A Guide To Water Well Casing and Screen Selection   -   29

9.2  APPENDIX II

Formation Sampling, Filter Pack and Aperture Size Selection

Selection of filter pack gradation and the corresponding well screen aperture size are determined by mechanical analysis of the finest aquifer material to be screened. Methods of obtaining formation samples vary according to the type of drilling employed.

Rotary samples are subject to contamination by caving and commingling of dissimilar materials in the drilling fluid. In the direct rotary system a careful mud program will maintain the walls of the borehole, reduce caving and facilitate removal of cuttings. With proper location, design and construction of mud pits, recirculation of material can be controlled. In order to correlate samples with the hole depth, the driller should record the length of time for returns to appear with each change of formation. In the reverse rotary system, high return drilling fluid velocity reduces sampling lag time, and accurate samples are readily obtainable. More accurate sampling under both methods is provided when drilling is stopped at all formation changes and circulation continued until an adequate formation sample is obtained.

With direct rotary drilling, samples should be taken directly from the shaker or a sample box. With reverse circulation, they may be withdrawn from the discharge pope through a 2-1/2 inch bypass fitted with a gate valve.

When drilling by the cable tool method, the principal tools utilized for taking formation samples are the bailer or mud scow. There is some tendency for the slurry produced by drilling to sort by grain size. Many drillers feel that the material found at the bottom of the first bailer is the most reliable. In unconsolidated alluvial formations, casing is carried more or less coincident with drilling, and samples are fairly representative of aquifer materials.

Unless otherwise required formation samples should be collected as follows:

1. At each ten foot interval below ground surface.
2. At each change of formation.
3. At each five foot interval within water bearing formations.

Divide the samples into three portions of approximately 1 pint each, and place them in suitable containers such as sample sacks, pint jars, or ziplock plastic bags. These are marked with well location, name or number of the well, depth interval represented by the sample and the date taken. One set is retained on the drill site for inspection, one delivered to the owner or his representative and the third retained for sieve analysis.

To prepare a sample for testing, remove the material from the container, place on a flat surface and mix thoroughly to assure that fine and coarse particles remain mixed. If the sample is wet, dry slowly over low heat, stirring frequently.

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