@Smokin Joe
So now I have questions. What is the purpose of injection a saline solution into the wells? Reduction of bacterial growth and a higher specific gravity to prevent inclusion of particulates, that would plug the injection site? And is it sodium chloride, some other salt--barium, calcium, etc. or a combination depending on the composition of the ground at the site? It seems like some bacteria crap sulfides and does that play in?
In the production end of the oil industry, the purpose is disposal of water that is too saline to release into the surface environment. The salt water is water that was produced with the oil, not some additive, and these wells are used to dispose of that water. Basically, the water is a remnant of the same marine environment the rock formed in. When oil is produced, some salt water is produced with it, that water native to the rock formation the oil came out of.
The salts present may be more concentrated than they are in seawater, but for the most part are Sodium chloride (halite), potassium chloride, and possibly some magnesium salts. This is naturally occurring, and not an industrial byproduct aside from being produced in conjunction with oil and gas. Hydrogen sulfide may also be present, the result of anaerobic activity in the same rock formation the oil was produced from, and in that case, usually the crude oil has H
2S in it as well, dissolved in the oil (referred to as "sour crude") and produced water. "Sweet" crude oil has no or extremely low Hydrogen Sulfide content (below 1PPM, the threshold for smelling Hydrogen Sulfide, which gives off an odor like rotten eggs).
Salt brines containing calcium chloride and calcium bromide are used in the industry as heavy brines to contain wellbore pressure when removing the drill string to change out downhole tools ("tripping out") and running those tools in ("tripping in), but are expensive, specially formulated brines and are recovered as much as possible to be reused for the same purpose on subsequent trips. These are used when the hydrostatic pressure exerted by saturated sodium chloride (regular table salt) solutions is not enough to keep the well from flowing--a situation which could lead to a blowout. The calcium chloride and calcium bromide brines weigh more per gallon when the solution is saturated than the sodium chloride brines, so they exert greater hydrostatic pressure for any given height of fluid column. Eventually, these may become contaminated to the degree they are disposed of along with production water.
Disposal wells are often oil wells that did not produce, but which had a suitable stratum to dispose of fluid in, although some are drilled for that purpose from the start. As a result, the well is constructed similar to an oil well, with surface casing to protect groundwater, and another casing string in the wellbore, like production casing in a producing well, down past the rock layer the fluid will be injected into. That casing is perforated, much the same as an oil well would be, to allow communication between the wellbore and the rock layer the fluids will be injected into. Casing strings are cemented in place, and the bond between the casing and the rock (cement quality) checked with a casing bond logging tool, which indicates any bad spots in that cement, should they exist. Those can be fixed by 'squeezing' (perforating the casing at the bad cement, setting packers above and below the perforations, and pumping cement into any voids behind the casing). Then the cement left in the wellbore is drilled out, along with the packers, and the wellbore is open for business.
A primer on disposal wells and seismic activity is available from the USGS at this link
http://earthquake.usgs.gov/research/induced/myths.php and more detail here:
http://media.wix.com/ugd/d3e01e_7a12408392f240c89943d3f500039004.pdf.
North Dakota has a good basic Q&A about disposal wells:
https://www.dmr.nd.gov/oilgas/undergroundfaq.asp Six states are considered to have 'at risk' areas of induced seismicity; North Dakota is not one of them. (The USGS lists two earthquakes in the state since 2008 that were of magnitude greater than 2.5, and one of them was outside oil producing areas and far too deep to be associated with oil production or fluid disposal).
Other injection wells have been used to dispose of fluids from other industries as well, from the chemical industry to nuclear weapons manufacturing. The above applies to the oil patch.