DEPTH Depth largely governs the type of drilling outfit chosen. Shallow holes up to 1500 ft. in depth can often be tested with a portable drilling outfit at less expense than with heavier equipment. Above 1500 ft. it is customary to use heavy frames or derricks that are usually allowed to remain in place after a paying well is obtained.
Portable drilling rigs of either the rotary or Standard cable-tool type are used in fields where shallow wells of 500 to 1500 ft. are the rule. They are also used in areas of hard, consolidated beds where drilling conditions are simple, and but little casing is needed, as in eastern Oklahoma, eastern Kansas, the Osage and the Greybull fields of Wyoming and in Kentucky, Ohio and Pennsylvania.
However, some deep tests have been made with portable rigs, notably one near Mineral Wells, Texas. This well, called No. 1 Oakes, drilled by Owens and Willis, south of Mineral Wells, Palo, Pinto County, Texas, was carried 4550 ft. with 2200 ft. of 6% in. casing and 2350 ft. of open hole. This is the deepest known test drilled with a portable National rig.
Portable rigs have all the drilling machinery conveniently mounted on a single frame. (See Figs. 3 and 4, pages 40, 41.) The boiler is also portable and mounted on wheels. The advan tage of such a system is obvious where a number of shallow wells are to be drilled.
In soft, unconsolidated beds where much casing is used, portable rigs are not desirable, and much heavier outfits are required. Derricks are then installed.
derrick is the frame structure used in drilling. Its main uses are: (a) To suspend the drilling cables and tools; (0) To house the drilling machinery; (c) To shelter the workers.
The essential parts of a derrick are: 1. The floor, 2. The crown block, 3. The legs, the girts, and braces supporting the crown block.
The type of derricks employed must be suitable to the drilling systems used. The choice is dependent on the conditions to be met in drilling. Derricks are either of wood or steel.
The wood derricks range in height as follows: 56 ft., 64 ft.,
72 ft., 84 ft., 96 ft., 106 ft., 114 ft. and 120 ft. The height varies (1) with the depth to be drilled, and (2) with the amount of casing to be handled.
The highest derricks are generally employed on deep rotary holes. Such high derricks allow the drill pipe or casing to be pulled in sections of four and five joints, thus greatly facilitating the pulling time.
Steel derricks are made of angle irons or hollow tubes. The angle-iron derricks are stiff, but where great strength is required in drilling they can be used.
Hollow, tubular derricks are used in some oil fields for drilling wells. (See Fig. 5, page 42.) In any area where high wind storms occur, steel derricks are decidedly superior to wooden derricks, as they present a minimum of resistance to the wind, due to the smaller surface exposed.
In a storm (in 1918) in Oklahoma and Kansas several thousand wooden derricks were blown down causing a loss of at least a million dollars.
Wooden derricks, while not so strong, are superior to steel for drilling purposes as there is greater elasticity in the rigs, which is essential where sudden strains and pulls are required. By double bracing the derricks their strength is greatly increased.
Tight guy lines on derricks are of much importance in strength ening derricks. Slack lines assist but little in holding derricks in place.
Rig Building.—In building a rig mud-sills are first laid down as a floor foundation and upon these sills are laid cross sills upon which the derrick floor is built.
The main sill supports the Samson-post, which in turn acts as the support for the walking beam. (See Fig. 6, p. 44.) One of the band wheel or jack posts also rests upon the main sill.
The other supporting post for the band wheel, and one of the posts supporting the calf wheel rest on another sill.
The bull-wheel shaft is supported by two posts called the bull wheel posts, which rest upon a cross sill called the bull-wheel sill. The bull wheels are controlled by the band brake.