MEASUREMENT OF WELL DEPTHS Accurate depths of oil wells are obtained in five ways, given in the order of their importance.
1. Steel-line measurements.
2. Sand-line measurements.
3. Cable measurements.
4. Casing measurements.
5. (New) Automatic method.
Steel-line measurements are made by a steel tape marked in feet like a surveyor's steel chain, and weighted by a plumb bob on the end. This line is let into the well and allowed to touch bottom. The " pick up " or feel on the line indicates bottom. The number of feet in depth is then read direct from the line. This method is considered the most accurate of those employed.
The sand-line method consists in first measuring a given length of sand line and using that as the unit of measurement. , This is called "stringing over." The bottom of the bailer is brought flush with the top of the casing. Then a string is tied on the line at the sand reel. Then the line between the strings and the bottom of the bailer is measured accurately. This measures on an 82-ft. derrick about 170 ft. Where accurate results are desired each derrick must be measured separately.
Measurements are now begun. The first string is run over the derrick. When a string reaches the top of the casing a new string is tied on at the sand reel. The number of strings tied on at the sand reel represents the number of units of measure ment. If the distance over is 170 ft. and 10 strings have been used, then the depth is 10 X 170 = 1700 ft. If 10 strings are in the hole and the eleventh is part way over the derrick, a string is tied on the line at the top of the casing at the point "pick up" is noticed. Then the distance between the last string in the hole and the "pick up" string is measured and the length added to the amount in the hole. Stringing over with the drilling cable is done much the same way.
Accurate measurements are possible with the sand line. However, the strings may slip. Then, too, unless the first unit over the derrick is carefully measured its error is multiplied every time a string goes into the hole. An error of 6 in. per
unit in a 3300-ft. hole would mean 20 times 6 in. or 10 ft. for that depth. Another error is due to the settling of the derrick, which increases with the length of the sand line in the hole. This sag may be as much as 3 ft. with holes 3500 ft. deep.
To overcome this error a measurement should be taken when the line first enters the hole and one on the last length in the hole. Average the two and use that unit as the true measurement.
Casing measurements, especially where the holes are cased to bottom, give accurate measurements of wells. Where each joint has been measured the sum of all the joints gives the actual depths. The measurements are taken from the top of one collar to the top of next collars after the pipe is "set up" in the derrick. In long strings of pipe there is undoubtedly some stretching. A string 3000 ft. long may stretch a foot, but that is negligible. With the rotary system measurements are taken on the drill pipe as with casing. Accurate results are obtainable in this way.
Automatic E. Van Ostrand of the United States Geological Survey has invented a measuring device that he used on the deep wells in West Virginia. It consists of a flat wheel 2 ft. in circumference that is held against the sand line. Two pins in the wheel strike small arms on a meter. This meter registers the number of feet the line travels, and one can read the depth directly in feet from the meter. This method is very accurate and should come into more general use, as the device is simple and inexpensive.
Dressing are dressed or sharpened by hand or by steam hammers. Hand-sharpening is used at present on tools drilling far off from machine shops, or in those fields where light bits are used. In the early days of the industry, hand dressing was the rule, but now all large operating companies have the bits sharpened in the machine shop. Dressing large bits by hand is man-killing work.