Drill String Definition
We can define the drill string as it is a group of pipes and tools. These components allow us to deliver the rotation, the weight and the fluids from surface to the bottom of the drilled hole. We can say that the drill string is an important part of the rotary drilling process in oil and gas wells.To understand more about the drill string we will have to go through two subjects :
- Drill string Components
- Drill String Design
Drill String FunctionsDrill string components serves several purposes and functions which includes the following:
- The drill string Provides a fluid conduit from the drilling rig to the Drilling Bits.
- Also drill string had another function which is to impart rotary motion to the Drilling Bits.
- Because of the existence of drill collars in the drill string, it will provide Weight On Bit (WOB) necessary to drill the hole.
- Lower and raise the Drilling Bits in the well.
- Drill String functions is a lot more but it depends about what do you need to do like it can allow formation evaluation and testing but this will be done by special tools for such application.
Drill string components
Drill string components falls under there main topics:
Drill String Component | Bottom Hole Assembly:The Bottom Hole Assembly (BHA) is one of the drill string components which consist of a Drilling Bit, which is used to break up the rock formations; Drill Collars, which are heavy enough to apply weight to the drill bit; drilling Stabilizers, which centralize the drill string in the hole; Reamers, Which is used to enlarge the drilled hole for certain reasons; Jars, which used to free any stuck may happen during drilling; Shock subs, which used to reduce the vertical oscillation (bouncing) of the drill string. The Bottom Hole Assembly may also contain other components such as a downhole mud motor and rotary steerable system (RSS), measurement while drilling (MWD), and logging while drilling (LWD) tools. The components are joined together using rugged threaded connections. Short "subs" are used to connect items with dissimilar threads.
Drill String Component | Transition Pipes:
This drill string component is mainly heavy weight drill pipe (HWDP). In the past, the two main components of the Drill String consisted of the Drill Pipe and the Drill Collars. The point where the relatively small OD and flexible Drill Pipe connects to the large stiff Drill Collars is called the Transition Zone.Oilfield studies have shown that almost all of the Drill Pipe fatigue failures are the result of an accumulation of fatigue damage occurring when the Drill Pipe joints were run in the Transition Zone, or were stressed above the endurance limit in crooked holes. Down hole data has indicated that the large change in diameter at the transition zone caused accelerated fatigue damage as a result of the concentration of cyclic bending stress reversals in the bottom joints of the flexible Drill Pipe, since the stiff Drill Collars bend very little from these stress reversals.
Drill String Component | Drill Pipes:
The longest component of the Drill String is the Drill Pipe. Each joint of drill pipe consists of the tube body and the Tool Joint (connection). Drill pipe joints are available in three length ranges:
Range Length, ft
Drill Pipes Range 2 is the most common.
Drill pipe component is generally described by tube OD, nominal weight, pipe grade, type tool joint, thread connection and classification. For example, a 5" drill pipe can be described as,
5'' 19.5 Grade ''E'' XH NC50 Premium
tube OD Nominal Minimum Upset Connection Classification
Weight Yield strength Tool Joint thread (wear)
Each description item is explained in API DRILL PIPES DESCRIPTION & SPECS Article.
The drill string components should be designed to deliver sufficient weight to the Drilling Bit and provide sufficient torsional and tensile strength to withstand the vigorous and dynamic conditions of drilling. The drill string components should also withstand burst and collapse pressure loads and be designed to minimize hole stability problems. There are many factors that must be considered in the design of the drill string such as:
- Total depth
- Hole size
- Mud weight
- Over pull
- Bottom Hole Assembly
- Hole angle
- Pipe weights and grades
- Corrosive environment
- Ability to fish tools out of hole
If you would like to have more details about the design you will find it in the DRILL STRING DESIGN Article.