The form of the daily drilling report can, of course, be designed to fit local conditions. The following form serves as an example which, with some alterations, can be adapted to most drilling operations.
Measurement of Wells.—Under extremely favorable and sim ple underground conditions, it is sometimes only necessary to obtain approximate measurements of depths in oil wells. How ever, many very expensive errors have been caused by incorrect measurements. A difference of only one or two feet may directly affect the success or entire failure of the operator in drilling a well, as, for instance, failure to land casing in a thin bed of shale at which it is necessary to shut off water. Therefore, the prudent operator will always obtain accurate measurements, since they involve but little expense and loss of time.
The most obvious method of measuring the depth to the bot tom of a well is to use a steel or aluminum tape with a weight attached at its lower end. Tape lines and reels are specially made for use in oil wells. The reel may be attached to the fly wheel of the drilling engine, or operated by hand.
The use of such tapes is common in some fields, particularly where drilling is done with Manila rope, which stretches to such an extent as to be unsuitable for accurate measurements. Meas urements by tape and attached weight are suitable where only the depth to the bottom is desired. It is frequently necessary to ascertain the depth to other points in a well, such as the lower end of a string of casing. The existence of many special condi tions along the wall or casing of a well cannot be readily detected by a weighted line. The magnetic attraction between steel tape and well casing interferes to prevent accurate measurement.
The widespread use of steel cables for drilling and bailing wells has led to the practice of using them for measuring, with the result that in some fields the special measuring tape is never seen. When measurements are made directly with the drilling or bailing cable, many successive measurements can be made without much trouble, and a more detailed and accurate record is therefore apt to be kept. Care and attention must be given
to the details if correct measurements are to be obtained with steel cable.
The following rules have been found sufficient to obtain results which are correct to within one foot.
1. All measurements should be made with steel tapes. Cloth or so-called "metallic" tapes can not be depended upon, as they are subject to great change in length by stretching. Measure ments made with a five-foot stick upon a sand line or drilling line for distances of more than 200 ft. are inaccurate. The reason for such inaccuracy is that it is difficult to make markings at an exact point on the line at the ends of the stick, and the great number of inaccurately placed marks quickly multiplies the error.
2. The depth of the well should, in all cases, be determined by running a bailer or string of tools to the bottom. The unit of measurement, when cable tools are used, should be the distance from the floor of the derrick along the sand line to a fixed point near the reel. This unit of measurement is commonly known as the distance the derrick "measures over," and details for such measurement are stated below. When the measurement of a well is made on the drilling line, it should be made from the floor to a point near the bull-wheel shaft, five feet above the floor, as determined by setting up a five-foot stick.
The depth of a rotary hole, before casing is put in, should be determined by measuring each stand of drill pipe with a steel tape, measurement to be made from the top of the tool-joint box to the bottom of the shoulder on the tool-joint pin.
3. The length of a string of casing should be determined by measuring to the shoe of the casing from the derrick floor. This measurement can be made on the drilling line by using an under reamer, a latch-jack, or any other tool which definitely locates the shoe of the casing.
4. A derrick should be "measured over" immediately before measuring the depth of well or of casing. A measurement made when the rig is new may not be correct after the rig and rig-irons have been in use for some time.