Application of aerated-fluid drilling in deep water

A Joint Industry Project is moving forward with field testing of a lighter-weight drill fluid applied from a semisubmersible rig in 1,500-ft water

Underbalanced drilling (UBD) has been proven to increase reservoir productivity in several areas around the world. Due to the advantages gained by using this technology, offshore application has also been increasing, but it has been restricted solely to fixed platforms. However, with the deepwater drilling trend, it is imperative that this technology becomes technically and economically viable for application from floater units as well.

This article presents the design and planning of an offshore well to be drilled in the Campus basin offshore Brazil in 454 m (1,490 ft) of water with aerated fluid. Although the operation is not planned to be underbalanced, the well will be the first to be drilled from a floating vessel using aerated-fluid technology. The article describes the approach taken to overcome the challenges posed, including the involvement of several companies in a Joint Industry Project (JIP) to speed up the learning process and steps related to critical areas, mainly hydraulics and equipment. The article finally discusses the most important points that still need improvement to make UBD a reality in offshore operations from floating units.

Overviewing principal conclusions from this presentation, the JIP, after its first year, has advanced toward the goal of drilling the first deepwater well with two-phase fluid. When fully developed, the UBD technique applied from floating vessels will bring several benefits to the industry, including the possibility of drilling horizontal/multilateral wells in depleted deepwater fields much more efficiently to increase field productive life.

The first well planned will be drilled with light-weight fluid ([N.sub.2] and liquid) but still in a slightly overbalanced condition. Actual underbalanced drilling operations will be considered in the subsequent steps.

However, use of a two-phase fluid drilling technique, even slightly overbalanced, from a floating platform in deep water will represent a tremendous step forward in advancing the technology.

INTRODUCTION, DEEPWATER APPLICATION CHALLENGES

Several UBD operations conducted onshore have illustrated the many advantages of this technology. The immediate one is generation of revenue even during the drilling operation phase, since it is possible to simultaneously drill the well and produce from the reservoir. But probably the most important benefit is the huge reduction in formation damage that will lead to higher-than-expected well productivity.[1]

Additionally, UBD has been used to avoid severe mud losses,[2] and it allows better formation evaluation and improves drilling performance.[3] Although mud-weight reduction during drilling has also been used to address these issues, in some situations this option could not completely solve the problem.

To take further advantage of UBD benefits, industry is now implementing this technique offshore. So far, applications have been restricted mostly to fixed platforms. In Lake Maracaibo, Venezuela, UBD has been conducted from barges. However, since the heave is not pronounced in the Lake, no special modifications were necessary.

Offshore, however, weather conditions are much more critical and, therefore, direct application of onshore UBD practices is no longer possible. To overcome the difficulties involved in UBD in a much more hazardous environment, a Joint Industry Project called "Offshore drilling with light-weight fluids" was initiated.[4] The purpose of the first phase was to undertake critical issues concerning safety and technical feasibility of applying aerated-fluid technology from floaters. UBD will be addressed in a second phase, after the main issues have been satisfactorily resolved.

Implementing aerated fluids in deep water faces several challenges. Use of a two-phase fluid to drill a well poses special difficulties if the operation is to be done from a floater. The main topics directly related to the application in deep water are:

* Designing/dimensioning a four-phase separator to fit on a platform

* Better hydraulics control to correctly determine actual equivalent circulating density (ECD)

* Configuring equipment to control the well (rotating diverter).

This last item involves several issues, including: 1) positioning of the rotating diverter; 2) type of rotating diverter to be used; 3) reliability of packer elements on the rotating diverter; and 4) the possibility of leakage in the riser and slip joint.

Other important points to be considered when drilling with aerated fluids, but not restricted to deepwater application are: operational procedures; cuttings transport; and wellbore stability, mainly in the shale sections. Regarding offshore operational procedures (connections, tripping, etc.), it is important to note that they will differ greatly from those normally adopted when drilling onshore.

Even though the first steps taken by the industry to apply UBD offshore were very important, its use on floaters will involve many more difficulties. For the first applications--that adapted the technology from onshore rigs to jackup platforms--the modifications needed were not as dramatic as the changes necessary to allow UBD on a semi rig. Due to space restrictions to accommodate all the necessary equipment, drillships are not yet being considered.

THE JIP: "OFFSHORE DRILLING WITH LIGHTWEIGHT FLUIDS"

Petrobras' main hydrocarbon reserves in Brazil are located in the Campos basin, most in deep water. In this area, drilling operations in deep water have been performed using floating vessels, therefore, UBD application for this scenario will need to consider this type of vessel. When the company started the project to use light-weight fluid drilling in the area, it was noted that modifications, both of operational procedures and equipment, were quite complex.

