Some drill bits are literally the jewels of the oil rig. They have evolved in shape, designs, components and applications over centuries. Drilling of oil and gas wells cannot happen without them. Let’s learn a bit about drilling bits.
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Drilling bits have been around for many centuries before their wide application in Oil and gas exploration. Used mainly for exploring shallow subsurface deposits of precious metals, freshwater and salt domes at different periods in time. In the early years of commercial oil exploration, the fishtail bits was widely applied as part of the percussion drilling system – a repetitive lifting and dropping of the heavy cable-tool bit which was quite slow, laborious and only useful for wells with soft rock formation which were drilled to very short depths. The advent of rotary drilling in 1845 by Pierre Pascal Fauvelle was a significant development which aided the modern bit design starting with the two coned roller cutter bit structure introduced by Howard Hughes and Walter Sharp in 1908.
Drilling bits are the tools connected to the end of the drill string which bores the well hole by breaking, cutting or crushing the subsurface rock formation into little pieces called Rock cuttings. The bits are hollowed structures allowing drilling fluid to jet through nozzles in the drill bit. The jetting fluid help to lift the rock cuttings from the wellbore to the surface. There are many variations in the designs of drilling bits. The boring mechanism of a drilling bit depend on the type of formation being drilled and ranges from Crushing (Elastic rock formation), Shearing (Plastic rock formation), to a combination of Shearing and abrasion (Abrasive rock formation).
The performance of a drill bit is evaluated on the rate of penetration (ROP) which determines the drilling rate. Other desired attributes are long life of bit (which lowers number of trips in and out of the well for bit replacement), full-gauge drilling which aids the drilling of straight uniform holes. The effective cost of the bit measured as $ per foot drilled is also a major important metric in determining the efficiency of the drill bit selected.
TYPES OF BITS
In the modern oil and gas industry, there are broadly two classes of drill bits, namely;
Rolling cutter drilling bits
Fixed cutter drilling bits
Roller cutters or rock bits are designed with the cutters on two but more commonly three (Tricone) cones, each cone is free to rotate independent of the others. The cutting structures are Tungsten Carbide Insert (TCI) cutters pressed on the cones or milled spiked tooth (made of steel and hardfaced with tungsten carbide substances) cut into the cone structure. An important element of rock bits is the presence of many moving parts (cutters, cones, bearing and lubrication system. During drilling, the rotary motion of the rotary table - drill string rolls the rock bit along the bottom of the well hole. The rock bit’s main cutting actions are chipping and crushing. The main advantage of rock bit when used in the right application is their price. They are usually 10 – 15 times cheaper compared to the fixed cutter bits of equal diameter size. Milled tooth bits are cheaper compared to TCI.
In general, soft formations are drilled with long and slim cutters with high spacing. Hard formations, on the contrary, requiring more weight on the bit, are drilled using short and flat cutters with a low spacing.
Fixed cutter bits - As the name implies, the bit structure have no moving parts. The design concept is to have a single piece-drilling head on which cutters can be inserted. The main cutting action down hole is gouging and scraping; both shearing actions. With cutters made of natural or synthetic diamonds, fixed cutter bits very expensive.
Fixed cutter bits are named by the type of diamond cutter inserts they carry. These include Natural Diamonds, Polycrystalline Diamond Compact (PDC – a combination of natural diamond and tungsten carbide), Thermally Stable Polycrystalline (TSP) and finally Impregnated diamonds. These bits have superior durability and resistance to wear due to their diamond based ultra-hard cutters compared to roller cones. They are also better suited for applications requiring very high RPM where rotation is supplemented down hole by positive displacement mud – motors (PDM) hence, they have found more use in horizontal and directional drilling. Another reason for their preference in horizontal drilling is the limited likelihood of junk in the hole from a loss of moving part which is common with roller cone types.
PDC bits show excellent performances mainly in soft and not very abrasive formations. TSP bits serve a similar range of formation as PDC, however, with better resistance to thermal degradation. They have a longer life of bit compared to PDC albeit with a lower rate of penetration. Impregnated diamond bits are designed to drill particularly hard and abrasive formations, when all the other types of bit wear out in a very short time, which might making drilling extremely costly. Impregnated bits do not have cutters in the strict sense of the word, but cut into the rock formation by abrasion.
DRILL BIT NOMENCLATURE
The International Association of Drilling Contractor – IADC is the global umbrella and network of companies and professionals involved in the drilling contract business. IADC has a 4 character naming code for identifying drill bits based on their design and application. The codes vary for the roller cones and fixed cutters. These are described in the picture pallet on the left.