Question

Which of the following is the role of stabilizer subs in the rotary drilling process?

• A. To prevent the drill-string from falling into the wellbore
• B. To keep casing equidistant from the wellbore wall
• C. To assist in keeping the drill collars centralized
• D. To isolate the wellbore fluid from the subsurface formations

Hint:

Stabilizer subs keep heavy drill collars centered in the wellbore.

Answer 1

The Correct Option is C

Step 1: Function of stabilizer subs:
Stabilizer subs are short, heavy walled sections with blades or ribs that are placed in the bottom hole assembly, mainly between and around the drill collars. Their main job is to hold the drill collars centered inside the wellbore so that the collars do not touch the borehole wall unevenly.
Step 2: Checking option A:
Option A says stabilizer subs prevent the drill-string from falling into the wellbore. This is actually the function of a slip or an elevator used at surface, not a stabilizer sub, so option A is incorrect.
Step 3: Checking option B:
Option B talks about keeping casing equidistant from the wellbore wall. That job is performed by casing centralizers, which are a different tool used during casing running, not by stabilizer subs which work on the drill string while drilling. So option B is incorrect.
Step 4: Checking option C:
Option C states that stabilizer subs assist in keeping the drill collars centralized. This matches the actual working principle of stabilizer subs. By keeping the collars centered, they control the bit tilt, reduce bending and vibration of the BHA, help control hole angle and direction, and reduce the chances of the collars getting stuck against the wellbore wall. This is the correct description.
Step 5: Checking option D:
Option D says stabilizer subs isolate wellbore fluid from subsurface formations. That isolation job belongs to casing and cement, not to a stabilizer sub, so option D is incorrect.
Final Answer:
\[ oxed{ ext{(C) To assist in keeping the drill collars centralized}} \]