To Fluid Substitute or Not in Geomechanical Workflows: Experimental Evidence
Bibliographic record
Abstract
Accurate static elastic properties are essential for reliable geomechanical modeling. However, dynamic log measurements are often compromised by the ‘gas effect’, which lowers dynamic moduli and introduces uncertainty. This study evaluates the true impact of fluid saturation on both static and dynamic elastic properties, using matched dry and mineral oil saturated sample pairs from five representative lithologies (Indiana Limestone, Austin Chalk, Berea Sandstone, Berea Buff Sandstone and Nugget Sandstone). Experimental procedures included cycled triaxial compression tests and ultrasonic velocity measurements. Results show a clear frequency-dependent divergence, with dynamic Young's modulus averaging 1.83 (dry) to 1.97 (saturated) times greater than its static counterpart. Critically, static Young’s modulus and peak strength exhibited negligible variation (less than 1.2%) between dry and saturated conditions using mineral oil, confirming that fluid saturation does not affect the intrinsic static properties of the rock. The Gassmann model’s assumption of fluid-independence for the shear modulus held for most lithologies (difference below 3%), supporting its routine application for log-to-core fluid substitutions. However, caution is advised for formations with complex mineralogy with Berea Sandstone showing a significant Gassmann error of –11.6%, and Austin Chalk of –5.1%. These findings provide the first direct experimental evidence that the apparent 'gas effect' is dynamic and not static and which affirmed fluid substitution as a necessary, standard procedure in geomechanical workflows.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.008 | 0.025 |
| Meta-epidemiology (narrow) | 0.001 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.001 |
| Science and technology studies | 0.001 | 0.003 |
| Scholarly communication | 0.002 | 0.003 |
| Open science | 0.002 | 0.002 |
| Research integrity | 0.001 | 0.001 |
| Insufficient payload (model declined to judge) | 0.003 | 0.001 |
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 source (direct Gemma or distilled Codex), 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".