Geomechanics · Module G3
Overburden and the Depth Frame
The vertical stress is the weight of all rock and fluid above a point, so it has to be integrated over true vertical depth from the correct reference surface. The module covers density integration, offshore water columns and air gaps, ways to fill the shallow section that was never logged, and the conversions between pressure, gradient and equivalent mud weight.
- Level
- Intermediate
- Position
- Module 3 of 8
- Video
- About 16 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
- Integrating bulk density over true vertical depth
- Air gap, water depth and the seabed
- Filling the shallow section above the first log
- Measured depth versus true vertical depth
- Hydrostatic gradients for fresh and saline water
- Equivalent mud weight in ppg, SG and kPa/m
Learning outcomes
After the module you should be able to:
- Build an overburden that starts at the right surface, on TVD, in the driller's units.
- Convert any pressure at depth into an equivalent mud weight
- Avoid counting the seawater column twice in an offshore well
A short preview
Equivalent mud weight states a pressure as the mud density that would exert it at the same true vertical depth: ppg equals psi divided by 0.052 times the depth in feet. A vertical stress of 9,360 psi at 10,000 ft TVD is 9,360 / 520 = 18.0 ppg. In a deviated well the same point might lie at 11,200 ft measured depth, and dividing by that depth instead gives 16.1 ppg, an error of almost two pounds per gallon that makes the overburden look weaker than it really is.
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.