CPG Drilling Operations & Well Logging 2 — Questions and Answers
Question 1: What is the primary purpose of a mud weight (density) window in well design?
- To maximize drilling speed
- To maintain wellbore pressure between pore pressure and fracture gradient (Correct answer)
- To reduce the cost of drilling fluids
- To cool the drill bit during rotation
Correct answer: To maintain wellbore pressure between pore pressure and fracture gradient
The mud weight window defines the safe range of drilling fluid density needed to prevent both wellbore influx (kick) and lost circulation (fracture).
Question 2: A gamma ray log reads 120 API units in a formation. This value most likely indicates:
- Clean sandstone reservoir
- Carbonate with dolomite
- Shale or clay-rich formation (Correct answer)
- Anhydrite layer
Correct answer: Shale or clay-rich formation
High gamma ray values (typically above 75–100 API) indicate shale or clay-rich formations due to the abundance of radioactive potassium, thorium, and uranium.
Question 3: During drilling, a 'kick' is best described as:
- Sudden increase in surface torque
- Unintended influx of formation fluids into the wellbore (Correct answer)
- Rapid bit wear in abrasive formations
- Pipe sticking due to differential pressure
Correct answer: Unintended influx of formation fluids into the wellbore
A kick occurs when formation fluid pressure exceeds the hydrostatic pressure of the drilling fluid column, causing reservoir fluids to enter the wellbore.
Question 4: The neutron porosity log measures porosity primarily by detecting:
- Electron density of the formation
- Thermal or epithermal neutrons slowed by hydrogen atoms (Correct answer)
- Bulk acoustic travel time
- Natural gamma radiation from the formation
Correct answer: Thermal or epithermal neutrons slowed by hydrogen atoms
The neutron log emits fast neutrons that are decelerated primarily by hydrogen; since hydrogen is concentrated in pore fluids, the tool responds to hydrogen index, which approximates porosity.
Question 5: What does a high standpipe pressure combined with zero flow-back indicate while drilling?
- Normal drilling conditions
- Washout in the drill string
- Plugged or washed-out bit nozzles
- Plugged bit or downhole tool restriction (Correct answer)
Correct answer: Plugged bit or downhole tool restriction
High standpipe pressure with zero surface flow-back suggests a plugged bit or severe downhole restriction preventing fluid circulation through the system.
Question 6: Which drilling parameter is monitored to detect a washout in the drill collar or bit?
- Sudden decrease in standpipe pressure (Correct answer)
- Increase in hook load
- Decrease in rotary speed (RPM)
- Increase in weight on bit (WOB)
Correct answer: Sudden decrease in standpipe pressure
A washout (erosion of the drill string or bit) creates an unintended bypass for fluid, resulting in a drop in standpipe pressure as less hydraulic resistance is encountered.
Question 7: The spontaneous potential (SP) log is most useful for:
- Measuring formation temperature
- Identifying permeable beds and estimating formation water salinity (Correct answer)
- Determining overburden stress
- Measuring acoustic velocity in shales
Correct answer: Identifying permeable beds and estimating formation water salinity
The SP log records the natural electrical potential between borehole fluid and formation water; deflections identify permeable beds, and the magnitude helps estimate Rw.
CPG Drilling Operations & Well Logging 2
What is the primary purpose of a mud weight (density) window in well design?