When a string of pipe is frozen and cannot be moved by pulling, driving is usually resorted to. It should be remembered that ordinary casing is not intended for such usage because of the fact that the ends of the joints do not butt. In other words, the blow is delivered upon the threads themselves and for this reason driving should be avoided as far as is possible. After driving, the casing-tongs should be applied and the string tightened again. Water, when not present in the well can be run in, materially aiding in holding back the cavings from the pipe. In case of a frozen string, the water can be bailed down, the mud started around the shoe and the pipe thus relieved. Should these means fail, the shoe-joint is sometimes slitted or perforated and pump-pressure applied to try to obtain circulation of the material behind the casing. This often proves effective and can be done cheaply. The use of a casing-spear for freeing pipe is not always advisable, for at best it is a dangerous tool, often 'bull-dogging' and sometimes plugging the hole. They are generally called into requisition as a last resort. In place of this dynamite is used to blow off the casing above the point of friction, and the top portion can be pulled out, a new shoe put on, and the pipe which is left in the hole side-tracked. Blasting, however, often does damage where none is intended, particularly to the water-string, if there be one in the well. The dynamite should be used • in small quantities ; 10 to 15 lbs. of a 40% strength of nitro-glycerine makes a fair charge for parting pipe. The casing-cutter often answers this purpose and eliminates the danger due to explosives, but the shock caused by the latter results in loosening the pipe more readily and is used oftener for this. reason. Ripping the pipe will often free it, as the mud is then admitted to the hole and bailed out.
Side-tracking the casing left in the hole is not difficult when. the formation is soft ; the bit will probably strike the pipe at first, but by continued work will finally slide past. The reamer can then be run, if necessary, to clear the hole for the casing to follow, and when once it passes the top of the shot portion, an ordinary rate of drilling can be maintained. When shooting or cutting, enough pipe should be left in the hole to insure its remaining in a vertical position, making side-tracking much easier than would be the case in which only one length remained. It will usually be found that the casing is more easily kept free when protected on one side by the lost pipe, and that a second freezing is not so likely to occur. It frequently happens, however, that two or even three lost strings are left in one well and while harder to avoid, they do not interfere seriously with drilling *rations. Considerable quantities of iron have to be drilled through in such work and often follow down the hole for several hundred feet. Such a task may take a period of several days or even a week, but this is generally cheaper than moving the rig and drilling a new well. As it is necessary for the casing to make a bend in passing lost pipe, there should be a space of at least from 60 to 75 ft. between the latter and the string previously ,landed, to permit an easy curve. Considerable pipe has to be drilled through when the two are closer, especially in the larger sizes, and occasionally it becomes necessary to abandon the well and move the rig away 20 ft. or more for a fresh start. In this case, pipe is either blasted or cut where it can be moved and used in the new well. The casing-splitter can also be used to part casing by driving it through a coupling once or twice, after which the pipe can be pulled apart. Where the latter parts at a defective coupling, a die-nipple (Fig. 104) can be run in and new threads cut by turning the string at the surface. After a good hold is obtained, a pull may be exerted and the whole column withdrawn.
Water in large quantities is often encountered near the surface and stands within a hundred feet or more from the top ; it usually gives no trouble in freezing pipe, maintaining its level when heavily bailed. Where the flow is small, the level should be kept constant by adding water when necessary in order to hold back the cavings and protect the casing. Where the source is deep and the flow strong, the hole
should be previously filled to prevent freezing or collapsing the casing when the new stratum is encountered. A constant circulation of water from the inside is helpful in holding back the cavings of water-sand and mud and when once begun should be continued until the string is landed. While cementing the water-string is now recognized as being the safest means of protection to the oil sand, many operators shut off the water by landing on a shell of limestone, sandstone, etc., or by driving the casing into a stiff bed of clay. In the former case, pipe is previously spudded as far as it will go into the shell and left to stand, when bailing follows to test for leakage. In making a landing in clay, a smaller bit is put on and 25 or 30 ft. drilled and the casing driven into the small hole after which the water is bailed. Additional clay is sometimes dumped into the well, thoroughly mixed and forced behind the casing by screwing a plug with a small valve into the top coupling of the latter after which it is raised, the valve closed, and the string lowered, forcing the clay behind the pipe. The clay gradually settles around the shoe, forming an impervious plug through which the water cannot penetrate.
For bailing water, the dart-bailer is used, usually 40 ft. in length for 6 and 8-in. casing, and a 2000-ft. hole can be bailed dry in 12 to 16 hours, occasional intermissions • being taken to keep the sand reel bearings from running hot.
In new territory where the character of the underlying strata is unknown, the standard-tool equipment is best for making tests of probable oil-bearing formation. Where a shell occurs over the stratum to be tried, the pipe can be landed temporarily upon it and the water bailed. Should there be a good showing of oil, the casing can be left permanently providing the water has been bailed out. When the showing is not sufficient, however, the casing can be loosened with a spear, blasting, cutting, etc., and carried on. In prospecting it is necessary at times to sacrifice a string of casing, good judgment being necessary to determine this. The drillers too should be especially reliable for this character of work, as a valuable deposit of oil may be overlooked in having the hole muddy or through lack of attention to changes of formation. A sand carrying a high gravity oil may be so washed as to give the appearance of water sand and it is often only by careful tests that the presence of oil is detected. In many oil fields the formation is so irregular that each oil-sand has to be tested separately for water, and while expensive, it is necessary, as the future success or failure of the property depends upon the initial tests made. The proper time for testing is when the • measures are first penetrated. Later on, if water should make its appearance, its definite source cannot always be located except by long and tedious trial, pumping, bailing, etc. In going into a known source of oil, a high water-level is usually - maintained to prevent the sand from heaving and sticking the drilling tools. When a sufficient depth into the sand has been obtained, bailing can proceed and the water be exhausted. Added knowledge of the strata, however, may be had by carrying no more water than is necessary to hold the sand down, for the presence of water in the sand is then more readily detected. Each sand in the well, if there be more than one, should be given a separate bailing, and where there is danger of encounter ing bottom-water, tests should be made at frequent intervals. After having reached the oil-sand, casing can then be released at the surface and often made to follow by bailing ahead instead of drilling with the tools; if it stops on a shell, a trial by pumping can be made to test the productivity of the sand before deepening. This character of work is often tedious, but its importance as a means of protecting the oil measures can hardly be over-estimated. A well should not always be judged by its first showing, for the initial gas pressure is often heavy, subsiding in a few clays, while other wells apparently `dead' often become good producers.