Hole openers and Surface pipe

Hole openers

As we increase the diameter of the hole, which is done in stages, we find the need for larger bits. The hole is constructed by the same method as used for drilling a hole in steel. The first step is to drill a pilot hole and increase the size in stages. Large diameter fishtail bits can be used successfully in the soft formations, but when we have to enlarge the diameter in consolidated formations we use a different type of roller bit. This bit has cutters mounted on the outer radius to enlarge the hole. It has either a pilot fishtail or a box for the use of a small roller cone bit for the 

pilot. For the enlarging of the hole below a casing to a diameter larger than the casing through which we are drilling, we use an underreamer. There arc several types of these tools, but basically we have only two, one being hydraulic where a jetting type of tool is used in soft formations to wash out a pocket. This is done by directing high pressure fluid through nozzles directed toward the face of the formation. This is not effective in hard or consolidated formations. The mechanical type has blades which extend out from the body of the bit when fluid pressure is applied. This type of bit is satisfactory in formations which are consolidated, but relatively soft. When necessary to underream hard formations a bit which has expandable rotary cones is employed. This particular type varies from one manufacturer to another . . . as to the type of rollers used. 

Surface pipe

Surface pipe is sometimes necessary and its purpose is to prevent caving in the immediate 

vicinity of the surface around the rig. This cav ing can be very hazardous if allowed to con tinue. It can develop to a point where the ma chine can actually sink into the ground. It is not normally required below the first hard or clay formation and is usually less than one hundred feet.