Well Screen

Fig. 1. Aquifers and monitoring wells where denotes the well screen and Hthe water-filled space in the monitoring well. (—) denotes the water table level, (—), the potentiometric surface, and ( ) the ground surface. Terms are discussed in text.

For sand, grit, or clay - use a hydrocyclone, sand trap, and/or install new well screen... 

Filter pack. Filter pack is the term used to describe the materials placed in the annular space between the screen and the formation (Figure 1). Aller et al. and Driscoll provide a discussion of the purpose and selection of filter pack materials. The filter pack serves a number of important mechanical functions. During well construction, it helps to keep the well screen centered in the borehole and acts to block the sealant from entering the well screen. The filter pack also improves hydraulic conductivity between the well screen and the formation, prevents formation materials from migrating to the well screen by acting as a filter, and helps support the screen and borehole. ... 

Unconsolidated or weakly consolidated sediments sometimes collapse around the well screen before the filter pack can be installed. This phenomenon is called formation collapse . Formation collapse can occur as a result of the inherently unstable nature of certain sediments or the disruptive nature of the drilling process. Formation collapse is most common below the water table. Although steps can be taken to minimize the amount of collapse, it may not be entirely preventable. The groundwater monitoring plan may need to accept natural formation material as the filter pack for some or all of the screen section. Well development activities (see Section 2.1.6) can be designed to maximize the effectiveness of the formation collapse materials as a filter pack. 

Well development refers to the process of removing drilling fluids and fine-grained materials from near the well screen. Their removal helps the subsequent collection of nonturbid samples and improves the hydraulic connection between the well and the aquifer. 

All travelers to endemic areas should be advised to remain in well-screened areas, to wear clothes that cover most of the body, and sleep in mosquito nets. Travelers should adhere to malaria chemoprophylaxis regimens and carry the insect repellant DEET (N, N,-diethylmetatoluamide) or other insect sprays containing DEET for use in mosquito-infested areas. 

A well point system consists of several individual well points spaced at 0.6 m to 1.8 m intervals along a specified alignment. A well point is a well screen (length 0.5 to 1.0 m) with a conical steel drive point at bottom. Individual well points are attached to a riser pipe (diameter 2.5 to 7.5 cm) and connected to a header pipe (diameter 15 to 20 cm). At the midpoint, the header pipe is connected to a centrifugal pump. As yield at different well points may vary, a valve at the top of each riser pipe is used to control the drawdown so that the screen bottom is exposed. The pump provides 6 to 7.5 m of suction, but friction losses reduce the effective suction to 4.5 to 5.4 m. 

Bacterial growth on well screen and formation. Emulsification of two fluid phases. 

Use gravel-pack well screen. Inject a slug of brine after every period of interrupted flow. 

Design of extraction points (e.g., diameter, depth, and well screen interval). 

Selection of active strains was achieved using the 20-well screening plate system discussed in Section 9.3.3. One screening plate containing 20 actinomycetes strains in malt extract medium was incubated for 2 days at 28 °C and 275 rpm. Dasatinib (2 /xLof a 100 mM solution in DMSO) was added to each well and the plate was incubated for one additional day at 28 °C. Then 1 mL of methanol was added to each well and the plate incubated at 28 °C at 150 rpm for 10 min and centrifuged at 3000 rpm for 15 min. The supernatant was analyzed by HPLC-UV-MS. LC-UV-MS data showed M20 and M24 were produced by seven strains, from which Streptomyces sp. SC15761 was selected for scale-up based on the yield of M20 (Figure 9.8). 

We now need to add the dissipation term. A Rayleigh dissipation function will suffice for this purpose, since the hydrodynamic interactions in the.elastomer should be well screened. Let F be a matrix such that XF has elements of the form... 

