A straight-hole drilling device has improved wellbore quality and reduced drilling time on a gas well in the Acambuco block of Northwestern Argentina.
The 28, 22, 16, and 121/4-in. hole sections were drilled to 4,700 m in 161 days, 30 days less than the best offset well performance and 49 days less than the regional average, saving the operator about $1.5 million in rig costs.
Wellbore quality was significantly better than in previous wells as shown by comparing wellbore displacement. At 2,300 m depth, the displacement was less than 20 m with use of the straight-hole drilling device as compared to almost 150 m in the closest offset well that was drilled with other traditional bottomhole assemblies.
This displacement was achieved with lower average dogleg severity, which contributed to reaching the 97/8-in. casing point without any drillstring failures or washouts. The 97/8-in. production liner was successfully run and cemented.
These achievements, along with the fact that no accidents occurred from spud to TD, has established the new "best in class" benchmark for drilling operations in the area.
Drilling through the complex overthrust geology in the Andes foothills of northern Argentina and Bolivia presents a number of distinct challenges.
Formations in the upper hole sections (22 in. and 16 in.) are typically hard, abrasive, and prone to circulation loss. Historically, these intervals cause major problems such as low penetration rates, hole deviation, and frequent drillstring failures.
Detailed analysis of the drilling performance, mud logging, and wireline data from offset wells identified potential problems. The operator set objectives for the upper hole sections, to achieve the highest penetration rate possible, while maintaining a near-vertical wellbore.
A three-part strategy was adopted to accomplish the objectives. The drilling team employed the Baker Hughes Inteq VertiTrak straight-hole drilling device, developed with ENI-Agip SPA of Italy, to reduce borehole deviation.
Secondly, the contract between operator and service company effectively transferred a portion of the drilling performance risk to the service company. It also combined lump sum and incentive payments to align the mutual objectives of both parties.
The third strategy called for service company employee integration into the operator's daily rig and office activities.
Well objectives
With 20 years' experience in the Northwest Basin of Argentina, the operator is familiar with the complex overthrust geology and problematic formations. This complex geology can seriously affect well economics.
The principal objectives for the well described in this article were to drill, evaluate, and test the Huamampampa and Icla formations and establish commercial production rates.
A secondary objective was to plan for a horizontal, multilateral contingency to ensure commercial production rates, if necessary.
Two previous wells were drilled using "natural drift." The surface location was offset from the target and the wellbore trajectory was permitted to drift up-dip into the target.
This technique, common in thrusted areas, is effective with good geological control, but risky where the geology is uncertain. This geologic uncertainty, coupled with potential requirements for a horizontal multilateral branch, led the operator to examine an alternative drilling strategy.
Geology
The wellsite is on the eastern part of the Subandean thrust belt within the Northwest Basin (Fig. 1). The thrust belt consists of north-south trending anticlines separated by wide synclines.
The Devonian age Huamampampa and Icla formations are relatively deep sandstones (Fig. 2). When set in a naturally fractured structural position, the reservoirs can be prolific. Examples include the Ramos and Aguarague fields with some wells capable of producing 35 MMscfd or more.
The first formations to be drilled are the Las Peñas and Upper Tarija, both of Carboniferous age. The Tarija formation is extremely abrasive and one of the most challenging formations in the basin to drill. Although it "drills" hard, the silty formation matrix promotes balling of the drill bit, particularly when drilling large hole sizes.
The next formation to be drilled is the hard and abrasive Tupambi, a sequence of interbedded sands and siltstones also of Carboniferous age.
The final challenge is overcoming borehole instability while drilling the Devonian Los Monos formation. The predominantly shaly section is relatively stable chemically but is stressed tectonically, as shown in cross section in which the true stratigraphic thickness of approximately 1,000 m has been uplifted and expanded to a section length in excess of 3,000 m in the well (Figs. 2 and 3).
Predrill analysis
The service company's drilling optimization team conducted a detailed analysis of offset wells to identify and diagnose barriers to improved drilling performance. Other wells in the basin further away were analyzed also to gain a better understanding of the general drilling environment. The analysis included information obtained from mud logs and wireline data.
As a result of the investigation, the drilling team gained valuable insights into the interdependence of bit selection, operating parameters, and other drilling practices on the performance of the offset wells.
This analysis enabled effective knowledge capture for improving future drilling performance. After a sophisticated and detailed step-by-step review of the relevant issues, the team identified the following areas, for more focused engineering study:
- Deviation control.
- Borehole instability.
- Vibration.
- Hard, abrasive formations.
- Balling tendencies.
- Mud losses.
- Low penetration rate.
- Short bit runs.
- Logistics.
