DEVELOPMENT-DRILLING CONTINUED Size of the Hole.—The idea in drilling is to start with a large enough hole so that when all reductions are made the completed hole will be large enough, not only to produce oil, but will also give room to manipulate cleaning out tools, bailers and pump tubing. A completed well or test hole of 6% in. in diameter may have a starting diameter of 152 in. or even more.
Drilling must be conducted so the hole will not be "pointed out" or reduced in size so rapidly that the well must be abandoned before it reaches the depth to which drilling is considered neces sary. In areas of Pennsylvanian rocks, or wherever beds stand up readily, the starting diameter can be less than in areas of soft, unconsolidated beds.
Casing holes require less casing for the same depth than do cable tools. Table 8, page 86, shows that in tests up to 3500 ft. the rotary starts with 122 in. casing, as against 152 in. for the cable tools. (Fig. 22, page 69 illustrates this point.) In rotary drilling casing sizes are reduced on the following plan. After the first casing is introduced, say a 14-in. size for a deep hole, then the next size must be a 10-in. casing. Twelve inch would go inside the 14-in., but as the diameter of the collar is 13 in., there is small room for circulation; also the friction of the upward-traveling water requires a heavy pump pressure. By allowing two extra inches of clearance good circulation is assured.
Crooked drill hole must be straight. Crooked holes are due generally to rapid drilling in steep dipping forma tions. If the bailer binds in going into the hole, it is a sign of either a caving or a crooked hole. If the hole is shallow a simple test is to reflect a ray of light from a mirror into the bottom of the hole, and see if it is straight or crooked.
If the casing, when .lifted, swings freely, it is a good to t of the hole being straight.
Drilling Time.—Drilling time is a factor influenced by the type of drilling method used, and by conditions, such as the hardness of the beds, accidents which cause delay, by the speed with which supplies are obtained, and by the individual efficiency of the drillers. Under favorable condi tions 100 ft. per day of hole may be made with cable tools. As high as 150 ft. has been made.
Under good conditions, a hole 3500 ft. deep in 60 days would be fast time with cable tools. Under average conditions six months would be considered good time.
With a rotary, drilling time is generally faster than with cable tools. As high as 500 ft. per day has been made with a rotary.
Completion of a well 3000 ft.
deep in 30 days is fast time.
Ninety days for the same depth is good, average time.