DRILLING METHODS oil or gas may be obtained it is necessary that a hole be bored or drilled through the formations in order that the producing horizon may be tapped. The most common methods employed for drilling arc the Rotary and Cable systems. The choice depends upon the character of the formations. Thus, in the majority of cases the strata are quite solid, so that the walls of the holes drilled are quite firm and will not cave imme diately; in such cases the standard cable or percussion system may be used. On the other hand, where the drilling is through soft formations that easily cave, the rotary method is the one adapted for the work. In some cases it is of advantage to combine the two methods so that a change from one to the other may be made whenever necessary.
The rotary system consists in turning a column of pipe, the lower end of which is fitted with the cutting tool, which when rotated will penetrate through the formations, and as the hole is deepened the tools are lowered by the addition of more pipe, which also acts as a preventive (or casing) against a caving hole. \Vat& is forced through the pipe, which is under pressure, temporarily holding the wall, also when it returns to the surface it will bring the cuttings with it. The returning water is examined and thus the various formations passed through may be recog nized.
The standard cable system differs in that the hole is drilled by the raising and dropping of a heavy steel tool, which is a "bit," and by means of additional weight given it by the stem, sinker bar, jars and rope socket, will cut through the formations. The tool is lifted by means of a walking beam which is attached to the wrist pin of the main driving shaft crank. The tool is attached to the drilling rope which in turn is attached to the walking beam by means of the temper screw. This operation can be carried out only after the hole is deep enough to permit the string of tools to be lowered into it so that it may be attached to the walking beam by means of the temper screw. Until the required depth is obtained, spudding is resorted to, which con sists in the lifting and dropping of the tools by means of a jerk line attached to the wrist pin, and the other end to the drilling rope by means of a spudding shoe which works freely along the line.. The jerking motion imparted to the rope lifts the tools in the derrick and as the hole is being deepened more rope is let out. The removal of the cuttings is accomplished by lowering a bailer with a dart valve at the bottom and the cuttings are brought up. (Fig. 113.) Both systems of drilling require a tall derrick for the purpose of drilling so that the tools and casing may be lifted up vertically before they can be inserted in the well. In fields where the
depths are not over 2000 feet, portable drilling machines are in use, which differ from the standard derrick in being lighter and using a high mast instead of the derrick tower.
When the location in the field is made and properly surveyed, the next step is the building of the rig, which is done by contract with people whose vocation is the erecting of such structures. When the rig is up (Fig. i4) the companies' or the drilling contractors' men will string their tools and soon the well is spudded in.
The drilling is mostly done by drilling contractors who agree to drill a well to a certain depth or through a certain sand for a specified price, which varies in different localities, depending upon the accessibility of the location, the depth of the well and other conditions that will affect the drilling, the price ranges between one to seven dollars per foot. The success of the drilling contractor depends upon the smoothness with which the opera tion has been carried out, numerous fishing jobs will eat into his profit considerably and in case of continued misfortunes may show considerable losses.
In the standard system of drilling various size casings are used at different depths to keep the wall up and to case-off water that may be encountered during the drilling. In most cases a certain amount of surface soil is to be drilled through before the underlying rocks are reached; this surface soil may consist of gravels, which are quite thick along river bottoms and in glaciated countries. If it is but a few feet, a n-inch hole is started, and an octagonal shaped wooden conductor or casing is inserted, but if the drift is thicker a large size casing known as the drive pipe is used. A short distance is drilled by spudding and the drive pipe allowed to lower down, and generally driven down by clamps attached to the drilling stem, and this continued until solid bottom is found. Where large boulders are encountered, care must be taken to drill through them, as driving the pipe against such boulders will cause the bottom of the casing to bend, especially if a light weight casing is used. Such boulders may be dynamited, but if all attempts to remove them or drill through them fails, the rig will have to be moved or " skidded " and a new hole started. Such boul ders may cause a crooked hole. as the tools hitting them a glancing blow may be diverted and thus the crooked hole started.