To speed up the learning process, the company decided to lead a Joint Industry Project. The key point in the project is to put together companies with experience in the technology and similar needs, with the ultimate goal of successfully implementing UBD in deep water.

The first steps were focused on accessing the feasibility of applying UBD technology offshore, establishing basic operational/safety procedures and equipment conversion. This phase of the project was concluded in December 1998. The next step involves performing a field test in which a well is drilled using aerated fluid.

For this test, the well will be drilled slightly overbalanced to avoid the complexity of handling oil/gas production offshore on a platform not connected to pipelines. The aim is not to evaluate formation damage or reduction of well productivity, but to make sure that all related issues of dealing with a two-phase drilling fluid have been resolved successfully and that the technology is feasible for deepwater application.

Due to the high cost of bringing onboard all necessary equipment ([N.sub.2] membrane unit, separator, compressors, busters, lines, etc.), the project is focusing on preparing a rig that will be totally adapted to UBD at all times. Ideally, operations on this rig will be easily switched from conventional, 100%-liquid drilling to aerated, two-phase-fluid drilling.

This will avoid the cost of mobilization, rig up, etc. Obviously, it will take some time to implement this concept. Ultimately, switching from liquid to a two-phase drilling fluid will be a matter of just installing a rotating diverter and opening the injection-gas valve. The amount of underbalance or overbalance to be used can be adjusted for each particular case.

Benefits. The benefits of having the capability of adding air or nitrogen to the drilling fluid at any time range from reducing formation damage to freeing stuck pipe due to differential pressure. Reducing drilling fluid density also may be a solution when lost circulation events are a concern. In these cases, instead of reducing fluid weight by diluting the whole system, adding some air will have a similar effect, but much faster, which will impact final well cost.

Other benefits usually associated with UBD use are an increase in the rate of penetration (ROP), more reliable hydrocarbon indication at surface due to the low ECD, and better log interpretation.

Project description. During the first phase, concluded in December 1998, studies were primarily focused on the topics noted here, and described below:

* Drilling hydraulics

* Cuttings transport

* Formation damage

* Four-phase separator

* Equipment (rotating diverter), and

* Operational procedures.

Drilling hydraulics. A real-scale test facility in Taquipe, Brazil, Fig. 1,[4] with a 1,279-m (4,200-ft) vertical well fully equipped with sensors and a data acquisition system, was used to evaluate ECD and temperature for different air/gas/foam/liquid ratios. The results were used to calibrate correlations of the hydraulic models. These tests also permitted a better understanding of the flow regime with various gas/liquid ratios, which is essential in determining the correct bottomhole ECD in the well. Knowledge of the ECD is critical for the operation, since failure to predict it may cause problems for the entire operation.

[Figure 1 ILLUSTRATION OMITTED]

Comparison among available hydraulics simulators was also performed to evaluate discrepancies and uncertainties. Each software gave a different result for a specific well configuration, emphasizing the importance of real-scale tests to determine the best correlation/model to be implemented and used for each case.

Cuttings transport. Both lab and real-scale experiments were conducted during the first year. The focus was to check influence of the gas phase on fluid-carrying capacity. This effect has been neglected in some studies in which it was assumed that only the liquid phase had carrying capability.

Lab experiments were conducted in a quasi-real-scale simulator which had the capability of simulating from vertical to horizontal wells. Cuttings bed removal could be investigated, determining the efficiency of each mixture injected.[5]

Experiments conducted so far have proven that addition of gas significantly improves liquid-carrying capacity. Therefore, gas volumes can be decreased from current standards, reducing overall well cost.

In the second project phase, a two-phase fluid option will be implemented on the in-house software to predict cuttings bed formation. The software will also allow prediction of cuttings removal efficiency for two-phase drilling fluids.

Formation damage. Besides studies to estimate damage that would be caused in a reservoir when conventional drilling is used, lab tests are being conducted to evaluate damage caused by foams. How the foam agent affects the rock, changing its wettability, might be an important factor. Problems faced so far are related to difficulties in adapting lab procedures to adequately mimic downhole conditions. Permeability determination should be done with oil or gas, according to what is expected in the reservoir.

Damage accessibility is important to quantify potential benefits of underbalanced drilling so that economics of the whole project can be done. Skin reduction can be used in reservoir simulators to determine economic benefits of UBD.

Four-phase separator. When drilling with an aerated fluid, it is essential to have an efficient surface separator. Ideally, this equipment should be a four-phase separator capable of separating water, oil, gas and solids (cuttings). This issue is critical for the project due to space restrictions on offshore platforms. A new conceptual separator is being developed, with a significant foot-print reduction compared to currently available systems. Fig. 2 schematically illustrates a vertical separator prototype tested at the Taquipe site.

