Multiscale Insights Into Borehole Instabilities in High‐Porosity Sandstones
Bibliographic record
Abstract
Abstract This paper presents a multiscale analysis of the classical borehole instability problem in high‐porosity sandstones using a hierarchical multiscale approach. A rigorous, two‐way message‐passing coupling of finite element method and discrete element method is employed, where the finite element method is employed to solve a boundary value problem and the constitutive material responses required at each of its integration points are derived by the discrete element method solution to an embedded representative volume element instead of by using an assumed phenomenological model. We employ this multiscale approach to examine the successive failure of a borehole subjected to gradually decreased support stresses at the inner borehole wall or increased far‐field stresses. To reproduce the material behavior of high‐porosity sandstone, representative volume elements with a high‐porosity structure and interparticle bonds are generated. It is found that stress concentration triggers the initial failure at the borehole wall, and the subsequent failure in form of deformation bands is driven by further stress concentration ahead of the band tips. The failure pattern around the borehole varies with initial stress state and global loading path. The comparison of the local stress paths of the initial failure points in various cases reveals that the change of failure pattern from shear failure to mixed‐mode failure and to compaction failure is dominated by the increased mean stress. Cross‐scale parametric studies show that the failure mode changes from clear compaction bands to multiple arrays of shear bands by changing the high‐porosity specimen to a low‐porosity one. The increase in cohesion strength expands the yield locus and enlarges the critical mean stress between different failure patterns. Hence, the failure mode may change from compaction failure to mixed‐mode failure or from mixed‐mode failure to shear failure due purely to the increased cohesion strength. The qualitative comparisons indicate greater length, larger area, and more severe damage of the diametrically opposite failure with the increase in minor principal stress σ 0 and principal stress ratio. The diametrically opposite failure mode is found a good indicator for σ 0 direction, but it may also occur under hydrostatic far‐field stress due to material anisotropy.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.001 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.001 |
| Insufficient payload (model declined to judge) | 0.000 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one teacher head, not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".