In connection with the problem of recovering lugs, as well as many other of the small tools that resist capture, the possible application of some form of a magnet appears to offer a wide and inviting field. Considerable experimental work along this line has been carried on, but the technical difficulties seem to have been too great for successful results, although the principle is sound and would be of great value if it could be applied under the peculiar conditions of pressure at the bottom of a deep hole filled with water and in the presence of bodies of casing, which have themselves in nearly all cases become highly magnetized.
Fishing for Casing. Among the accidents that may hinder the progress of drilling a well, and involve no small expense as well as loss of time, are the mishaps that occur to the casing, especially in those fields where the sides of the holes cave badly and give rise to the constant danger of cavings falling in and binding the pipe. The extent to which conditions of this nature may endanger the casing depends entirely upon the ground. Some formations `stand up' and are so compact and closely cemented that no dirt falls in, while others disintegrate rapidly and unless the pipe is moved up and down at frequent intervals, so that the materials fall to the bottom of the hole, it soon becomes bound with so much loosened dirt that it resists all efforts to move it.
Frequently, when casing has become 'frozen' in this way and cannot be pulled up, it may be driven down for a few feet and then pulled back to its original position, driven again and so worked up and down• until it is loosened. The driving is accom plished by inserting a drive-head (Fig. 94) in the coupling at the top of the string of casing and striking this with heavy clamps attached to the drilling-tools, raising and lowering the tools either by direct drive .from the bull-wheel shaft or with the jerk-line and spudding-shoe. Another resource that may be tried is that of bailing the water from the inside of the pipe, causing the pressure of the water on the outside, between the pipe and the wall of the hole, to tend to force the sands that are binding the pipe down to the bottom of the hole. In either of these methods, precautions must be taken to prevent the sudden descent of the pipe for any considerable distance after it has become free, because of the danger of its bending or telescoping. The usual device is a wire
sling suspended from the casing-hook and attached either to the ends of the spider or to each of the two links of an ordinary elevator.
Frequently it is necessary to apply more forcible measures be fore the casing may be dislodged, and for this work spears that take hold of the pipe, and by means of which it may be jarred, are universally used. Usually they are run into the hole, on a drilling-line and string of tools, until the desired depth is reached ; they are then pulled up till the slips engage with the inside of the pipe and jarred until the pipe is moved.
The most simple form of spear for this purpose is the common bulldog-spear (Fig. 172), which is rarely used, however, because it may not be pulled up in the pipe after the slips have once taken hold. Many improved patterns, such as those shown in Figs. 195 and 196, are so constructed that when it is desired to free the spear and withdraw it from the pipe, a downward jar of the tools causes the slips to become disengaged and fall into a recess in the body of the spear, where they remain while it is being pulled out. A dozen or more styles of 'trip' spears, as these are known, are made for service of this kind, some with two and others with four slips, and all work along the same lines of being lowered to the desired point and then raised, at which time the slips engage with the pipe. When lowered a second time, the slips trip back into a recess and remain there, and the spear must be pulled from the hole and the slips 'set' again before they can be made to grasp the pipe. The most common type is made so that the slips grasp the pipe for an upward pull, and is known as the `jar-up' spear. For jarring down on pipe the oblique plane holding the slips is reversed.
Often the point at which the pipe is bound will be found to be at the casing-shoe, which, by reason of its slightly greater diameter, is holding back cavings that would otherwise pass to the bottom of the hole. Or it may be that the shoe has been lowered into an opening just small enough to bind it. In such cases a few taps with a casing spear usually succeed in knocking it loose. At other times the friction may be so great that jarring must be continued for several hours, or days, before the pipe starts to move.