Modern drilling operations used to create boreholes in the earth for the production of oil, gas and geothermal energy typically employ rotary drilling techniques. In rotary drilling, a borehole is created by rotating a tubular drill string having a drill bit secured to its lower end. As drilling proceeds, additional tubular segments are added to the drill string to deepen the hole. While drilling, a pressurized fluid is continually injected into the drill string. This fluid passes into the borehole through one or more nozzles in the drill bit and returns to the surface through the annular channel between the drill string and the walls of the borehole. The drilling fluid carries the rock cuttings out of the borehole and also serves to cool and lubricate the drill bit.
One basic type of rotary rock drill is a drag bit. Some drag bits have steel or hard faced edges, but primarily they have a main body into the outer surface of which are embedded extremely hard cutting elements. These cutting elements are typically made of natural or synthetic diamonds. As the drag bit is rotated, the cutting elements scrape against the bottom and sides of the borehole to cut away rock.
Another basic type of rotary rock drill uses roller cone cutters mounted on the body of the drill bit so as to rotate as the drill bit is rotated. The angles of the cones and bearing pins on which they are mounted are aligned so that the cones essentially roll on the bottom of the hole with controlled slippage. One type of roller cone cutter is an integral body of hardened steel with teeth formed on its periphery. Another type has a steel body with a plurality of tungsten carbide or similar inserts of high hardness that protrude from the surface of the body somewhat like teeth. As the roller cone cutters roll on the bottom of the hole being drilled, the teeth or carbide inserts apply a high compressive load to the rock and fracture it. The cutting action of roller cone cutters is typically by a combination of crushing, chipping and scraping. The cuttings from a roller cone cutter are typically a mixture of moderately large chips and fine particles.
When drilling rock with a roller cone cutter, the fracture effect of loading on the teeth of the rock bed is limited due to the rock matrix surrounding the borehole. Failure of rock is prevented in a large degree by the restraint to movement offered by the surrounding rock. Thus, it appears in usual drilling operations that small cracks are created in the rock which return to the surface of the bottom of the wellbore creating chips instead of propagating deep into the rock itself. Thus, the bit tooth of the usual rock bit presses on the rock surface tending to create small cracks which propagate downward, but by virtue of the resistance to fracture offered by the surrounding rock matrix, a crack follows the path of least resistance and emerges at the surface on the bottom of the wellbore, thus creating the small chips.
U.S. Pat. No. 3,055,443 to Edwards discloses a combination drag bit and roller cone cutter which removes the lateral restraint on a core to be drilled. The drag bit component cuts a single annular kerf forming a core which is received within a hollow body member and drilled by multicone rolling cutters arranged within the hollow body member. Windows are provided in the bit body adjacent to the multicone cutters to provide an egress for chips formed by the destruction of the core. This bit design causes rapid failure of the drag cutters, however, since drilling fluid escapes through the windows and results in insufficient fluid flow to cool the drag bit component.
In the practice of the present invention, the resistance of the rock to fracture is removed or reduced by employing an improved drill bit which destroys the rock rapidly and efficiently. The drill bit of the present invention overcomes the prior art problems of rock chip removal and cutter cooling associated with cutting a single annular kerf and removing material within the kerf. Furthermore, the drill bit of the present invention cuts multiple annular kerfs which result in more rapid drilling rates than those achieved by cutting a single annular kerf.