[Figure 2 ILLUSTRATION OMITTED]

Rotating diverter. Not only the type of rotating diverter, but also its position, as well as configuration of all related equipment involved in controlling the well are important issues to be addressed. From the beginning, it was decided that the rotating diverter would be placed at the surface since, presently, no subsea rotating equipment is available on the market. Therefore, the discussion was focused on which position at the surface would be better to place the equipment. The decision on the best position will be based on the following criteria, i.e., the system must:

* Be safe and simple, to control the well

* Require minimal rig modifications

* Allow easy and simple switching from aerated to conventional (100% liquid) drilling fluid.

Out of the four possible positions for the rotary diverter initially considered, the two most likely are: 1) below the ball joint; and 2) on top of the slip-joint.

Each option has its advantages and disadvantages. There are also some restrictions due to the need for adaptations of the rotating head to be safely placed at the chosen location. An intense discussion among the JIP participants took place to define the best configuration. After six months of interaction, with meetings and electronic communication, the second option appears to be the one that will be used in the first well, but a definite decision had not been made at this writing.

Operational procedures. When discussions regarding operational procedures began, it was noted that many conventional procedures, regularly adopted onshore, would no longer be admissible offshore. One example is the method used to pull the drillstring. Onshore, normally, if one wants to avoid influx during tripping, fluid inside the well will be kept within certain levels that will allow the operation to be done safely, but also without over-pressuring the formation. However, in a deepwater environment, it was noted that this procedure would always keep the entire riser empty during tripping. It is clear that, depending on water depth or resistance of the riser to collapse, this procedure would not be possible.

This topic, and other practical ones, are still being discussed with offshore specialists. Presently, the authors feel that only after the field test will these issues have a clear recommendation.

FIELD TEST, RIG/ WELL SELECTION

With a step-wise method to conduct the project in mind, the first well to be drilled with an aerated fluid from a floating vessel will be located in a water depth less than 500 m (1,640 ft). This restriction is due to limitations of the riser. The collapse pressure that the riser can sustain when full of gas limits maximum water depth to 500 m. Although this restriction may be overcome with a differential pressure valve, for this first application, every potential problem is being eliminated or at least minimized.

The drilling schedule is another important restriction since the idea is not to interfere with the regular schedule of the company. Two possible rigs were visited to check for layout feasibility; both could handle the operation. Diamond Offshore Drilling's Ocean Whittington semisubmersible was selected and plans are being made to adapt and modify the rig.

Lines, separator instrumentation, and all related equipment needed were designed. All modifications are planned with the goal of allowing the rig, in the future, to have the capability of using a two-phase fluid any time, with minimal modifications. Actually, after the rig is prepared, nitrogen generation equipment will be the only part of the system that will need to be transported onboard. All other equipment will already be in place. Switching from liquid to two-phase drilling fluid will be just a matter of small rearrangements.

The well selected as the best candidate will be drilled in the Albacora field, in Campos basin. Even though all equipment will be installed immediately after running the subsea BOP stack, aerated fluid will be used only in the last well section when drilling into the reservoir. The well is planned to reach 31 [degrees] at 2,989 m MD, 2,725 m TVD, (9,800 ft MD, 8,938 ft TVD).

If successful, this first test will allow Petrobras to consider drilling, with a reduced overbalance, several development wells planned for Campos. Since, in many cases, the reservoirs are already depleted, use of light-weight, two-phase fluid is expected to reduce risk/costs of drilling such wells.

FUTURE NEEDS

After conclusion of the project's first phase in January 1999, the JIP member companies decided to continue the venture. Several more items, as noted here, were added to the original list and will be addressed in the second phase of the project.

Formation damage. Probably the most important advantage of lightweight fluid drilling is increased production rate due to lower formation damage. However, before drilling, while performing feasibility studies, it is difficult to estimate expected formation damage reduction to be obtained with use of a particular light-weight fluid

In the second phase, the effect of differential pressure and timing on the variation of permeability for specific reservoir rocks will be evaluated. The objective is to develop a quantitative model that could help in the feasibility analysis of potential applications. If successful, the same study will be undertaken for different lithologies.

Wellbore stability. Instability of reservoirs is not considered today to be critical, assuming the rock is not completely disintegrated. The biggest problem will be sand production if UBD is used. But the important issue here is related to the shale. Since shale has extremely low permeability, a lower-than-necessary fluid pressure will cause stresses on the wellbore wall that can easily cause rock failure. The main question to be answered is, how low can the pressure go while still keeping the rock in place and avoiding collapse?

