After the successful landing of the drive pipe, drilling is resumed with the next smaller sized tool and continued until for some reason or other another string of casing is to be inserted. The different points where new strings of casings are needed are well known in a developed pool, and the size of the hole with which the wells are started depends upon the number of times the hole is to be cased, as each successive casing is of a smaller diameter. Each string of casing reaches from the top of the well through the 'object which is to be shut off. and the bottom landed in a solid rock.
The object which is to be cased off, whether it is caving or water, must from going below the casing and thus into the hole, and for this special casing shoes are used or a " packer " placed at the bottom of the casing. The old system of packing off water consisted in placing a seed-bag, which as the name indi cates is a bag filled with seeds, which will expand or swell up when in contact with water, so that if such a seed-bag is inserted between the wall of the hole and the casing below the source of the water, it will prove to be a means of shutting off the way for the water to enter the sand. Seed-bags are seldom used at the present time, and in place of them specially pre pared packers are used; they come in different styles and sizes, depending on the peculiarities of the different conditions. The object of the packer is the same as that of the seed-bag and it is accomplished as follows: a hollow rubber cylinder is placed between two flanges, and the weight of the casing above will cause the rubber to bulge outward and come in contact with the wall of the hole, forming a strong seal. (Fig. rii.) During the course of drilling, various difficulties may present themselves; it may be that the drilling cable will break, or the joined parts of the tools may become unscrewed, the stem may break, a string of casing may drop in the hole, or some other similar accident may occur; in each case a "fishing job" is on hand. By the use of various fishing tools, depending on the nature of the job, lost objects may be recovered. If not, they may be drilled past and cased off, or the rig skidded and a new hole started.
The general system of casing consists in screwing the joints of pipe together each about 20 feet long, and lowering them into the hole. There are various ways in which the joining together of the several joints may be accomplished.
Quite often it is necessary to drill a hole while it has con siderable water in it, and before casing came into use all wells were drilled "wet." The process is not only slow, but dangerous,
as showings of oil and gas may be overlooked, and if discovered it is hard to determine whether there is any water in the producing sand with the oil or not.
It is the duty of the driller, one which is quite often hastily done, to keep a record or log of the well. This consists in noting the various formations drilled through, the casing points, and the showings of water, oil or gas. The importance of good logs cannot be overestimated. The accuracy with which these records are kept determines the accuracy of the convergence sheet. The points where oil and gas showings are found must be known as it may become desirable to shoot such showings and in order that it may be done properly the exact distance from the top of the hole must be known. Measurements should be made with a steel tape made for that purpose.
A typical well record will show the various lengths and sizes of the casings; the points where water and cavings are encoun tered as well as the various sands and formations drilled through. The driller may be able to determine the kinds of strata that are being penetrated; experience is the best teacher for this, but we may put down one or two characteristics that may be noticed which will help to determine the kind of stratum is being struck by the bit; thus a rapid letting out of the screw shows that soft rocks, such as shales or coals are being drilled, while the opposite would indicate that the stratum is hard. The drill, when pulled out, will have a muddy appearance when drilling through soft, clay-like material, and a polished stem and bit when hard shale is being cut up, while solid hard formations will roughen the bottom of the bit, the "feel" of which will indicate such a con dition.
The oil man's vocabulary contains the word "shell," which must be carefully understood, as it is rather loosely used by the driller. The meaning of the word in this connection is applied to any comparatively thin and hard formation. Thus it may be a thin limestone or sandstone or hard shale; anything that will retard the drilling tools for any length of time. It means very little ordinarily, except that a hard stratum was found. Many records show the following: "No sand found, its place taken by a shell." Such information is rather loose and must be guarded against. Of course the best method of knowing what has been drilled through is by examining the bailings as they come up from the well; whether it is a shale or sandstone or limestone can easily be determined. Quite often dark shales are mistaken for coal.