The "distance over" can be determined in the following man ner, using a bailer and sand-line: (a) Run the bailer into the well a short distance and tie a string (target) on the sand-line level with the surface of the floor, using a steel square or other straight edge to determine the cor rect position.
(b) Tie a second string or strand of rope tightly on the sand line at a fixed point near the sand-line reel.
(c) Lower the bailer into the well until the second target is within easy reach from the derrick floor. Attach the end of a steel tape to the sand-line at the target. Raise the bailer so that another target may be fastened to the line at the end of the tape. Lower the bailer, detach the tape; hoist the bailer and attach the tape at the third target; hoist the bailer and set a fourth target. Repeat the operation until the tape reaches the first target originaly set at the level of the floor. The tape must be shorter than the height of the derrick, so that it will not go over the pulley at the crown block.
When a target is tied to the drilling or sand-line, paint should be applied to it above and below the target, to show any dis placement of the target.
To measure into the well, after the unit length or "distance over" is determined, hold the bottom of the bailer-dart, when raised, level with the surface of the floor, set a target at a fixed point near the reel, lower the bailer until the target is level with the floor, and set a second target at the reel. Correct count of the targets is most easily kept by detaching and keeping each one as it reaches the floor.
The depth can be less conveniently measured when the bailer is pulled out of the well by setting the first target even with the floor, while the bailer is on bottom, hoisting until the target reaches the reel, set new targets at floor level and remove old ones as they reach the reel.
Measurements of depth are frequently determined from the amount of casing put into a well. This method is not accurate under ordinary conditions. The total length of a string of casing can not be correctly determined by adding together the lengths of separate joints because it is impossible to know exactly how far each joint will screw into the collars. Measurements of sep arate joints, after each one is screwed into place and before it is lowered into the well, will not give an accurate total, because the entire string may be still more tightly screwed together.
Strings of casing 2000 ft. long have sometimes been shortened as much as two feet by additional screwing together after being placed in the well.
Special measurements, such as that to the casing shoe, are sometimes satisfactorily made with the pump tubing. The tub ing is accurately measured as it is put into the well, and a hook is fastened to it near its bottom. The hook is caught on the casing shoe by lifting or lowering and turning the tubing, and after the measurement is ascertained, the tubing may be with drawn whenever convenient.' Identification of Strata.—It is necessary to identify the strata penetrated by a well in order to know where drilling should stop and where casing should be landed and perforated.
The identification must be made by some system which affords comparison with other records of known geological conditions in the vicinity. The comparison may be made either with evidence furnished by logs of neighboring wells or with a record of strata where they are exposed at the ground surface. In either method the most obvious characteristics, such as color, texture and hardness of the rock, will be most useful in preliminary records or logs.
Samples of drill cuttings vary with the kind of tools used. Cable tools ordinarily yield better samples than rotary tools, but the physical conditions governing the collection of either kind of samples are not ideal and their effect must therefore be considered.
With cable tools the bailer can be used to collect the drill cut tings at almost any place in the well. However, the action of the drill will usually pulverize the rock to such an extent that its appearance may be greatly changed, and, furthermore, the sample will usually be mixed with water and the finer particles are easily lost. A positive identification requires that the entire contents of the bailer be saved so that the mud or slime can be examined as well as the larger fragments of rock. In some locali ties the upper formations will continually contribute to the adulteration of the cuttings collected at the bottom of the well. There may be cases where casing will have to be landed in order that clean and representative samples may be obtained.