All of the challenges listed were addressed in detail. The drilling optimization team's principal recommendation, however, was to use straight-hole drilling technology, designed to correct well deviation automatically.
Although untested in Argentina, the straight-hole drilling device that was planned had solved similar challenges in a tectonically active area in southern Italy.1-3
Straight-hole drilling device
Experience in the area indicates that attempting to drill a vertical well using conventional technology is ineffective.
Traditionally, this has either meant drilling with pendulum-style bottomhole assemblies, with low weight on bit or steerable directional assemblies, both of which retard rate of penetration. The straight-hole drilling device provides a unique solution to the problem (Fig. 4).
The device incorporates a downhole motor with a modified bearing section, computerized instrumentation, and a hydraulic steering system. The hydraulically controlled steering head maintains the inclination, and the relevant directional data are continuously pulsed to surface via a mud pulse telemetry system.
For corrective steering, the straight-hole drilling device is operated in sliding mode. As soon as the measurement-while-drilling (MWD) system senses any inclination, the tool generates selective hydraulic pressure to the appropriate steering ribs and corrects the inclination back to vertical.
This fully automated downhole process effectively maintains a vertical wellbore. It is also possible to rotate the tool from surface when steering is not required.
An earlier version of the tool was successfully used onshore Italy in hard formations that have strong deviation tendencies. As the structural setting and rock strengths in northern Argentina are similar to those of southern Italy, the team incorporated the straight-hole drilling device into the plan and felt confident that it would be successful from a technical point of view.
Detailed analysis of the specific application led to several design changes from previous versions of the tool.
The new tool features modular components. It is easily modified to fit any hole size between 26 and 16 in. Modular construction makes repair and maintenance of the tools cost effective and allows successful operation in logistically challenging environments with fewer tools.
The new tools also featured three hydraulically operated steering ribs. All previous tools used four independent steering ribs. The reduced number of ribs allowed increasing component size, which decreased chances of rib failures by 25%.
Contracting philosophy
The team had confidence that application of the straight-hole drilling device was sound technically. It was unclear, however, whether use of the tool would be economical. With this in mind, the operator and service company implemented a novel commercial arrangement to manage risk.
The companies established a benchmark using the last two wells drilled by the operator along with all wells recently drilled on the structure. Even wells drilled by other operators were included. This analysis established the best-offset performance, average performance for drilling on this structure and average regional performance (Fig. 7).
The commercial proposal to introduce the straight-hole drilling device technology was centered on a lump sum, performance basis. The contract called for the operator to pay "standard services" price if average performance was achieved and the service company to earn the "premium services" price only if the "best offset" performance was achieved.
For performance surpassing the best offset, the operator's savings were shared with the service company as a further incentive for success (Figs. 5 and 6).
The performance basis was mutually agreed upon. The contract was structured to include only times where the service company had a direct impact on performance. Time required for logging and running casing was normalized out of the calculations.
The nonproductive time associated with contractor services was included in the performance calculation as further incentive to provide quality service. Nonproductive time for operations the service company was not responsible for was excluded.
Teamwork
To introduce and build support for the new drilling philosophy and tool implementation, the operator held a 3-day, team-building, technical-limits session for field personnel and rig crews.4
This session accomplished two major objectives:
- It provided a explanation of the straight-hole drilling device, including an evaluation of the risks and benefits of drilling in 100% slide mode.
- It engaged the field personnel in the well program and ownership of an aggressive, self derived performance target.
Additional trust and camaraderie developed between the team because the service company engineers sat directly opposite the operator drilling staff. Also, service company employees were an integral part of daily activities, including daily meetings on the rig and in the office.
Performance
The straight-hole drilling device significantly improved wellbore quality. It reduced drilling torque and drag as well as minimized ledging.
Improved borehole quality led to reduced wear on both the casing and the drillstring, which resulted in direct cost savings on hard banding and drill pipe protectors. The tool increased hole stability by drilling an in-gauge, near-vertical hole with less mechanical destabilization due to drillstring rotation.
An additional benefit of 100% sliding is reduction of vibration levels that significantly reduced the possibility for drill string failures and led to direct savings in drill string inspection costs.
Finally, further improvements in rate of penetration and bit life were realized with the more stable drilling environment and improved weight transfer to the bit. The well reached the 97/8-in. casing point faster than the previous best in the basin (Fig. 7).
The well was successfully tested upon completion. Evaluations of the results and the decision whether to add a horizontal multilateral are pending. The improved wellbore quality resulting from use of the straight-hole drilling device is expected to reduce the risk of performing further completion operations.
Future improvements
While this project was successful, the following conditions were identified as challenges to further improving drilling performance:
- Inability to apply desired weight-on-bit (WOB) due to loss of motor performance with the rotor nozzle that was installed to limit bit speed.
