Failure Mechanisms in Deepwater Chalks: Rock Stability as Function of Pore Pressure and Water Saturation
Notice bibliographique
Résumé
Abstract This paper describes the mechanical behavior of unsupported open-holes (such as wellbores, perforations and cavities) in selected chalk samples from a European North Sea hydrocarbon reservoir. The main objective was to characterize cavity failure as a function of stress and fluid saturation. A series of compaction tests were performed on Thick-Wall Cylinders (TWC) of deeply buried North Sea Chalks, complemented with Uniaxial Compression Strength (UCS) tests. Comparison of oil vs. brine saturated specimens demonstrated water-induced "softening", drastic weakening, and then collapse of the cylindrical cavity. Images from X-Rays and thin-sections depict fracture-bands extending from the pilot-hole. These near-wellbore shear bands are interpreted to be pathways for enhanced permeability. The results and graphic images allow improved understanding of rock failure processes in chalks, as influenced by the varying degrees of water saturation and loading conditions. A poly-axial shear failure criterion is shown to predict TWC onset of instability. The knowledge gained on the failure mechanisms provides the exploitation engineer with scenarios on how flow-paths and well-inflows are affected by drawdown and water-floods. They also give diagnostics for declining productivity, which could lead to casing damage and wellbore failure. A field example is in wellbore acid fracturing, some of which lead to excessive liner deformations and even wellbore failures. Such downhole deformations are thought to be similar to those observed in these laboratory tests, mechanisms related to pore pressure depletion and/or stimulation. Acid-fracturing plans and models could be revised with the aid of processes, laboratory data, failure criteria, and results described here. Introduction Several oil fields in the North Sea have undergone periodic acid stimulation campaigns over the years. Typically, the results obtained after each acid fracturing treatment are excellent; several wells reported productivity index (PI) improvements ranging between 40 and 70. However, it has been commonly found that these results are short lived; the wells experience a severe decline in PI, and return to pretreatment production rates in as short as 1 year. Other problems have been observed in these fields; excessive casing deformation has been reported to occur in numerous wellbores. Previous field investigations have shown a correlation between casing deformation and perforation location. Casing failure was observed very near perforations subjected to acid-frac treatments. This suggests that large rock deformation, which causes casing failure, is a consequence of the acid stimulation process itself. The TWC laboratory program performed in this study provides some insight on the behavior of boreholes in chalks under stress.
Récupéré en direct depuis OpenAlex et désinversé. Les résumés ne sont pas conservés dans cette base de données : les index inversés représentent 8,6 Go des 9,3 Go de texte de la base, et le serveur dispose de 13 Go libres.
Comment cette classification a été obtenuedéplier
Prédiction machine sur la base complète
Imitation des enseignantsNi prévalence calibrée, ni vérité terrain. Validation humaine à venir. Le volet Gemma est une étiquette directe du modèle pour chaque travail de la base, lue sur la notice réduite au titre. Le volet Codex est un classifieur appris des 10 348 étiquettes directes de Codex et calibré sur les taux pondérés de l'échantillon; les champs sans appui suffisant ne portent aucun appel Codex. Le mode candidate est l'union des deux volets; le consensus est leur intersection. Ces sorties portent le statut machine_predicted_unvalidated et ne sont pas des étiquettes humaines.
Scores du classifieur distillé par catégorie (deux têtes)
| Catégorie | Codex | Gemma |
|---|---|---|
| Métarecherche | 0,000 | 0,000 |
| Méta-épidémiologie (sens strict) | 0,000 | 0,000 |
| Méta-épidémiologie (sens large) | 0,000 | 0,000 |
| Bibliométrie | 0,001 | 0,000 |
| Études des sciences et des technologies | 0,000 | 0,000 |
| Communication savante | 0,000 | 0,000 |
| Science ouverte | 0,000 | 0,000 |
| Intégrité de la recherche | 0,000 | 0,000 |
| Charge utile insuffisante (le modèle a refusé de juger) | 0,001 | 0,000 |
Scores machine (provisoires)
Les deux têtes enseignantes du modèle étudiant, lues sur ce travail. Un score ordonne la base pour la relecture; il n'affirme jamais une catégorie, et le statut de validation accompagne chaque rangée tel quel.
Scores de référence d'un modèle non mature (critères de maturité non atteints, 7 itérations). Un score ordonne; il n'affirme jamais une catégorie.
score_only:v0-immature-baseline · tel quel depuis la passe de notation : score_only signifie que le nombre peut ordonner les travaux, et qu'aucune étiquette de catégorie n'en découleClassification
machine, non validéePrédiction automatique; un appel candidat d’une seule source (Gemma direct ou Codex distillé), pas un consensus.
Le détail, modèle par modèle et score par score, se trouve en fin de page sous « Comment cette classification a été obtenue ».