Drilling BitsIn the oil and gas industry, a drill bit is a tool designed to produce a generally cylindrical hole (wellbore) in the earth’s crust by the rotary drilling method for the discovery and extraction of hydrocarbons such as crude oil and natural gas. This type of tool is alternately referred to as a rock bit, or simply a bit. The hole diameter produced by drill bits is quite small, from about 3.5 inches (8.9 cm) to 30 inches (76 cm), compared to the depth of the hole, which can range from 1,000 feet (300 m) to more than 30,000 feet (9,100 m). Subsurface formations are broken apart mechanically by cutting elements of the bit by scraping, grinding or localized compressive fracturing. The cuttings produced by the bit are most typically removed from the wellbore and continuously returned to the surface by the method of direct circulation.
Selecting the proper bits for a well is an important decision that affects the overall well cost. Many factors must be evaluated:
- bit cost
- expected parameters (Le., turbines, rotary drilling, air operations)
- formation types and properties
- mud systems
- rig cost
The emerging technology for bit design adds additional significance when evaluating these parameters. Drilling optimization can be viewed in three distinct phases:
- Selecting the proper bit for the drilling conditions.
- Monitoring the drilling conditions on the prospect well so drilling performance is at least equal to or above the average in the particular area.
- Implementing a bit weight-rotary speed program from theoretical calculations that will improve drilling performance above the best wells in the area.
Generally, the last phase is difficult to implement in a one- or two-well drilling program. Unfortunately, the first two phases are often neglected, even though they require only a relatively simple evaluation of offset well data. This text will be restricted to covering the first two phases of drilling optimization.
TYPES OF DRILLING BITS
Rolling Cutter Bits
- Roller Cone Bit Design: Roller Cone Bit Bearing Assemply, TCB Cone Geometry Design, TCB Cutting Structure, Fluid circulation
- Bit Classification
- Running Procedures
- Bit Performance
Fixed Cutter Bits
Dull bit grading is a standard method to grade the bit and it will also help personnel to understand bit performance better. However, it is sometimes very difficult to understand the meaning of the codes showing in the grading because you’ve never seen these dull characteristics before. Especially, there are new people who start to work in the oilfield therefore it is very critical to have them learn the correct way to do the bit grading in a short period of time.
is a method of determining optimum weight on bit and overall bit performance. A given amount of weight is put on the bit and the drawworks brake is tied off so that no more weight is applied to the bit as it drills. The time it takes for the bit to stop drilling ahead with the given amount of weight is measured. Different weights are applied and the times are compared to determine which amount is best.
The penetration rate that can be achieved by a bit has an inverse effect on the drilling cost per foot. The main factors that affect penetration rate are
- bit type
- formation properties
- drilling fluid properties
- bit weight and rotary speed
- bit hydraulics