Any fundamental study of the rheology of concentrated suspensions necessitates the use of simple systems of well-defined geometry and where the surface characteristics of the particles are well established. For that purpose well-characterized polymer particles of narrow size distribution are used in aqueous or non-aqueous systems. For interpretation of the rheological results, the inter-particle pair-potential must be well-defined and theories must be available for its calculation. The simplest system to consider is that where the pair potential may be represented by a hard sphere model. This, for example, is the case for polystyrene latex dispersions in organic solvents such as benzyl alcohol or cresol, whereby electrostatic interactions are well screened (1). Concentrated dispersions in non-polar media in which the particles are stabilized by a "built-in" stabilizer layer, may also be used, since the pair-potential can be represented by a hard-sphere interaction, where the hard sphere radius is given by the particles radius plus the adsorbed layer thickness. Systems of this type have been recently studied by Croucher and coworkers. (10,11) and Strivens (12). 

Remedying the problem of clot formation and its detrimental effect on sample transfer has also been addressed in a recent publication from the Johnson Johnson Pharmaceutical Research and Development Laboratories161 where workers designed a novel 96-well screen filter plate consisting of 96 stainless steel wire-mesh screen tubes (Figure 1.42). 

FIGURE 1.42 Example of 96-well screen filterplate design.161 (Reproduced with permission from the American Chemical Society.)... 

Extrapolation of laboratory results to the field scale is at best qualitative because of the complexity of the geochemical and hydrological systems in the Bengal Basin. Movement of As-contaminated groundwater towards a well screened in the uncontaminated aquifer is likely to contain both vertical and horizontal components of flow. For modeling purposes presented here, a relatively slow interstitial groundwater velocity of 3 m/yr (Stollenwerk et al. 2007) is compared with a more rapid interstitial velocity of 30 m/yr (McArthur et al. 2008). 

Shallow water table contour (wells screened above 40 ft)... 

Hand-augered U 15 M,R water, or vapor) l-in.-diameter well screen capability accessibility excellent Accurate sampling difficult in coarse sediments or loose sand physically demanding fluid levels easily... 

In addition, monitoring wells screened across LNAPL within confined aquifers will exhibit an exaggerated thickness. This exaggerated thickness reflects relatively high confining pressure, which forces the relatively lower density fluid upward within the borehole. Thus, the measured thickness is a function of the hydrostatic head and not the capillary fringe, which has been destroyed by the confining pressures. 

Apparent/actual LNAPL thickness ratios can be very high at the perimeter of LNAPL pools, notably, under low permeability conditions. Once a well is installed, it can take several months before the LNAPL migrates from the formation into the well reflecting the presence of low-permeability soils in the zone of LNAPL occurrence. As clearly shown in Figure 6.8, a well screened at the perimeter of a known LNAPL pool initially had no detectable LNAPL until 4 months after installation, whereas upon detection the apparent thickness slowly increased with time up to 15.71 ft. 

Well points are commonly small diameter pipes (2 to 4 in., 5 to 10 cm) attached to a short length of well screen (2 to 3 ft, 60 to 90 cm), which are installed in sets and are connected to a common suction pump. This type of system is best suited... 

Several attempts have been reported by recovery teams to recover LNAPL by the use of vacuum wells set above the fluid surface. While these efforts have attained marginal success, several factors have been seen to interfere. Vacuum wells set in the vadose zone tend to encourage airflow from the surface downward through the soil, as well as to extract vapors. The result is often the enhancement of biological degradation near the wells, which tends to cause the well screens to become plugged with biomass. Routine maintenance of the wells is required to keep them functioning properly. 

Bioslurping uses a vacuum to extract NAPL and water from the subsurface soil (Figure 7.18). A small-diameter well is installed so that the well screen is open slightly above and below the water table. A smaller-diameter dip tube is inserted into the well and sealed against the outer well casing. The dip tube does not extend to the bottom of the well, but ends below the water table (or oil-water interface). The maximum depth of the bottom of the slurp tube is the suction-lift capacity of the vacuum pump. 

---