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A mechanistic model is formulated to predict the mixture behavior for upward two-phase flow in concentric annulus. The model is composed of a procedure for flow pattern prediction and a set of independent mechanistic models for calculating gas fraction and pressure drop in bubble, dispersed bubble, slug and annular flow. Small-scale experimental data from the literature validate the predictions of the model. A full-scale experimental investigation is also executed to complete the evaluation. The experiments are performed in a 1278 m vertical well in the Petrobras research facility in Taquipe, Brazil, with an 88.9 mm×159. 4 mm (3.5 in. ×6.276 in.) annulus. Test matrix covered the whole range of possible combinations of liquid and gas injection rates for an underbalanced drilling operation in a similar geometry. The overall model performance is in good agreement with the experimental data. Introduction Upward two-phase flow through an annular channel is encountered in distinct applications such as heat exchangers, power plants and production of oil and gas. However, the amount of industrial use is not reflected in research efforts. In fact, literature presents a very small number of studies related to it1–6. In the past, the interest of the oil industry for this subject was restricted to some high productivity wells flowing through the casing-tubing annulus2. In addition, some studies were motivated by oil wells lifted by sucker rod pumps5. Recently, it is gaining more relevance as grows the popularity of the underbalanced drilling technology. Considering that accurate prediction of downhole pressure is a key factor for a successful UBD operation7, the knowledge of the two-phase flow through annuli becomes more relevant. Because of the complex nature of the problem, most of the calculation approaches in current practice for UBD are based on empirical methods. As a result, the possibilities of use are restricted to particular conditions without well-defined borders8. In this scenario, similarly to the trend observed in two-phase flow in pipes, the application of mechanistic models is supposed to be the natural way for improvement. The mechanistic or phenomenological approach postulates the existence of different flow configurations and formulates separated models for each one of these flow patterns to predict the main parameters as gas fraction and pressure drop. Since the basic laws of fluid mechanics are behind the development, the results can be extended to conditions different than those used for the development. Literature Review Sadatomi et al.1 performed experiments in a 15 mm×30 mm (0.59 in. ×1.18 in.) annulus and evaluated bubble rise velocities. They also utilized the Lockhart & Martinelli relationship9 for studying pressure drops. However, their investigation did not cover all flow configurations. Caetano2 developed a mechanistic model for dealing with vertical upward two-phase flow in concentric and eccentric annulus. He also performed a comprehensive experimental investigation in a 42.2 mm×72.6 mm (1.66 in. ×3 in.) annular space using air-water and air-kerosene. This was an extensive study, but work is still needed for improvements. The sub-model for annular flow regime, for instance, tended to overestimate the total pressure gradient. As an example, the model predicted the total pressure gradients 66% higher in average than the measured values for the air-kerosene mixture. Kellessidis and Dukler3 investigated the flow pattern map for upward two-phase flow. They also performed experimental tests in a 50.8 mm×76.2 mm (2 in. ×3 in.) annular channel, although the study was limited to flow pattern definition. Literature Review Sadatomi et al.1 performed experiments in a 15 mm×30 mm (0.59 in. ×1.18 in.) annulus and evaluated bubble rise velocities. They also utilized the Lockhart & Martinelli relationship9 for studying pressure drops. However, their investigation did not cover all flow configurations. Caetano2 developed a mechanistic model for dealing with vertical upward two-phase flow in concentric and eccentric annulus. He also performed a comprehensive experimental investigation in a 42.2 mm×72.6 mm (1.66 in. ×3 in.) annular space using air-water and air-kerosene. This was an extensive study, but work is still needed for improvements. The sub-model for annular flow regime, for instance, tended to overestimate the total pressure gradient. As an example, the model predicted the total pressure gradients 66% higher in average than the measured values for the air-kerosene mixture. Kellessidis and Dukler3 investigated the flow pattern map for upward two-phase flow. They also performed experimental tests in a 50.8 mm×76.2 mm (2 in. ×3 in.) annular channel, although the study was limited to flow pattern definition.

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Pre-salt carbonate reservoirs from Santos Basin represent a great opportunity and probably the most important recent oil discovery. Tupi area (estimated to have recoverable volume of 5 to 8 bboe), which is the most known amongst several other leads in the cluster, is going to be a great insight for the production project. From the production point of view, this new frontier has technological challenges that are being addressed by PETROBRAS and partners. Special attention is being dedicated to anticipate solutions to potential problems. This will require a balance between innovative and field proven solutions. This paper addresses the most critical points, where Petrobras is making a great R&D effort, which are: Well technology, where casing stability, well cost and productivity are addressed, in a scenario of water depths beyond 2,200m, target depths greater than 5,000m, crossing salt layers that can reach 2,000m in thickness. Poorly known microbial carbonate reservoir, heterogeneous in vertical profile, spread over very large areas, with wettability concerns, requiring careful evaluation of the performance of the waterflooding method and EOR. Wax deposition, due to low temperature in the ocean bottom, imposing limitations to the subsea layout. Gas processing and exporting technologies concerning environmental issues: CO2 compact removal units aiming the minimization of emissions to the atmosphere. Production units placed in more than 2,000m water depth, with oceanic conditions quite severe and dealing with high CO2 content stream. These challenges deserve all Petrobras attention, but also count with the confidence in achieving good technological solutions, supported by the history of successful developments of the company. Introduction The history of success built by Petrobras in deep water Brazilian coast - mainly Campos Basin - was largely supported by successive discoveries of heavy oil in turbidite sandstones, step by step towards gradually deeper and deeper water depths (Carminatti et al., 2009). The Pre-salt discoveries of Santos Basin represent discovery of huge volumes of light oil (28 to 30 degrees API), with high gas content, in a very short time span, close the most important consuming centers in Southeastern Brazil, and formation tests in the first wells have presented very high flow rates with no indication of barriers. All are excellent news, although PETROBRAS and partners recognize that the Pre-salt of Santos Basin represents a challenging scenario: ultra-deep water (greater then 2,000 meters), deep carbonate reservoirs (deeper than 5,000 meters), spread over very large areas, with high gas-oil ratio (GOR in Tupi area greater than 200 m3/m3), CO2 content (8-12% in Tupi), high-pressure and low temperature, laying immediately bellow a thick salt layer (more than 2,000 meters of salt), located around 300 km from the coast, with oceanic conditions more severe than Campos Basin. This scenario demands using sometimes present day technology in the limit, and other times, adaption and development of technologies specific for such conditions. This paper addresses the most critical points to the development of production in the Pre-salt of Santos Basin, where Petrobras is making an R&D effort to develop and qualify new technologies, including well technology, reservoir technology, flow assurance, gas processing and exporting technologies, and production units. Additionally, the paper discusses briefly how PETROBRAS has organized its structure to face these technological challenges, including the creation of a technological program - PROSAL - focused on Pre-salt objectives.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThe accurate prediction of the downhole pressures and the returning flow rates in low-head drilling (LHD) and underbalanced drilling operations (UBD) is a major concern in the oil industry. The present work shows an original formulation of a dynamic two-phase flow model based on the classic driftflux set of conservation equations. However, differently from the traditional approach, the closure of the system is obtained by using measured data acquired during the execution of the operation. The innovative concept consists of utilizing mechanistic models that are dependent on unknown parameters. These parameters and the model state are calculated and updated on a regular basis to minimize the differences between model predictions and measured data.The potential of this new approach is discussed by presenting some transient examples of application. The current study represents a continuation of previous work on real time data interpretation 1 , and a step in the development of a methodology for the introduction of learning while drilling process in the hydraulic design and follow-up of LHD and UBD operations.