CWD Well Drilling Techniques & Equipment 2 — Questions and Answers
Question 1: What is the primary purpose of drilling fluid (mud) in rotary drilling?
- To lubricate and cool the bit, remove cuttings, and stabilize the borehole (Correct answer)
- To dissolve rock formations and speed penetration
- To chemically treat groundwater before sampling
- To seal off artesian pressure zones permanently
Correct answer: To lubricate and cool the bit, remove cuttings, and stabilize the borehole
Drilling fluid simultaneously cools and lubricates the bit, carries cuttings to the surface, and supports borehole walls to prevent collapse.
Question 2: Which drill bit type is best suited for drilling through very hard, consolidated rock formations?
- Drag bit
- Tricone roller cone bit (Correct answer)
- Fishtail bit
- Spudder bit
Correct answer: Tricone roller cone bit
Tricone roller cone bits use three rotating cones with hardened teeth or inserts to crush hard rock formations effectively.
Question 3: In cable tool (percussion) drilling, what component delivers the impact energy to break rock?
- Swivel joint
- Drill stem and bit assembly (Correct answer)
- Kelly bushing
- Annular preventer
Correct answer: Drill stem and bit assembly
In cable tool drilling, the drill stem and bit assembly is repeatedly lifted and dropped to pulverize rock through percussion.
Question 4: What does 'weight on bit' (WOB) primarily control during rotary drilling?
- The rotational speed of the drill string
- The rate of penetration and bit wear (Correct answer)
- The viscosity of the drilling fluid
- The annular velocity of cuttings
Correct answer: The rate of penetration and bit wear
Weight on bit is the downward force applied to the bit; too little slows penetration and too much accelerates bit wear or causes deviation.
Question 5: When drilling with air rotary methods, what is the minimum air velocity typically required in the annulus to lift cuttings to the surface?
- 500 ft/min
- 1,000 ft/min
- 3,000 ft/min (Correct answer)
- 5,500 ft/min
Correct answer: 3,000 ft/min
An annular air velocity of approximately 3,000 ft/min is generally required to efficiently lift cuttings out of the borehole.
Question 6: What is the function of a stabilizer in a rotary drill string?
- To increase drilling fluid density
- To prevent borehole deviation and centralize the bit (Correct answer)
- To measure formation water temperature
- To connect the kelly to the drill pipe
Correct answer: To prevent borehole deviation and centralize the bit
Stabilizers are placed in the bottom-hole assembly to centralize the drill string and minimize borehole deviation.
Question 7: Which situation most commonly requires the use of a downhole hammer (DTH) in air drilling?
- Drilling through unconsolidated sand and gravel
- Penetrating very hard, abrasive rock formations (Correct answer)
- Installing surface casing in soft clay
- Developing a newly completed well
Correct answer: Penetrating very hard, abrasive rock formations
Downhole hammer (DTH) drilling delivers percussive blows directly at the bit face, making it highly effective in very hard, abrasive rock.
CWD Well Drilling Techniques & Equipment 2
What is the primary purpose of drilling fluid (mud) in rotary drilling?