The necessity for reaching greater depths than could be drilled with the spring-pole or Canadian pole-tools called for heavier tools and improved methods, and so there came about a gradual evolu tion to the use of, horse power and steam ; from the spring pole to the walking beam with its rigidity and positive motion ; from rods screwed together to manila rope and wire cables. At the same time were developed many special drilling and fishing tools, and the greatest single improvement of all, the use of casing or pipe for holding back caving ground that tends to fall in and fill the hole, and for excluding from the oil-sand the water from overlying strata.
Much of this growth has occurred as different requirements of the various new fields were encountered, so that while the basic methods of drilling along the lines either of the standard tools or the rotary are followed everywhere, yet local conditions and the inherent inventive ability of the oil operative have resulted in any number of special applications of these methods, devised to over come the specific obstacles encountered.
A volume of this kind cannot include descriptions of all the in genious schemes at the command of the old driller experienced in many fields. At best, few branches of engineering carry the hazard and chance that accompany drilling for oil. A little carelessness, an unavoidable accident or defect in tools or may result in plugging a hole, with the loss of months of work. A plugged hole has slight salvage value and the need for keeping out of trouble, rather than of getting out, is constantly before the oil man ; and while fishing jobs are inevitable, yet care and proper precautionary steps are features of exceptional value in this work.
The two methods of drilling most commonly employed are known as the standard, or cable-tool method, and the hydraulic, or rotary method. The former employs a walking beam to churn the hole by an up-and-down motion imparted to tools suspended from a line connected with the end of the beam. When the hole has been advanced several feet, the cutting tools are withdrawn and a bailer, or sand pump, is run in on the end of another line, for the purpose of removing the cuttings. The rotary method of drilling is a cutting process by which a suitable bit, attached to the end of a column of pipe that is turned by machinery at the surface, is made to scrape away the bottom of the hole. Thin mud is pumped down inside the pipe and through an opening at the bot tom, from where it returns to the surface on the outside of the pipe, bringing with it the drill cuttings. The process is practically continuous except for the necessity of pulling the pipe from the well when the cutting-bit has become dull and must be replaced with a sharp one.
Each of these methods is successful when used for drilling in ground to which it is adapted. In general, the cable-tool method preferred where the series of strata to be pierced is hard and the severe impact of the walking-beam motion is needed to churn the hole. In soft and loose material, the rotary method is usually su perior, and while it entails a greater expense for labor, fuel, and maintenance of machinery, yet the speed it often attains and other advantages described in detail in the chapter devoted to drilling, often warrant the added expense from the standpoint of commercial feasibility. It is rarely, however, except in the Gulf Coast districts,
that it is employed in drilling wildcat wells.
It should be noted here that the term 'wildcat' does not possess the unsavory meaning associated with it in the mining world, where it suggests dubious financial operations rather than progres sive mining activity. In the oil fields, a 'wildcat' well is a prospect or test well, drilled sufficiently far from proved territory to raise the question as to whether or not oil will be found. Much wild catting is carried on by many of the old substantial companies.
Properties and Uses. Petroleum is a liquid belonging to a series of hydro-carbon compounds of complex chemical composition ranging from the gaseous to the solid state, namely, natural gas, petroleum, mineral tar, and asphalt. These pass by insensible gradations from one to the other with no strict line of demarcation between them ; and among the petroleums, wells only a short dis tance apart will frequently show remarkable differences in compo sition and gravity. In the United States, the oil which has a paraffin base generally occurs east of the Mississippi while west of it usually is found the heavier oils with an asphalt base.
Within the limits of individual fields, the value of petroleum is generally rated according to its weight, or specific gravity, the greater value going with the lighter oils that contain a higher per centage of the more valuable products. By specific gravity is meant the relation in weight between any given volume of oil at 60° F. and that of an equal volume of pure water at 39.2° F. This is generally designated in oil field practice according to the Beaume scale, in which the weight is represented by degrees, the higher num bers being those of the lighter oils, and 10° Beaume the equivalent of water. The gravity is determined by the use or a Beaume hydrometer (Fig. 8), a glass column which, when immersed in oil, sinks to a depth dependent on the density of the oil. A scale on the glass shows the depth of immersion and gives a direct reading of the gravity, except for a correction that must be applied if the tempera ture of the oil is greater or less than 60° F. A thermometer is generally combined with, and made a part of, the hydrometer. The temperature cor rection varies with oils of different gravities and published tables of correction must be used when precision is desired, but for ordinary oil field work a reduction of 1° in gravity for every of tem perature above 60° F. is sufficiently close for oils around 18° Beaume. With 25° Beaume oil the cor rection is 1° Beaume for every 16° above 60° F., with corresponding additions of course when the temperature of the oil is below 60° F.