Several wells have been abandoned when drilling underbalanced due to shale collapse. Without a good understanding of the failure mechanisms and a better prediction of allowable drawdown, risks of UBD in shale intervals are high. In addition to this mechanical failure, shales also interact with the drilling fluid, and quantification of this interaction is essential, to have a complete picture of all processes involved in shale instability.

Besides the above issues, other points being considered for the second year of the project are:

* Improvement of operational procedures after drilling the first well

* Testing of the four-phase separator

* A new version of the "Offshore light-weight drilling manual"

* New tests at the real-scale facility.

And finally, depending on results of the field test, other wells may be programmed to be drilled in the near future.

ACKNOWLEDGMENT

The authors express their gratitude to these supporting companies in the Joint Industry Project, "Offshore drilling with light-weight fluids," for permission to publish this article: Amerada Hess, Air Drilling Services, BP, Mobil, Petrobras and Williams Tool Co. We also acknowledge each task leader within the project, at this stage: Paulo Hora, Jose L. A. Vidal, Valter Silva Jr., Sara Shayegi, Paulo R. C. Silva, Andre L. Martins, Afonso M. F. Lourenco, Joao C. Queiroz, Carlos H. S. Martins, Mario Villas-Boas, Felipe Redo, Osvaldo do Vale, Geraldo Spinelli, Antonio L. S. de Souza, Ricardo C. M. Portella, Fabio S. N. Rosa, Marcelo O. Andrade, Eugenio Rosa, Fernando A. Franca and Marcelo Friedman.

This article was adapted from the paper, "Application of aerated-fluid drilling in deepwater," paper SPE/IADC 52787, presented at the 1999 SPE/IADC Drilling Conference held in Amsterdam, the Netherlands, March 9-11, 1999.

LITERATURE CITED

[1] Lage, A. C. V. M., E. Y. Nakagawa, A. A. Souza and M. M. Santos, "Recent case histories of foam drilling in Brazil," paper SPE 36098, presented at the 4th SPE-LACPEC, Trinidad & Tobago, April 1996.

[2] Negrao, A. E and A. C. V M. Lade, "An overview of air/gas/foam drilling in Brazil," paper SPE 37678, presented at SPE-LACPEC Petroleum Engineering Conference, 1997.

[3] Cunha, J. C. and F. S. N. Rosa, "Underbalanced drilling technique improves drilling performance: A field case history," paper SPE 47802, presented at the 1998 IADC/SPE Asia Pacific Drilling Conference, Jakarta, Indonesia, Sept. 7 9, 1998.

[4] Nakagawa, E. Y. and A. C. V. M. Lade, "Light-weight fluid drilling: R&D main needs based on present experience," presented at the 3rd International Underbalanced Drilling Conference & Exhibition, The Hague, the Netherlands, October 1997.

[5] Martins, A. L., C. H. M. Sa, A. M. F. Lourenco and W. Campos, "Experimental determination of interracial friction factor in horizontal drilling with a bed of cuttings," paper SPE 36075, presented at the 4th SPE-LACPEC Pet. Eng. Conf., Port of Spain, Trinidad & Tobago, April 1996

Edson Yoshihito Nakagawa, holds a BS in civil engineering from Escola de Engenharia de Piracicaba (Brazil), an MSC in PE from the University of Ouro Preto (Brazil) and a PhD in PE from Louisiana State University. He has worked for Petrobras since 1980. Initially involved in drilling projects in Amazon and Campos basins, he moved to Petrobras' Research Center in 1984, where he continues to research topics related to well design, well control, UBD and implementing new drilling technologies.

Helio Santos earned BS and MSC degrees in civil engineering from PUC-RJ in 1982 and 1989, and a PhD in geological engineering in 1997 from the University of Oklahoma. He joined Petrobras in 1983, and worked as a drilling engineer in offshore fields until 1986, when he moved to the Petrobras Research Center. His main research interests are applications of geoscience in petroleum engineering problems, such as wellbore instability, sand production, subsidence, etc. He is also involved in projects for drilling optimization and special drilling techniques.

Jose Carlos (JC) Cunha, a senior petroleum engineer with Petrobras since 1979, holds a BS in civil engineering from Juiz de Fora Federal University, an MSC in PE from the Ouro Preto Federal University, and a PhD in PE from the University of Tulsa. As a drilling engineer, he has worked in various Petrobras operational units in Brazil and also for Petrobras International in Africa, the U.S., South America and Europe. Currently, he is the coordinator of the PROCAP-2000 project for implementation of extended reach wells in deep water, and he also works in other projects related to development and/or implementation of new weft technologies in Petrobras operational units.

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