Casing. In drilling where the ground is rocky and firm or where the materials in the series of strata are bound together so that fragments do not cave in from the walls of the hole, the drill ing may frequently be carried for hundreds of feet in 'open hole.' More often, however, the beds of clay, shale and sands, 'with some of them containing water, are so fragile and loose that they crumble and fall in to such an extent that drilling operations must be discon tinued unless they can be held back. In such ground there is al ways the further danger of the cavings burying the drilling tools. These conditions have led to the adoption of various forms of tubes for lining the hole. A second and very important feature of the value of such linings is their use for excluding from the oil-sands the water held in strata nearer the surface and which, if not pre vented from entering the oil sand, will displace the oil by reason of its greater specific gravity and eventually ruin the well.
Casing as now used in the oil fields is made of either iron or steel and the kinds and sizes differ considerably with the conditions obtaining in different parts of the world. The complete column of pipe as placed in the well is known as the 'string' of casing and in some fields one string suffices to finish the well. More often, if any considerable depth is attained, the pressure (commonly known as the 'friction') of the crumbling materials against the pipe becomes so great that the pipe is bound tight and cannot be moved farther either up or down. A second string, small enough to go inside the first, must -then be put in before drilling is continued ; and fre quently four or five, or even more, may be necessary in reaching depths of over 2000 ft. in difficult ground. .
For the first well drilled in unproved ground; the number of strings of pipe that will be required in reaching a certain depth is unknown ; but in a field that has been drilled and the drilling condi tions learned, the starting-size becomes merely a question of the size with which it is desired to finish the well. Strings of 10-in. and 8A-in. pipe are sufficient in some American fields, while with others the well will be begun with 18-in. casing. In Russia, where the sands cave badly, holes are started with a diameter of 36 in. in order to finish them 16 inches.* Two general classes of casing are in common use for oil-well service—riveted steel pipe and screw casing. Riveted, or 'stove pipe,' casing is made of steel or iron sheets, riveted at the seams, and is used especially for the first string to be inserted in a well. It is made by cutting the sheets into the proper size, punching and countersinking the rivet-holes, then rolling to shape and fastening with rivets. The pipe most commonly used in the United States has two thicknesses of sheets; so placed with respect to each other that the end of one sheet is set opposite the centre of the other, so that at the end of a joint the inside sheet projects for half its length beyond the outside sheet, leaving a corresponding recess at the other end (Fig. 55). This double-riveted casing is made in joints
2 or 3 ft. in length, and, for ease in handling, several of these joints are riveted together into sections of from 10 to 21 ft. before placing in the well.
Frequently the pipe is 'picked' before inserting it in the well. This consists in denting the outside with a heavy sharp-pointed pick, and is done to take up any slack between the outside and in side sheets and assist the rivets to prevent it from pulling apart. Since nearly all casing is driven from the surface before reaching its final depth, it is advisable to place on the bottom of the first, or 'starter' joint, a steel shoe of slightly greater diameter than the outside of the pipe itself (Fig. 56). This cuts away any irregu larities projecting from the side of the hole and clears a passage for the casing. Stovepipe casing shoes are made from 3 to 14 .in. in length and are riveted directly to the starter joint. The latter is usually made of three thicknesses for the first 18. ft., and when, a steel shoe is not used, the innermost sheet is lapped back over the outside for 6 or 8 in. and riveted there. This is known as the 'turn back' starter and while it is not as rigid as the solid steel shoe and does not contribute as well to the strength of the starter-joint it has the advantage of a smaller outside diameter, thus reducing the size of hole to be drilled by the cutting tools.
The merits of riveted pipe are mainly that its smooth, uniform outside surface is a great aid in carrying the casing down through loose and sandy materials which tend to fall in and hind against the couplings on screw casing. Screw casing, however is more easily handled and may be raised and lowered at will, while the riveted pipe, when once started in the hole, is not raised and can be lifted out only by the use of a spear.
Screw casing is made of either iron or steel plates, welded at the seam, and takes its name from the threads that are cut at each end of the joint. With the exception of a few types, a threaded sleeve, or coupling, connects two joints by screwing over the threads at the ends. Couplings are invariably made of iron, but the pipe itself may be obtained of either iron or steel and individual tastes or ideas of operators rather than any specific drilling condi tions usually govern which is used. Steel has the advantage of a slightly lower cost and is said to be stronger than iron. It is, how ever, more subject to weakness with age from the chemical and electrolytic action of alkaline and sulphur waters.