Drilling Underbalanced From A Floating Unit In A 1,500-m Of Water Depth Exploratory Well: Planning, Equipment, And Rig Modifications
Notice bibliographique
Résumé
Abstract The present study describes the steps taken to drill the final section of an exploratory well in Santos Basin in the underbalanced (UB) condition, in 1,500-m (4921-ft) of water depth. The first well in the area reached a fractured carbonate reservoir in which massive losses of circulation caused so many operational difficulties that the formation evaluation was not conclusive. As consequence, in the third quarter of 2005, another well will be drilled, reaching the same reservoir in the UB condition. Pre-planning and planning activities, additional required equipment, riser modifications, and new procedures are presented and discussed. Introduction Despite the fact that underbalanced drilling (UBD) has been applied for many years in the oil industry, implementing this technique from floating units is still a challenge. Several thousands of wells have been drilled underbalanced on world basis and more than 4,000 wells in Canada alone1, but almost exclusively restricted to onshore fields. Literature reports some offshore experiences that are limited to fixed platforms or jack-up rigs1–4. Operational difficulties associated to safety, logistics, and equipment placement have been keeping UBD aside from floating rigs. At the end of 2000, Petrobras, leading a JIP with other operators, service providers and consultants, overcame an important barrier with a well in Albacora Field (297-m or 974-ft of water depth) being drilled with aerated mud from the semi-submersible rig Petrobras 17. In spite of not drilling underbalanced, the principles of operation were close because a two-phase mixture composed of nitrogen and water based mud was pumped down through the drillstring and returned to surface under pressure using a rotating control head on the top of the riser, which was decoupled from the diverter assembly, and the return taken through hoses back to a typical UBD surface system5. After reaching that milestone, Petrobras started the development of technology to achieve the dual gradient drilling (DGD) condition by pumping gas at the bottom of marine riser in ultra-deep water. Differently from the mechanical lifting systems6, the use of gas consists of diluting the mud returns at the seafloor. From the point of the gas injection upward to the surface, the density of the drilling fluid is lighter than the effective mud density below the seafloor7. In ultra deep-water, the DGD condition provides an effective approach for managing the challenges associated with the narrow operational margin between the curves of pore pressure and formation fracture. For illustrating this capability, Fig. 1 presents two distinct curves of equivalent circulating density (ECD). The red curve represents the conventional single gradient drilling (SGD) method and the blue one stands for the DGD. In this example, which is based on an exploratory case from Capos Basin, in Brazil, the conventional SGD method needs a 9.2 lbm/gal water based mud (WBM) for reaching the final depth of the 12–1/4" section of the well at 4,200 m. Because of the friction pressure losses, the bottom hole ECD is slightly lower than 9.4 lbm/gal.
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Prédiction distillée sur la base complète
Imitation des enseignantsNi prévalence calibrée, ni vérité terrain. Validation humaine à venir. Apprise à partir de 10 348 étiquettes directes de Codex et de 10 348 étiquettes directes de Gemma. Le mode candidate est l'union des têtes enseignantes seuillées; le consensus est leur intersection. Ces sorties portent le statut machine_predicted_unvalidated et ne sont ni des étiquettes humaines ni des étiquettes directes de modèles de pointe.
Scores Codex et Gemma par catégorie
| 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 | 0,000 | 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) | 0,000 | 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 tête enseignante, 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 ».