Drilling Engineering: Main Problems and Solutions

Lesson Summary

A quick look at common drilling problems like sticking and lost circulation, and how to handle them.

This lesson is part of the Drilling & Completion track and lasts 1:40 minutes. Watch the video first, then review the key points and test your understanding with the questions below.

Key Points

1. 1Differential Sticking
2. 2Lost Circulation
3. 3Wellbore Instability

Written Lesson

Why do Drilling Problems matter?

Rig time is very expensive, and most Non-Productive Time (NPT) comes from predictable recurring problems: stuck pipe, lost circulation, wellbore instability, and poor hole cleaning. These problems are interrelated: an unstable hole fills with cuttings, cuttings cause sticking, and attempting to free with high pump pressure may fracture the formation, leading to losses.

Differential Sticking

This occurs when part of the drill string, usually the Drill Collars, rests against the Filter Cake across a permeable zone, and the mud pressure in the well is higher than the formation pressure. The pressure differential pushes the pipe against the wall and holds it as if stuck by suction.

Distinctive signs: it occurs after pipe movement stops, such as during a connection or survey; mud can be circulated normally, but the pipe cannot be moved or rotated.

F (lbf) ≈ ΔP (psi) × Contact area (in²) × Friction factor

Calculated Example: Pressure differential between mud and formation is 500 psi, contact length is 20 ft (i.e., 240 in) with an effective contact width of 2 in, giving an area of 240 × 2 = 480 in². The normal force is 500 × 480 = 240,000 lbf, and with a friction coefficient of 0.2, the required additional overpull is about 48,000 lbf. This is why tension alone may not suffice, and we resort to reducing the differential pressure, pumping lubricants, or freeing with a Jar.

Prevention: Keep the pipe moving, do not increase mud weight beyond necessity, maintain a thin and low-permeability filter cake, and use spiral Drill Collars or Stabilizers to minimize contact area.

Mechanical Sticking

• Pack-off: Accumulation of cuttings or cavings around the BHA; indicated by rising pump pressure with difficulty in rotation and circulation.
• Key Seat: Pipe wears a groove into the wall of a curved section, where the wider BHA wedges during pulling.
• Hole Geometry: Undergauge hole due to a worn bit, or plastic formations like salt and swelling shales that narrow the hole over time.
• Foreign objects falling into the well (Junk) or unset cement.

Lost Circulation

This occurs when mud pressure in the well exceeds the formation's strength, or when the formation is cavernous or naturally fractured. Losses are usually classified as Seepage, partial, or total (no mud returns to surface).

The mud weight window is bounded by pore pressure at the bottom and fracture pressure at the top. Crucially, the actual pressure during circulation is the ECD, not just the static mud weight.

ECD (ppg) = MW + APL ÷ (0.052 × TVD)

Calculated Example: 11.0 ppg mud at 8,000 ft TVD, and Annulus friction pressure of 250 psi. ECD = 11.0 + 250 ÷ (0.052 × 8,000) = 11.0 + 250 ÷ 416 ≈ 11.6 ppg. If the fracture gradient of the weakest exposed zone is equivalent to 11.5 ppg, the well will take mud during circulation, even though the static mud weight is below the fracture limit.

Treatment starts by reducing pump rate and friction pressure, then using Lost Circulation Materials (LCM) of appropriate sizes, followed by cement plugs or special materials for major losses. With every loss, monitor the well level as its drop can cause a Kick.

Wellbore Instability

• Chemical: Shale layers absorb water from water-based mud, swelling and disintegrating; the solution is Inhibitive mud or oil-based mud.
• Mechanical: Mud weight too low to support the wall causing collapse, or too high causing fracturing.
• Signs: Large splintered cavings on the shakers, increased torque and drag, and the need for Reaming while running in hole.

Hole Cleaning

Cuttings that are not removed accumulate, especially in deviated sections where they form a bed on the low side of the hole. Cleaning efficiency depends on pump rate, mud properties, pipe rotation, and circulation time before trips. Monitoring the volume of cuttings on the shakers compared to the rate of penetration is one of the simplest early detection tools.

The figures and examples in this lesson are for educational purposes. On the job, always follow your company procedures and accredited training center instructions.

Lesson Glossary