This invention relates to rehabilitation of existing water wells.
Water wells are created by drilling from the surface of the earth down into a water producing zone(s). The drilled hole is cased with a suitable string of steel or PVC casing from surface down through the water zone. Typical installations can have the diameter of the casing through the water zone be from six to sixteen inches or more. The portion of the casing adjacent the water zone is typically perforated with holes or machine slots to allow water entry into the casing. After the casing has been set and depending upon the water zone strata, a clean porous zone of gravel can be pumped and set around the slotted or perforated section of the casing, also referred to as the gravel packed area, out to the lining of the well bore. The purpose of the gravel pack is to filter out the fine sediment from the surrounding formation, while allowing the water to enter into and through the perforated well casing.
After a period of time, and due to a variety of different reasons, the slotted or perforated well casing, and possibly the gravel pack can become plugged with material such as calcium carbonate, iron bacteria, etc. An early indication of plugging is a deterioration of specific output capacity of the well. Well rehabilitation companies and well owners alike quantify this deterioration by measuring the amount of drawdown reported in units of gallons per foot of drawdown (gpm/ft) over a period of time. As drawdown deteriorates over time, the decision is made to rehabilitate the well using any of various methods well known in the prior art. These methods include acid washes, reperforating the portion of the casing across the water zone and scrapers to scrape scale build-up about the inner wall circumference of the casing. Methods also include utilizing a low pressure jet spray which washes the plugging material with water or water in combination with an acid.
One example of prior art well cleaning with a jet spray is disclosed in U.S. Pat. No. 5,060,725 issued to Buell. Buell teaches the use of sufficient hydraulic horsepower to supply a plurality of jet nozzles with at least 0.77 barrels per minute per jet nozzle to clean perforation tunnels in a well casing or liner.
Although perhaps suitable for oil well cleaning, with respect to water well cleaning the quantity of fluid being introduced by Buell down hole is not recommended or commercially used today. The reasoning is that introduction of such a great volume of water could potentially include unintended contaminants as well as the volume of the wash disrupting the integrity of the gravel pack.
Another example of prior art well cleaning is disclosed in U.S. Pat. No. 5,060,725 issued to Alford. Alford teaches a well cleaning tool comprising a jetting tool used in combination with a surge block. This reference, in respect to use of the jet nozzles, teaches away from cleaning gravel pack screens directly and instead uses the radially orientated jet nozzles to turbulently clean the inside wall of a casing.