Petroleum drilling basics: A rotary oil well typically has a main boring (hole) diameter of about 20–30 cm. Approximately how deep are such commercial oil wells commonly drilled (order of magnitude in kilometres)?

Introduction:
Petroleum wells are drilled with relatively modest bore diameters compared to their great depths. The question asks for a realistic order-of-magnitude depth in kilometres for commercial oil wells having a main boring diameter of roughly 20–30 cm.

Given Data / Assumptions:

• Main boring (hole) diameter: 20–30 cm.
• We seek the common depth range of commercial wells in km.
• Order-of-magnitude answer is sufficient.

Concept / Approach:
Commercial onshore wells frequently range from about 1 km to several kilometres in depth depending on geology, reservoir pressure, and target formation. Ultra-deep wells exist, but are not representative of typical field development wells. We therefore look for a range that reflects common practice rather than extremes.

Step-by-Step Solution:

Verification / Alternative check:
Many sedimentary basins host productive formations between ~1.5 km and ~3.5 km. Shallow (0.1–0.5 km) is too small; 7.5–12.5 km or 15–20 km represent extreme or impractical depths for most fields.

Why Other Options Are Wrong:

• 0.1 to 0.5: Too shallow for most commercial oil reservoirs.
• 7.5 to 12.5: Ultra-deep and atypical for routine development.
• 15 to 20: Unrealistic for standard oilfield practice.
• 0.5 to 1.0: Still too shallow for most commercial reservoirs.

Common Pitfalls:
Confusing record-setting “deepest wells” with normal development depths; assuming larger diameter always implies shallower depth.

Final Answer:
1.5 to 4.5