Fracking

Geologic formations may contain large quantities of oil or gas, but may have a poor flow rate

due to low permeability, or from damage or clogging of the formation during drilling. This

occurs most times for tight sands, shales and coal bed methane formations. Within the past

decade, the combination of hydraulic fracturing with horizontal drilling has opened up shale

deposits across the world and has brought large-scale natural gas drilling outputs. The fracking

process occurs after a well has been drilled and steel pipe has been inserted in the well bore. The

casing is perforated within the target zones that contain oil or gas, so that when the fracturing

fluid is injected into the well it flows through the perforations into the target zones. Eventually,

the target formation will not be able to absorb the fluid as quickly as it is being injected. At this

point, the pressure created causes the formation to crack or fracture. Once the fractures have

been created, injection stops and the fracturing fluids begin to flow back to the surface. Materials

called proppants usually sand or ceramic beads, which were injected as part of the frac fluid

mixture, remain in the target formation to hold open the fractures. Typically, a mixture of water,

proppants and chemicals is pumped into the rock or coal formation. There are, however, other

ways to fracture wells.  Sometimes fractures are created by injecting gases such as propane or

nitrogen, and sometimes acidizing occurs simultaneously with fracturing. Acidizing involves

pumping acid (usually hydrochloric acid), into the formation to dissolve some of the rock

material to clean out pores and enable gas and fluid to flows more readily into the well. Some

studies have shown that more than 90% of fracking fluids may remain underground. Used

fracturing fluids that return to the surface are often referred to as flowback, and these wastes are

typically stored in open pits or tanks at the well site prior to disposal.

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Figure 1 Showing various aspects of fracking

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Groundwater Prospecting

Successful prospecting for groundwater requires knowledge of the manner in which water exists

in the water-bearing ground formations. Without this knowledge, effective and efficient water

exploration is impossible, and well drilling becomes challenging. The aim of the prospecting

work must be clearly defined. Available hydrogeological information about the study area should

be collected and organized. This may include: geological maps and reports, topographical maps,

logs of boreholes, surface geological reconnaissance, meteorological records, and hydrological

data. A survey of the study area should be done, preferably towards the end of the dry season.

This survey should also include the knowledge of local men and women on the history of water

sources, water quality and land uses. They also know the flood-prone areas not suitable for well

development. In some cases a survey may be all that is needed for an experienced hydrogeologist

to define water sources for small community supplies and no further investigation will then be

required. If essential data are lacking, some fieldwork is necessary. The survey should provide

sufficient data to form a basis for drawing up a hydrogeological map showing: the distribution of

aquifers; any springs or signs of springs present; depth of water tables and piezometric levels;

yield of existing groundwater sources; and the quality of the water from them. Sometimes, it is

possible to prepare such a map on the basis of an examination of outcrops and existing water

supplies. In other cases, it may involve the use of specially drilled boreholes and geophysics.

Drilling special test boreholes will usually only be required when an aquifer is to be fully

exploited and knowledge is therefore needed of the hydraulic permeability and water storage

capacity. The survey should provide sufficient data to form a basis for drawing up a

hydrogeological map showing: the distribution of aquifers; any springs or signs of springs

present; depth of water tables and piezometric levels; yield of existing groundwater sources; and

the quality of the water from them. Sometimes, it is possible to prepare such a map on the basis

of an examination of outcrops and existing water supplies. In other cases, it may involve the use

of specially drilled boreholes and geophysics. Drilling special test boreholes will usually only be

required when an aquifer is to be fully exploited and knowledge is therefore needed of the

hydraulic permeability and water storage capacity.

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Applicability of well construction method

Method Maximum Depth

(m)

Diameter(cm) Geological

Formation

(suitability)

Geological

Formation

(unsuitable)

Dug 60 90-500 Clay ; silt;

gravel; soft

sandstone ;soft

fractured

limestone

Igneous rock

Bored 25 5-40 Chalk ;gravel ;so

ft

sandstone ;fractu

red limestone;

alluvial

formations

Igneous rock

Driven 15-20 3-5 Clay; silt; sand;

fine

gravel ;sandston

e(in layers )

Any formation

with

boulders ,cement

ed gravel,

limestone ,igneo

us rock

Jetted 80-100 10-30 Clay; silt; sand;

pea gravel

Any formation

with boulders,

cemented

gravel , sand

stone ,limestone

,igneous rock

Sludgd 50 3-10 Clay ;silt;

sand ;gravel; soft

sandstone ;fractu

Any igneous

rock formation

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red limestone;

alluvial

formations

Percussion-

drilled(cable

tool)

300 10-60 Clay, silt sand;

gravel; cemented

gravel ; boulders

(in firm

bedding );sandst

one ;limestone

and ingenious

rock

None

Rotary-

drilled( fluid

circulation)

