Drilling equipment and Drilling bits

Drilling equipment

This method of drilling requires the use of a drilling machine as illustrated (Fig. 1) which consists of a mast with sheave wheels mounted at the top for the purpose of operating a hoisting mechanism. Hanging in the mast or derrick by a block assembly is a square or keyed drive called a kelly. At the top of the kelly is a swivel to which is connected a rubber hose or steel with flexible joints as pressure dictates. This kelly is driven by a rotary drive assembly which has inserts or blocks to fit the kelly. The kelly is larger than the drill rods in order to drill down a few feet below to enable a section of drill pipe to be added as the hole progresses deeper. To the bottom of this kelly are attached hollow drill rods and by the use of attachments, subs, the rotary bit. As wc drill deeper additional rods are added to the string just below the kelly. The bit has holes in the bottom through which the fluid can flow. Moving back on the machine we find the draw works which operates the hoisting mechanism. Also mounted either on the machine or sitting beside it is found the pump which forces the drilling fluid through the lines to the swivel down the kelly and out the bottom of the bit.

Drilling bits

The different types of bits can best be de scribed by illustration. We start with the simple 

star or fishtail drag type of bit (Fig. 2). This bit is used in soft unconsolidated formations such as 

clay and sands. They have fluid courses which tend to jet the formations and the blades are very 

effective on sticky clays. 

The cone type of roller bits have two to four cone type of cutters mounted on roller bearings which have teeth in varying lengths that inter mesh (Fig. 2a). This bit has fluid courses which depending upon the design either jet directly on the formations or wash the cutters clean when the stream is directed on the cones themselves. The bits vary from long tooth cutters for use in soft formations to short intermeshing cutters which tend to chip in very hard material. The harder the formations the shorter the cone teeth. As a general rule we rotate soft formation bits 16 at higher speeds and have much lower bit weight. As a general guide soft formation bits will be used at speeds from 50 to 150 RPM with bit weights from 1,000 to 4,000 pounds per inch of bit diameter. The very hard formation bits will be used at speeds of 30 to 50 RPM with 2,000 to 5,000 pounds per inch of bit diameter. 

 Varying manufacturers have different designs of the fishtail such as finger bits to many types 

of roller bits. This is a wide field and experience in an area will solve the problem of the most 

efficient type of bits to use in a particular area and formation.