Before putting in the casing, a four-way reamer-bit is run to bottom to insure its free passage. The drill-stem is then stood back in the derrick and the casing inserted as rapidly as possible so that circulation can be started again before the walls begin caving. Slide tongs (Fig. 124) are used to support the elevator on the rotary table when inserting or withdrawing pipe. When a hole cannot be cir culated, the casing must be withdrawn to a point above the friction, and the pipe rotated back to bottom, where it is later cemented. In deep wells, where the weight of the casing is more than the draw works can safely carry, calf-wheels are installed and the lines transferred to it. Another engine becomes necessary to move the calf-wheels, but when it is considered that the success of the well depends upon shutting out the water, this additional cost need not be considered. Ten-inch casing is usually set for the water-string, although 8/ and are frequently used.
Heavy gas-pressures are generally encountered in the oil-sands or at some point not far above, and the rotary method is the ideal one for this character of work because the pressure can be overcome by heavy mud. When a heavy pressure becomes evident, the blowout preventer is attached to the water-string, while a back pressure-valve is placed in the drill-stem at the bottom. The blowout-preventer is a heavy gate with four projecting clips which can be set up to the drill stem by means of a long handle operated outside of the derrick.
The clips fit snugly around the stem when closed, preventing the escape of gas or mud, while the body of the preventer has two screwed openings which communicate with the lead-line. This valve is also made to close when there is no drill-stem in the hole. The back pressure-valve screws into the pipe-couplings between joints, and is so arranged that a pressure below is resisted while the top-pressure can force it open. It often happens, in extreme pressures, that gas is not sufficiently checked and that there is danger of a blow-out. Heavy, clay mud can be admitted to the drill-stem by attaching a gate to the casing at the floor, while two or three joints extend above it to a second gate at the top. A hose is attached to the latter and the clay pumped into the column above the floor, when the upper gate is closed and the lower one opened, allowing the mud to slip down the hole. In this way the gas-pressure can he gradually checked until it gives no trouble. This method is called `lubricating' and by its use the heaviest gas-pressures can be controlled.
The greatest source of trouble when using a rotary is twisting off the drill-stem, that is, applying so great a torsional strain to the stem that the column twists in two. Frequently the relief from the
strain or 'backlash,' as it is called, spins the stem in the reverse direction, often parting it a second Freezing the drill-stem does not often occur, but when it does, a larger string can•be rotated over it to the bit, freeing it so that the whole column can be removed. When the entire drill-stem resists washing, pulling, etc., a larger string with left-hand threads in the couplings is run and a few joints unscrewed at a time by operating the rotary in the reverse direction until the hole is clear. It often happens that the casing is frozen while being run previous to landing. Where this happens, the same methods as in standard tools can be used. A rotary shoe (Fig. 125) is usually placed on the bottom of the first joint of casing.
While the rotary is not always reliable for prospecting, yet a driller with wide experience in judging the returns makes this method nearly as safe as with the cable tools, especially where the drilling is done in daylight so that the ditch can be more carefully inspected. If the cementing-point is uncertain, a smaller rotary-bit should be used and when ready to set the casing, the hole can be enlarged to bottom in the usual way. After the oil-measures have been drilled through, the walls of the hole are left in the mudded condition until the liner is set into place. This is done by attaching the perforated pipe to the drill-stem by a left-hand coupling, which, when unscrewed, leaves an adapter or guide at the top, to prevent lodgment of the bailer, tools, etc., when cleaning out. In the southern fields, where the oil-sand is often a coarse gravel, a screen or strainer pipe is used, while in California, round or slotted perforations are generally considered to be better adapted to the fine sands usually found. When running the liner in, 2-in. tubing is used to carry the water to bottom instead of allowing it to return through the top perforations. The tubing is set upon a ring attached to the lower collar of the liner, and extends to the drill-stem, where it is attached to the latter with a bushing. The liner always extends 50 to 75 ft. up inside the larger string.
After being lowered to within a few inches of bottom, clear water is pumped into the well until the returns show only traces of mud. Then the liner is set on bottom and the drill stem detached from it. The latter is then pulled out and the well bailed and prepared for production.