Petroleum Geomechanics

Instructor: Prof. Richard R. Hillis


Geologists, geophysicists, reservoir engineers, exploration supervisors and managers working with overpressured, naturally fractured or tight gas reservoirs, or reservoirs that will be subject to fracture stimulation or water-flooding operations. Explorationists working in areas of active tectonism, or concerned with seal integrity. Engineers concerned with deviated and horizontal wellbore stability.


Contemporary, or in situ crustal stresses affect a range of petroleum exploration and development-related issues from the occurrence of structural permeability (desirable in tight reservoirs, but undesirable in seals) to wellbore stability. The course will promote an integrated understanding of the origin of the contemporary stresses within the earth's crust, and of the wide-ranging implications of contemporary stress data. The application of this understanding is applied to such questions as: "how can I predict dynamic seal integrity prior to drilling?"; "how can I predict zones of enhanced structural permeability in reservoirs?"; "why deplete an overpressured reservoir before fracture stimulation?"; "why should injector wells not be aligned in the maximum horizontal stress direction?"; "when are horizontal wellbores more stable than vertical ones?", and; "in what direction should a well be deviated in a naturally fractured reservoir?" Case studies will be presented from the North Sea, Australasia and SE Asia. The topic requires a multidisciplinary, team approach to problem-solving with input from geology, geophysics and engineering. A basic knowledge of geology, geophysics and drilling practices is assumed. Individuals from any of these areas will benefit from exposure to the multidisciplinary nature of the topic. All the topics are addressed in lecture format and re-inforced with practical exercises.



Origin and significance of crustal stresses

2. Basic Rock Mechanics

Stress and strain, stresses and forces, resolving stresses and forces on a plane, Mohr's circle of stress, states of stress, modes of rock deformation

3. Basic Fault Mechanics

Friction, cohesion, Coulomb failure, Mohr failure, Griffith failure, fault/fracture meshes, Andersonian classification of faults

4. Stress in the Crust

Plate tectonics, regional stresses, local stresses, stress rotations, World Stress Map, examples of stress studies from Western Europe/North Sea, Australasia and SE Asia, reference state of stress, tectonic stress.

5. Determining Contemporary Stresses from Oilfield Data

The stress tensor, principal stresses, units (densities, gradients and pressures), overburden stress, borehole breakouts, tensile fractures, vertical and deviated wells, formation integrity, leak-off and hydraulic fracture tests, fracture gradient relations, frictional limits on stress.

6. Pore Pressures

Pore pressure, hydrostatic gradients, overpressure, effective stress, development of overpressure, types of overpressure, pore pressure/stress coupling.

7. Applications of Contemporary Stress Data

Deviated and horizontal wellbore stability (stable drilling directions and mud weights), fracture stimulation (problems of stimulation in heterogeneous and overpressured formations, fracture twisting and the importance of depletion), structural permeability and natural fluid flow in the sub-surface (dynamically sealing faults and fracture permeability), EOR planning (injection and production well positioning), recent tectonic style.


Five days



Course notes are provided. A computer with access to Excel software is required.