Well drilling methods 



The method used to drill a well or test boring affects the reliability of the log, the accuracy 

 of the samples collected, and the subsequent correlation of well samples. It is important there- 

 fore to be acquainted to some extent with the various methods of drilling, and procedures for 

 obtaining samples, in order to properly evaluate drilling information for geologic purposes. The 

 cable-tool and hydraulic-rotary methods are the principal methods used for drilling on Long 

 Island. Jetting and drive-point methods have been used only to construct shallow, small 

 diameter wells. 



Cable-tool or spudding method: Most of the water wells greater than 4 inches in diameter 

 and less than 600 feet in depth are constructed by this method. The well casing is driven down 

 by dropping a heavy weight upon it or is bailed down by removing material at the bottom of 

 the casing with a bailing device. The latter has the effect of causing the casing to settle slowly 

 of its own weight as material is removed. Most wells require a combination of driving and 

 bailing. A heavy steel bit, raised and lowered by a cable, is used to drill through hard layers 

 of tough clays, pyrite, cobbles, and large boulders. This method is also known as drop-tool 

 drilling or churn drilling. 



Samples collected by this method of drilling are in most cases fairly reliable for geologic 

 studies if properly collected. The materials penetrated by the casing are picked up in the 

 bailer, hoisted to the surface and dumped on the ground. Certain precautions are necessary 

 to prevent contamination or mixing of samples from different horizons. This is most apt to 

 occur when the hole is drilled or bailed too far ahead of the casing in a caving or weak forma- 

 tion. Another source of contamination is careless dumping of material from the bailer upon 

 previously deposited cuttings and subsequent collection of mixed samples from the pile. Con- 

 tamination of samples from the critical Pleistocene-Cretaceous contact zone is a frequent 

 source of difficulty in correlating well samples. Contamination takes place when the casing is 

 driven across the contact before the well is thoroughly bailed out at the change of formation. 

 When accurate undisturbed samples are desired for geologic or mechanical analyses, drive 

 core samples are taken. The coring device generally consists of a 2-inch pipe about 3 feet 

 long with a hinge valve at the bottom, which is driven into the undisturbed beds below the 

 casing by means of a heavy drill stem attached to a set of drilling jars. 



Rotary method: Most of the deep wells on Long Island are drilled by the hydraulic- 

 rotary method. The hole is cut by a rotating bit connected to a string of hollow steel drill rods. 

 A drilling fluid composed generally of a suspension of red-colored Raritan clay mined in 

 New Jersey mixed at times with clay conditioners is pumped down through the hollow rods, 

 emerges through holes in the bottom of the bit and flows back up to the surface, laden with cut- 

 tings, in the annular space between the drill rods and the wall of the hole. A casing is not inserted 

 until drilling of the hole has been completed. Continuous circulation of drilling mud enables 

 material pentrated by the drill to be brought to the surface and by mudding the wall of the 

 hole prevents major caving before the casing is inserted in the hole. Many of the wells in the 

 old Flatbush franchise area of the New York Water Service Corporation in Kings County 

 were drilled by this method as were also a number of other deep wells on Long Island. 



One of the chief advantages of this method of drilling is the great reduction in drilling 

 time for deep wells. However, most of the samples collected by this method have proved to 

 be unsatisfactory for geologic studies as the unconsolidated sediments on Long Island fre- 

 quently cave during drilling operations. In addition, the sorting action of the hydraulic process 

 separates most of the fine sands and clays from the coarser materials. The samples are collected 

 in a trough or mud ditch at the land surface, quite frequently in a highly contaminated condi- 

 tion due to caving of the hole, re-circulation of material, or incomplete removal of material 



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