Hole Cleaning, Torque Management Lead to Longest Extended-Reach Well in Brazil
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The Peregrino project, located in the post-salt zone of the Campos Basin, presents immense challenges for drilling extended-reach wells. This paper shares challenges imposed by the well profile and downhole environment on the design and execution phases. Information is included about the solutions used that led to reaching the target depth as planned, including the well-planning methodology, bottomhole-assembly (BHA) design, drill-bit selection, data monitoring, and procedures for maximizing the transfer of power downhole and minimizing rock-cutting energy.
The Peregrino oil field is in the BM-C‑7 area of the Campos Basin. Of 47 wells drilled from two fixed platforms in the field (Peregrino A and B), 20 were from Peregrino A, where the longest horizontal sections have been drilled. The average horizontal length was 1419 m, and the operator is pushing the technical limits to extend the overall lateral stepout, increase reservoir lengths, and maximize hydrocarbon exposure.
Drilling horizontal sections beyond the field average length has several additional challenges including mud quality along the sections, high friction factors, and surface-equipment operational limits. For extended-reach wells, hole-cleaning efficiency and proper torque management have been shown to be key factors for flawless execution.
Considering the typical well setup in offshore Brazil, Well A-27 had a considerable total length. The planned length of the well was 8400-m measured depth (MD), with a possible extension to 8740‑m MD. The landing of a 9⅝-in.-casing shoe was planned to be at 6520‑m MD with approximately 82° of inclination, and the 8½-in.-section lateral displacement was to start at 5673 m from the wellhead (Fig. 1 above).
With a possible horizontal section of 2220 m, a rotary-steerable system was to be used, with the addition of a drilling dynamics sub and a modular motor, to reduce the demanded weight on bit (WOB) and surface rate of revolution while maintaining sufficient energy at the bit. Management of torque and equivalent circulating density (ECD) would be helped by the use of 4-in. drillpipe, which would also provide lower torque values with less cross-sectional area. The final measure required to reduce overall friction along the horizontal well path was to add lubricant to the drilling fluid to reduce metal-to-metal friction at the drillpipe/casing interaction. Lubricant was also to be added while drilling, with the idea that drops of lubricant deposited into the mudcake would help reduce friction when drillpipe tool joints pass through.
Well A-27 presented mud losses when pulling the BHA out of hole after reaching total depth (TD) in the 12¼-in. section. To cure such losses, a balanced cement plug was displaced close to TD. A cleanup run then was performed to dress off the top of the cement plug. Using the same BHA, operations were resumed to deviate the well sufficiently to avoid crossing lost-circulation zones.
The sidetrack was performed successfully, and the TD for the 12¼-in. section was called at 6345-m MD, approximately 175 m before the planned 12¼-in. TD. The 9⅝-in.-liner shoe was set at 6342‑m MD. No problems were reported during the liner run, and no losses were observed during the cement job, which confirmed the cement plug’s success in curing the losses.
The 8½-in. BHA was configured to provide essential logging information such as azimuthal resistivity, ultradeep azimuthal resistivity, azimuthal gamma ray, photoelectric effect, formation density, and porosity. The oilfield services company also provided other measurements for enhanced understanding of the downhole drilling environment, such as borehole caliper; downhole torque; WOB; bending moment; downhole vibrations; ECD; and directional data, including near-bit inclination.
The horizontal section was initiated at 6345-m MD at 20 m/h. Mud weight was 9.2 lbm/gal. WOB was between 4,000 and 5,000 lbf. Rotational speed was 230 rev/min at the bit (80 rev/min from the topdrive and 150 rev/min from the motor). Torque variation was 26,000–28,000 lbf‑ft. ECD was 10.1 lbm/gal, and drilling performance was between 20 and 40 m/h. Close monitoring of drilling torque and hole cleaning was performed during drilling of the entire horizontal section.
Hole cleaning was the key to success when drilling this horizontal section, and additional focus on the monitoring of the drilling torque and the weight transfer enabled outstanding drilling performance. In addition, the inclusion of 2400 m of 4-in. drillpipe in the drillstring configuration was a critical element in allowing certain surface equipment, such as the topdrive, to operate within specifications without overheating. The 4-in. drillpipe also would aid in efficient weight transfer from surface to bit while using low WOB.
The addition of lubricant in the mud helped to keep drilling-torque values below 40,000 lbf-ft for most of the 8½‑in. section.
Drilling parameters were monitored closely using downhole data from the drilling dynamics tool, which provided a measured amount of weight applied on the bit. The at-bit measurements allowed for a good understanding of the downhole drilling environment and a real-time assessment of WOB applied so as not to exceed buckling limits while drilling.
Power loss is unavoidable in extended-reach drilling and is one of the more common bottlenecks to be overcome to drill successfully and within budget. Trajectory and friction are the main causes for the loss of power, and high torque and drag are the result. Poor hole cleaning can increase torque and drag in addition to elevating the risk of pressure-related problems. These concerns must be addressed carefully during the planning phase and controlled by monitoring surface and downhole data while drilling. Crucial tasks during the planning phase include BHA design and simulations of torque, drag, and mechanical loads. The execution process was concluded with monitoring of available data to adjust the course of action. Once more, the weight transfer, torque at the bit, and downhole pressure were important data, considering that references of weight must have a constant calibration with surface values from rig equipment. In addition, dynamic dysfunctions, especially lateral and torsional vibrations, were monitored to understand if drilling was progressing efficiently within drillpipe and BHA limits and to observe drill-bit behavior and life.
After drilling the 8½-in. section, the drill bit displayed no damage to the cutters, supporting the conclusion of efficient drilling and a controlled downhole dynamic environment.
Conclusion and Lessons Learned
The following four aspects were of particular importance for success in drilling the longest horizontal section in Brazil:
- Predicting and controlling torque and drag, hole cleaning, and the effect of hole cleaning on pressure-related problems while drilling a depleted reservoir
- Monitoring of downhole WOB and torque on bit
- Managing available power downhole with an adequate balance between the applied WOB and the drillstring rotational speed from the surface
- Providing additional power through a positive-displacement motor attached to a rotary-steerable BHA
The interrelation between these variables is not necessarily straightforward, and mismanaging one aspect could lead to a cascade effect on the others and an increase in cost per meter.
Despite the challenges, the project delivered a longer-than-base-case horizontal section. The key to success was the use of established software and integrated engineering evaluation in the planning phase from operator and service provider, combined with proper modern data monitoring while drilling. In addition, the use of analytical tools such as the power curves in the post-job analysis proved to be a valuable integration method for analyzing results and providing improved simulation models for future extended-reach wells. Careful ECD management and WOB transfer to guarantee integrity of the drillstring design and minimize torque and drag are two aspects of utmost importance for achieving the observed results. Moreover, the use of the modular motor in the rotary-steerable BHA enabled an increase of power downhole to counteract the natural loss of power transfer resulting from trajectory and drag and to counteract the decrease of rock-cutting efficiency resulting from factors such as subtle formation changes or drill-bit wear. In fact, the motor enabled a roughly constant power level throughout the section. It provided the availability to collect downhole and surface data for real-time simulation and evaluation of drillpipe integrity. Finally, the 4-in.-diameter drillpipe was a highly beneficial part of the drillstring configuration.
Hole Cleaning, Torque Management Lead to Longest Extended-Reach Well in Brazil
01 May 2018
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