Geomechanics · Module G6
Rock Strength and Horizontal Stresses
The strength of the rock and the two horizontal stresses decide whether a borehole stays in gauge. The module covers log-based strength correlations and how to choose between them, the poroelastic equations that add tectonic strain to the effects of overburden and pore pressure, the difference between minimum stress, fracture gradient and leak-off pressure, and the frictional limits that make a stress state admissible.
- Level
- Intermediate
- Position
- Module 6 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
- Compressive strength, friction angle and tensile strength
- Choosing a strength correlation by lithology
- Poroelastic horizontal stresses with tectonic strain
- Fracture gradient from Poisson's ratio
- The frictional stress polygon
- Setting parameters zone by zone
Learning outcomes
After the module you should be able to:
- Produce strength and horizontal stresses that sit inside the stress polygon.
- Compute the minimum and maximum horizontal stress at one depth
- Test whether a stress state is physically admissible
A short preview
Friction on existing faults caps how different the principal stresses can become. With a friction coefficient of 0.6, the ratio of the biggest to the smallest effective stress cannot exceed about 3.1. At 10,000 ft with 10,000 psi of overburden and 4,650 psi of pore pressure, the effective vertical stress is 5,350 psi, so in a normal-faulting setting the minimum horizontal stress cannot drop below roughly 4,650 + 5,350 / 3.1 ≈ 6,370 psi. A model that predicts less is describing rock that would already be slipping on its faults.
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.