In California, a disc-bit has been invented which cuts through shells with greater rapidity than does the ordinary fish-tail bit. Two heavy arms extend from the body, and at the lower ends are two saucer-shaped steel discs which revolve on pins, a water connection in the bit providing for circulation. This bit is rotated on bottom, the discs turning and cutting at the same time. The Sharp and Hughes bit (Fig. 119), however, is practically the only rotary bit invented which will cut the hard limestone and sandstone shells as quickly as the same work could be done by the standard drilling tools. In fact, this bit will cut hard rock at the rate of about 1 ft. per hour, which is fully as fast or faster than can be done with the cable tools. Its use is confined only to hard formations, but in these it excels any rotary cutting-tool yet made. Two heavy lugs are held together by a collar, and the cones of specially-made steel with 60 or more rows of cutting-teeth revolve on pins on the inside of each lug. The lubricator pipe, 12 ft. long, is filled with a special bit-oil which is forced down into the bit by the pressure of circulating water above the plunger. The lubricator, when filled, will carry a supply for 24 hours. The cones act as a milling tool, and upon being rapidly revolved, cut their way through the shell. For reaming the hole preparatory to inserting casing, a four-way bit with water connection is used to remove any projecting boulders or shells on the walls which might interfere with the passage of the casing.
For the larger size holes, a 6-in. drill-stem does the boring. Many - operators prefer a heavy pipe, 28 lbs. per foot being the usual weight, while others use a 20-lb. upset pipe, the ends of the joints being heavily reinforced at the couplings for about 6 inches. This pipe, while light in weight, gives excellent service and the danger of twisting it off is not so great as with a heavier pipe, due to the fact that it is elastic enough to permit the bit turning over a projecting boulder instead of throwing a severe torsional strain upon the drill stem. Tool joints (Fig. 120) are placed at every third or fourth joint, depending upon whether the drill-stem is pulled in three or four-length stands. The joints are tapered (Fig. 121) as in those of the standard drilling tools, with a hole through the centre to allow passage of the drilling water. A shoulder 1 in. wide holds the joints together when once screwed up. These joints save considerable time when pulling or lowering the drill-stem, as they are easily coupled or loosened, while the wear on pipe joints and collars is eliminated. For drilling through 8 or 10-in. casing, a 4-in. drill stem can be used while a drill-stem is run in a smaller sized hole.
The drill-collar (Figs. 122 and 123) into which the bit is screwed has a pipe connection at the upper end and a tool-joint connection at the lower end; these collars are often made of solid billets and are 36 in. in length with stock. They are sometimes both babbitted and riveted to the lower joint of the drill-stem, making a stiff connection that is not readily twisted off.
On beginning to drill, the first joint of the drill-stew is plumbed and securely anchored in the derrick by braces until the hole is well started. When four or five joints have been added, the grip-ring
attachment is laid aside and the patent drill-stem before described is substituted. The pumps are run fast enough to carry the drillings to the surface, at the same time keeping the hole clean. Where there is not enough clay present in the formation to 'mud up' the walls, this material can be hauled from a nearby well or bank, mixed and pumped into the hole until the caving ceases. The drill-stem is not forced, but a part of the weight is carried on a swivel to prevent a crooked hole and to 'mud up' the wall properly as well as to allow a free flow of water through the bit. Quite often the water does not return, by reason of the presence of a poi-ous stratum, in which case enough clay is pumped into the well to get a complete circulation. This may take several days to accomplish, but it is necessary before drilling ahead can be resumed. When the bit becomes so dull that it does not readily cut the formation, the drill-stem is pulled and placed back in the derrick in 'stands.' Under severe con ditions, where no other form of bit is procurable, it is necessary to substitute a fresh bit as often as every few inches, but under ordinary conditions, when drilling in blue clay or blue shale, one bit can be used for making from 50 to 100 feet.
The top formation usually is easily drilled, the average rate being 100 to 150 ft. a day. In most districts, however, boulders are encountered. They often can be forced into the walls and side tracked, but when this fails, they must be ground up, withdrawn by a basket, or blasted. If the boulders are driven ahead until a shell is encountered a Sharp & Hughes bit can be used to grind them up, while often a charge of dynamite will save time by blasting them into the wall.
Sand of course is ideal for rotary drilling, the only precaution necessary being to watch the returns and see that the walls are `mudded up.' Shales also drill easily, and clay, while sometimes slow, gives no particular trouble. Formations lying at an inclined angle should be drilled slowly, as with the standard tools, to prevent the hole from going crooked. The drilling-returns from a well furnish evidences of the formation being passed through, and the ditch should be closely watched for any changes. Experience is necessary to judge oil-sands, water-sands, etc., and in many instances where the character of strata is uncertain, the safest method is that of shutting out the water above the strata by cementing a string of casing and later testing by bailing or pumping. Should the formation prove unproductive, the casing is lost, but this is a necessary additional expense where any uncertainty exists. Wells capable of making from 8000 to 10,000 barrels per day have later been discovered in territory where the returns from the sands had been misjudged. Gas makes its appearance known in the trench by froth or foam on the water, and this often indicates the presence of a sand. Some sands, however, show little gas, and for the additional reason that an oil-sand when washed, resembles a water-sand, it will be seen that the returns cannot be too carefully inspected.