Factors in the rock mass that influence the borehole stability

Further relevant factors

Stresses around a borehole

For a borehole aligned parallel to an in situ principal effective stress the normal stresses perpendicular and parallel to the borehole alignment are identical with the effective principal stresses in the rock mass and there are no shear stresses in that direction.

For a boreholealignment deviated from the orientations of the in situ principal effective stresses results in different magnitudes and orientations of the normal stresses in relation to the borehole and in shear stresses parallel to the borehole axis.

A modified borehole orientation can result in a different failure mechanism.

3D Stress configuration and strength (GLAST)

Description of the load dependent strength with the Mohr-Coulomb criterion

Compression test ()

→ too pessimistic

Extension test ()

→ too optimistic

Description of the load dependent strength with the Tauber criterion

Collation of the strengths under any 3D stress states in an envelope surface (characterized with only three parameters).
The strengths used to define the failure envelope are determined in compression & extension tests.

Use of the “classical“ Mohr-Coulomb rock strength criterion for borehole stability analyses can result in considerable reduction of the reliability assessment; the Tauber failure criterion defines the strength under 3D stress conditions (GLAST), which largely excludes the uncertainty in many current assessment approaches.

Identifying zones of instabilities

The relative positions of the failure envelope and the current 3D stress state define the Safety Factor.

A Safety Factor ≤ 1 shows that the strength has been reached or exceeded and this could mean that macroscopic failure/sanding occurs in that region.

The overall stability assessment around the borehole is illustrated with a colour corresponding to the Safety Factor. This shows the radial and tangential extent of unstable regions.

Special evaluations of the stress conditions around the borehole wall, and of their 3D orientations, are used to assess the type of failure.

Common types of failure

Influence of planes of weakness on the borehole stability

Borehole in undisturbed rocks

Borehole parallel to a plane of weakness

Borehole trajectories closed to or parallel to the orientation of zones/planes of weakness increase the risk of instabilities.

Causes of failure in deviated boreholes

Trajectory & failure intensity

Trajectory & shear stresses

Instabilities in boreholes deviated from a principal stress axis can result from critical shear stresses parallel to the borehole axis, especially in the direction of the largest difference between the maximum and minimum far field stress components.

Effects of shear stresses parallel to the borehole axis

Critical shear stresses parallel to the borehole wall result in breakout & peeling.

A critical normal stress deviator is not necessarily required.

3D in situ stress state and borehole stability

Spatial effective stress magnitudes (dark blue → min, dark red → max)

Failure intensity in the near borehole zone (dark blue → min, dark red → max)

Different intermediate principal stress magnitudes, even with unchanged maximum and minimum stress components, can lead to very different failure intensities.

Failure affected by MOPP⁺ (see 3D Biot coefficient)

Initial pore pressure effectiveness

Initial borehole stability

MOPP⁺ & pore pressure effectiveness

MOPP⁺ & borehole stability

Loosening, particularly in low porosity rocks, increases the pore pressure effectiveness (MOPP⁺), and thus changes the effective stresses. This can lead to a time-dependent propagation of failure and to increasing borehole instabilities..

Optimization of drilling parameters (e.g. mud pressure)

low

optimal

high

very high

Stress in the borehole wall (red - radial stress, green - tangential stress , black - paraxial stress)

Stability around the borehole (red - Zone of instability)

With an appropriate collation of individual stability analysis results, further analyses can be made of the trend of the development of instability (for example resulting from varying mud pressure, borehole alignment, etc.).

Summary of borehole stability

The borehole stability is (in addition to technical and technological parameters) affected by rock mechanical boundary conditions. They have very different, partly opposing effects with respect to the risk of failure. But this can be evaluated with expert rock mechanics analyses, and can therefore be mitigated.