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Water Quality Data
STANDARD GUIDELINES FOR DESIGN
AND PROPER CONSTRUCTION
OF A WATER WELL
By Roy F. Senior, Jr.*
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CUTTING & SETTLEMENT PITS
The drill cutting and settlement pit must be excavated to dimensions adequate to permit sufficient time for fine sands to drop out of the viscous fluid as the cuttings are discharged into the pit and to pass the full length of the pit before returning to the well bore.
If fine sands (fines) are not settled out (or removed mechanically by a de-sander) they will return to the well bore and deposit fines on the walls of the well, plugging the water passages and creating irreparable damage.
In Ground Water and Wells, published by Johnson Filtration Systems, Inc., the author suggests that a reverse rotary pit system should be three times the volume of the hole in order to properly settle solids from the drilling fluid.
EXAMPLE: A 28" diameter well bore that is 1200' deep will hold about 38,000 gallons of water. Using "Johnson's" formula a pit large enough to accommodate about 115,000 gallons is needed. A pit 70' long x 10' wide x 8' deep will hold about 42,000 gallons of water. Using two pits of this dimension will process approximately 84,000 gallons of water. The cuttings laden water will travel 70' across one pit and 70' back through the other pit before returning to the well bore. This distance allows considerable settlement time, and in most cases, will settle out fine sand.
To prevent drill hole walls from plugging, the fluid system must be maintained with a sand content of below 2% at all times. This type of plugging is often caused by less qualified contractors using poor drilling procedures or conventional rotary drilling methods. Then to compensate for this, they use a gravel pack many times coarser than the formation demands resulting in void areas between these coarse sand or gravel particles, not only large enough to pass the sand deposited on the wall into the production water, but also to large to sufficiently stabilize the sands on the drill hole wall., thereby continually passing producing fines into the water. This creates what is commonly known as a "sand pumper."
Pits of sufficient size, proper construction methods and the correct selection of gravel pack size is vital to the efficiency of a well and cannot be over emphasized.
CONVENTIONAL ROTARY vs.
REVERSE ROTARY DRILLING METHOD
In conventional mud rotary drilling, damages due to formation plugging will occur without fail because the sand laden fluids must pass by the walls to reach the surface and therefore will irreversibly plug some of the production zones, even under the best of conditions.
Knowing this, one can make the assumption that wells should be drilled by the reverse rotary method, because the efficiency of the wells drilled by the conventional rotary method usually cannot approach the efficiency of wells drilled properly by the reverse rotary method. Even though installation costs may be less expensive, it will cost much more during the entire life of the well due to the increased pumping cost.
* Note: These remarks were presented by the author at a District
workshop in 1992.
Last updated January
2001