Geomechanics · Module G7
Wellbore Stability — the Mud Weight Window
Drilling a hole concentrates the far-field stresses at its wall. This module works out the hoop, radial and axial stresses around a vertical well, applies shear failure criteria to find the collapse pressure and a tensile criterion to find breakdown, and turns the resulting physical window into an operating one by allowing for kick, swab and surge margins.
- Level
- Intermediate
- Position
- Module 7 of 8
- Video
- About 17 minutes, narrated
- Format
- Narrated video lesson with captions and slide reading view; with the certified option, a quiz whose pass unlocks the next module.
What the module covers
- Kirsch stresses at the borehole wall
- Mohr–Coulomb and Mogi–Coulomb failure
- Collapse pressure and a tolerated breakout width
- Tensile breakdown and lost circulation
- From physical window to operating window
- Reading a mud weight window plot
Learning outcomes
After the module you should be able to:
- Turn the stress model into a physical and an operational mud weight window.
- Work out the hoop stress around a vertical borehole by hand
- Apply margins that turn a physical window into an operating one
A short preview
Around a vertical hole the hoop stress peaks in the direction of the minimum horizontal stress, where it equals three times the maximum horizontal stress, minus the minimum one, minus the mud pressure. With horizontal stresses of 8,000 and 6,500 psi and 5,200 psi of mud, the wall carries 24,000 − 6,500 − 5,200 = 12,300 psi. Each extra half pound per gallon at 10,000 ft adds 260 psi of mud pressure and takes the same amount off that peak, which is how heavier mud holds a weak hole open, until the wall at right angles starts to fail in tension.
Who it is for
Drilling, completion and reservoir engineers, petrophysicists and geoscientists who build or review pore pressure and wellbore stability predictions for well planning.
Prerequisites: None required; working familiarity with well logs helps. The hands-on steps use the Geomechanics workspace, available from the Professional plan.
Take the module in the Learning Hub
Geomechanics: Specialty course, sold separately: eight modules that build a calibrated 1D mechanical earth model from logs, from elastic properties and pore pressure to the mud weight window and its range. The lesson opens in the Learning Hub once the track is purchased; graded quizzes and the certificate come with the certified option.