250 10-60 Clay; silt; sand

(stable);

gravel ;cemented

gravel ;sandston

e;

limestone ;and

igneous rock

Problems with

boulders

Rotary drilled

(down the hole

air hammer )

250 10-50 Particularly

suitable for

dolomite ;basalts

; metamorphic

rock

Loose sand,

gravel, clay ,silt,

sandstone

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Wellhead protection

Wellhead protection means protecting the area surrounding public drinking water supply wells,

and in turn, protecting drinking water supplies. Groundwater is and will continue to be the source

of drinking water for many communities. Protection of this vital resource is important, for

example, expanding development may bring with it more potential sources of contamination;

growing populations may stress the quantity of water available; and intensive agricultural

practices may increase the need for more proactive management strategies. Whether faced with

an existing impairment to the water source or seeking ways to prevent contamination, wellhead

protection makes good economic and environmental sense!

In general, wellhead protection involves:

forming a local team which will assist with protection of public supply wells in their

area;

determining the land area which provides water to public supply wells;

identifying existing and potential sources of contamination;

managing potential sources of contamination to minimize their threat to drinking water

sources; and

developing a contingency plan to prepare for an emergency well closing and to plan

for future water supply needs

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WHAT CAUSES GROUNDWATER CONTAMINATION?

Natural Sources

Groundwater contamination can occur in many ways and from many sources, both natural and

human induced. Groundwater commonly contains one or more naturally occurring chemicals,

leached from soil or rocks by percolating water, in concentrations that exceed federal or state

drinking water standards or otherwise impair its use.

Dissolved Solids and Chloride

One of the most common water quality concerns is the presence of dissolved solids and chloride

in concentrations that exceed the recommended maximum limits in federal secondary drinking

water standards: 500 mg/L (milligrams per liter or approximately equivalent to parts per million)

for dissolved solids and 250 mg/L for chloride. Such concentrations are found at the seaward

ends of all coastal aquifers and are quite common in aquifers at depths greater than a few

hundred feet below the land surface in many parts.

Iron and Manganese

Although not particularly toxic, iron and manganese in concentrations greater than the limits for

federal secondary drinking water standards (0.3 mg/L for iron and 0.05 mg/L for manganese) can

impair the taste of water; stain plumbing fixtures, glassware and laundry; and form encrustations

on well screens, thereby reducing well-pumping efficiency.

Nitrate-Nitrogen

Most groundwater not affected by human activity contains less than 10 mg/L nitrate-nitrogen, the

maximum concentration allowed by federal primary drinking water standards. Nationwide,

nitrate nitrogen concentrations of less than 0.2 mg/L generally represent natural conditions,

whereas values greater than 3mg/L may indicate the effects of human activities. Although

relatively nontoxic, nitrate may be reduced by bacteria to nitrite in the intestines of newborn

infants and cause the disease methemoglobinemia. Nitrate also can react with amines in the

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human body to form N-nitrosamines, carcinogenic chemicals known to induce tumors in

laboratory animals and thought to be linked to human cancers.

Human Activities

Contaminants can enter groundwater from more than 30 different generic sources related to

human activities. These sources commonly are referred to as either point or nonpoint sources.

Point sources are localized in areas of an acre or less, whereas nonpoint sources are dispersed

over broad areas. The most common sources of human-induced groundwater contamination can

be grouped into four categories: waste disposal practices; storage and handling of materials and

wastes; agricultural activities; and saline water intrusion.

Waste Disposal Practices

Perhaps the best-known sources of groundwater contamination are associated with the storage or

disposal of liquid and solid wastes. The organic substances most frequently reported in

groundwater as resulting from waste disposal, in decreasing order of occurrence, are:

Contamination Caused by Wells

Improperly built wells can result in contaminated groundwater , by forming a pathway or a

conduit for pollutants entering a well from surface drainage or by allowing communication

between aquifers or varying quality .Unuesd wells sometimes are simply abandoned , or

truncated just below the ground surface and plowed over , or otherwise destroyed improperly .

such wells can contaminate groundwater in several ways :

contaminates enter the well from the surface

the well casing can erode , allowing poor quality water or contaminats to move vertically

from one aquifer to another

the well might be used for direct (and illegal) disposal of waste

References

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http://groundwater.ucdavis.edu/files/136257.pdf

http://dnr.wi.gov/topic/Groundwater/documents/pubs/gwcntsrcs.pdf

http://ethesis.nitrkl.ac.in/1336/1/PRITI_RANJAN_SAHOO__(10501031)_,_B_TECH_thesis.pdf

http://www.earthworksaction.org/issues/detail/hydraulic_fracturing_101#.VDr1mPldWSo

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