension, immisci-bility, capillary pressure, wettability, saturation, residual saturation, relative permeability, solubility, and volatilization. The two most important factors that control their flow behavior are density and viscosity. 

Saturation (v) is the volume fraction of the total void volume occupied by a specific fluid at a point. Saturation values can vary from zero to 1 with the saturation of all fluids equal to 1. Residual saturation (Sr) is the saturation at which the NAPL becomes discontinuous and immobile due to capillary forces. Residual saturation is dependent upon many factors, including pore size distribution, wettability, fluid viscosity and density ratios, interfacial surface tension, gravity and buoyancy forces, and hydraulic gradients. 

The residual saturation capacity of soil is generally about one third of its waterholding capacity. Immobilization of a certain mass of hydrocarbon is dependent upon soil porosity and physical characteristics of the product. The volume of soil required to immobilize a volume of liquid hydrocarbon can be estimated as follows ... 

The viscosity of separate LNAPL products varies significantly, ranging from far less to many times that of water. Flow of LNAPL in the unsaturated zone is largely dependent upon viscosity and soil grain size. Finer-grained materials have a higher residual saturation of water, which restricts the number of pores available for LNAPL entry in this region. 

Another similar approach would be to follow Wilson et al. (1988) in using different residual saturations for the vadose and water-saturated zones. Wilson et al. (1988) found sr values of 9 and 29% in the vadose and saturated zones, respectively. Hence, one could revise Equation 6.8 as follows ... 

In the above equations, Pc is the capillary pressure, /, is the Brooks-Corey pore-size distribution index, Sr is the residual saturation of the free product phase, a is the van Genutchen fluid/soil parameter, and the superscript n is the van Genutchen soil parameter. 

In general, as viscosity of the hydrocarbon increases and grain size decreases, the residual saturation increases. Typical residual saturation values for unsaturated, porous soil are tabulated in Table 5.5. 

The American Petroleum Institute (1980) has presented some similar guidelines for estimating residual saturation. Basing its work on a typical soil with a porosity of 30%, the API gives residual saturation values noted as a percentage of the total porosity of the soil as follows ... 

Comparisons of the estimated volume to the actual volume recovered prove to be the only reasonable procedure for assessing the recoverable volume considering all the variables involved. These comparisons indicate that the volume of hydrocarbon retained in the aquifer is higher than published residual saturation values. Based on experience for gasoline and low-viscosity hydrocarbons, the recoverable volumes have ranged from 20 to 60% of the pore volume in fine to medium sands. 

In summary, when undertaking a project such as the recovery of LNAPL, treatment of the coproduced water, prior to reinjection, may not be beneficial or technically necessary. A large percentage of the spilled or leaked petroleum hydrocarbon (40 to 60%) will be retained in the unsaturated zone as residual saturation. This residual hydrocarbon cannot be recovered by conventional withdrawal techniques. Without removing this continual source of contamination to the groundwater system, dissolved contamination will continue. Therefore, in most cases, it may be pointless and extremely costly to treat the coproduced groundwater prior to reinjection while the free- and residual-phase hydrocarbon contamination exists. 

In virtually every situation where the source of a hydrocarbon spill occurs at or near the ground surface, some quantity of hydrocarbon is retained in the soil as residual saturation within the vadose zone above the water table. Most unsaturated soils are capable of retaining hydrocarbons in a quantity equivalent to approximately 30% of their holding capacity (the amount of water that a particular soil can retain at saturation). In most cases, however, by the time remediation of the impacted soil is implemented, the residual hydrocarbon is less than maximum. 

Hoag, G. E. and Marley, M. C., 1986, Gasoline Residual Saturation in Unsaturated Uniform Aquifer Materials Journal of Environmental Engineering, Vol. 112, No. 3, pp. 586-604. 

The different capillary pressure, saturation, and permeability relationships of the different models can be compared. To do this, the effective permeabilities from some of the different models are plotted as a function of the capillary pressure in Figure 8. In the figure, the capillary pressure at which the effective permeability no longer changes is where the medium is fully saturated. Also, the values of the effective permeability are dependent on the diffusion media being tested. Furthermore, the value of the effective permeability at the right end of the curves corresponds to the saturated permeability, except for the model of Weber and Newman, who use a gas-phase residual saturation. 

Naimochelin C (Fig. 15, 54) from the myxobacterium Nannocystis exedens contains two L-Lys and two ( )-cinnamic acid units. The reported mono- and di-methyl esters (nannochelin B and A) may be artifacts from the work-up (198). A synthesis is described (29) (see Sect. 8.4). The ochrobactins (Fig. 15,55) isolated from the sea-shore bacterium Ochrobactrum sp. (214) with the spacer L-lysine are membrane active due to the fatty acid residues (saturated Cg and (2 )-unsaturated Cg and Cio) cf. lipopeptidic siderophores in Sect. 2.8. 

Autoxidation without Discharge. To compare our results with normal autoxidation, the reaction was carried out using a reaction mixture similar to Run 4 without silent discharge. Low conversion of cyclohexene (0.051% ) was observed at 60°C., indicating that the discharge oxidation was hardly affected by the normal autoxidation process under the present reaction conditions. The major product was 3-cyclohexenylhydroperoxide, and minor products were 3-cyclohexenol, 3-cyclohexenone, cyclohexene oxide, and trace amounts of residue saturated materials such as cyclo-hexanol and cyclohexanone were not detected. The conversion of cyclohexene was raised to 0.15% when the reaction temperature was elevated to 140°C. however, the kinds of product were not changed. 

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