WATER WELL DEVELOPMENT AND TESTING PROCEDURES & METHODOLOGY
Successful completion of the well development and testing phase of water well construction project through these guidelines will produce a well that maximizes the naturally occurring water bearing formation available at the site, and the installation of pumping plant best matched to the well.
Immediately preceding the construction phase of a new well initial well development is a critical element in producing a well that has the highest pumping level for a given production rate of water. Competent well development is how it is possible to achieve maximum flow rates of the well. Well testing is how we the well measure performance of the well; at variable flow rates, through pump step tests at predetermined constant rates. It is though and evaluation of the standardized constant rate tests that it is possible to select pumping equipment establishing horsepower requirements making it possible to install a pumping plant capable of the highest energy efficiencies. The long constant rate test evaluates the effects of long term pumping on an aquifer, is the basis of determination that the pumping equipment proposed to be installed in the well will remain efficient during sustained pumping.
Preliminary Well Development
Preliminary well development occurs as the last phase of well construction as the drilling rig is removed from the site. The intention of these procedures are to remove the heaviest weight drilling fluid from the well and move at least a small amount of water from the intervals of water bearing formation though the gravel pack, into the well there by removing the heaviest drilling fluids and sediments from the gravel pack. The effectiveness of the preliminary well development phase is greatly enhanced by following a drilling fluids control procedure that monitor the solids content and conditions the drilling fluid prior to the removal of the drill pipe which precede the installation of the casing and gravel pack material.
Well Development by Pumping
In the well design phase of the project a rough determination of the wells potential pumping rate, anticipated standing water level and pumping level is estimated. It is by reviewing this data and experience of completing other wells in a general area that the depth, pump column diameter, horse power requirements and selection of a temporary development and well testing turbine pump bowl assembly is made.
The selection of all the temporary test pump equipment is an approximation, because none of the aforementioned selections can be accurately made until the end of the well testing phase. The objective is to measure the new well performance and selected permanent pumping equipment that most accurately matches the conditions of the well.
The temporary development and test pump equipment is set deep enough to lower the static head pressure inside the well to achieve sufficient entrance velocities of water to thoroughly remove drilling fluids, natural sediments, and sort the gravel pack material establishing the free movement of water though the gravel pack material and limiting the entrance velocities of the water precluding the movement of sand and silts into the well.
This is accomplished by initial production of water from the well at low pumping rates, cleaning the gravel pack, water bearing formations and slowly increasing the pumping rates until a production rate close to the anticipated pumping rate is reached. The pumping rates are only increased as sand and silt production is not measurable or very low and water appears clear of heavy clay particles.
The well is then developed further by rapidly bringing the water to the surface through the use of a high horsepower variable speed diesel driven engine, where by after the development water reaches the surface the pump power is withdrawn and the water back flows into the well creating a massive surging action. Rapid and repeated surging actions of the development remove drilling fluids and sediments which may have become lodged in the gravel pack and promote further sorting of the gravel pack material.
The development pumping and surging phase of the work is continued until no further evidence of drilling fluid color is observed and the pumping water level ceases to continue to rise. It is not good practice to pump the well at rates higher than 120 per cent over the intended finial pumping rate or at a rate that continues production of measureable sand can be detected.
Well Testing, Selection of Step Tests and Constant Rate Tests
At the end development pumping phase preliminary measurements are taken. Based these initial measurements of well performance, and the customers needs and horsepower requirements a step drawdown test is designed. Step drawdown tests consist of pumping the well at 3 to 4 flow rates over a 2 hour period, measurements are take every minute for 10 minutes, then every 5 minutes for 30 minutes, and every 10 minutes for 30 minutes followed by once every 30 minutes. It is though an evaluation of the step drawdown information that the most efficient rate for the well can be determined. Step drawdown information is the basis for the determination of the long constant rate test.
The yield (specific capacity) of the well is calculated on 3 or 4 points from the step drawdown test, due to the duration of the each of the step drawdown test the accuracy of the yield calculations are greatly enhanced. It is though the calculation of reliable step drawdown information that horsepower requirements based on gallons per minute and total dynamic head from each of the test can be made and a selection of pumping equipment can be made. The most efficient pumping equipment can be installed in the well when the point (production vs. drawdown) can be determined.
Constant Rate Test – Aquifer Testing
It is during the constant rate test, in most cases should not be less that eight hours and can be run longer depending on conditions a determination can be made as to how well the performance or the well can be maintained under sustained pumping. In essence it is possible to do a long term test of the new well at the production rate permanent equipment will be engineered to meet. By doing the long constant rate test it is possible have assurance that the performance of the well will be able to maintain the conditions where the permanent pumping equipment will remain efficient.
If after long term pumping the drawdown of the well increases; so will horse power requirements for a given amount of water. Additional drawdown will also push pumping efficiencies down should additional drawdown occur that is not considered in the pump bowl unit engineering.
Water costs can be greatly increased by the installation and operations of pumping equipment that are engineered on incomplete well and aquifer testing information.