- Premature seal failure on bits due to excessive bit speed, especially in larger hole sizes.
- Vibrations prevented application of higher WOB, limited by the bottomhole assembly (BHA) design.
- Building tendency was not totally corrected by the drilling tool.
- Severe washing was observed on tools due to a combination of high solids content in the mud and plugging with loss circulation material.
After extensive laboratory testing and analysis of the challenges, the following changes were identified for the next well:
- Replace the straight-hole drilling device power section with a lower-speed 91/2-in. motor to increase bit life.
- Make seal and grease improvements to extend bit bearing life.
- Increase weight on the straight-hole drilling device from 80,000 to 100,000 lbf. This was made possible by changing the low speed motor, thus allowing higher weight to be run in combination with lower rotating speed.
- Improve steering capabilities of straight-hole drilling device by increasing internal hydraulic pressure. Working in tandem with updated system electronics and increased hydraulic pressure, the tool was modified to deliver an increased lateral steering force.
- Improve drill pipe screen quality, discontinue use of a nut plug, and use a shock sub or thruster where appropriate.
Use of the straight-hole drilling device tool resulted in a higher quality wellbore without sacrificing ROP in the shallower sections. This, in turn, allowed for optimized drilling performance in the critical 121/4-in. and 81/2-in. hole sections.
Acknowledgements
Thanks are due Pan American Energy (PAE) acting as operator of the UTE Acambuco (52%), O&G Developments Ltd. (22.5%), YPF SA (22.5%), Apco Argentina Inc. (1.5%), and Northwest Argentina Corp. (1.5%) for permission to publish this article and to Baker Hughes Oasis, Baker Hughes Inteq, and Hughes Christensen for their cooperation and support.
References
- Calderoni, A., Savini, A., Treviranus, J., and Oppelt, J., "Outstanding Economic Advantages Based on New Straight-Hole Drilling Device," SPE Paper No. 56444 presented at the 1999 SPE Annual Technology Conference and Exhibition, Houston, Oct 3-6, 1999.
- Poli, S., Donati, F., Oppelt, J., and Ragnitz, D., "Advanced Tools for Advanced Wells: Rotary Closed Loop Drilling System-Results of Prototype Field Testing," SPE Paper No. 36884, prepared for presentation at the 1996 SPE European Petroleum Conference, Milan, Italy, Oct. 22-24, 1996.
- Ligrone, A., Oppelt, J., Calderoni, A., and Treviranus, J., "The Fastest Way to the Bottom: Straighthole Drilling Device-Drilling Concept, Design Considerations, and Field Experience," SPE Paper No. 36826, prepared for presentation at the 1996 SPE European Petroleum Conference, Milan, Italy, Oct. 22-24, 1996.
- Bond, D.F., Scott, P.W., Page, P.E., and Windham, T.M., "Step Change Improvement and High Rate Learning are Delivered by Targeting Technical Limits on Sub-Sea Wells," IADC/SPE Paper No. 35077, prepared for presentation at the 1996 IADC/SPE Drilling Conference, New Orleans, Louisiana, Mar. 12-15, 1996.
The authors
Mike Barnes has been a drilling engineer for BP for 12 years, including experience in the US, North Sea, Colombia, and most recently Argentina. Barnes has a BS in chemical engineering from Colorado State University.
Carlos Vargas has been a drilling engineer with BP for 9 years. He has drilling experience in deep wells in Colombia and Argentina. Vargas has a BS in petroleum engineering from Universidad de America, Colombia.
Fernando Rueda has been a drilling engineer for BP for 7 years, with experience in Colombia and Argentina. Rueda has a BS in petroleum engineering from Universidad de America, Colombia.
Juan Garoby began working in the oil field as a drilling engineer in 1992 for Tecpetrol Argentina, working first in the Northwest Basin and in South Argentina after 1995. In 1996, he joined YPF SA as a drilling and workover operations coordinator in the Los Perales field in San Jorge Basin, South Argentina. He joined the Baker Hughes' OASIS group in August 1997 as a drilling optimization engineer, and since 1999 has been working as the OASIS operations manager for Latin America. Garoby has a degree in petroleum engineering from Buenos Aires Technological Institute.
Mario Pacione joined Anadrill/Schlumberger in 1995 as an MWD operator, based in Neuquén Argentina. He has worked as a directional driller since 1996 in almost all the basins in Argentina. In 1997, he joined Hughes Christensen as a field engineer, based in Neuquén and afterwards in Bolivia. In 2000, he transferred to the Baker Hughes Oasis group as a drilling optimization engineer. Mario Pacione obtained his degree in electronic engineering in 1995.
Albert Huppertz is a drilling engineer and project manager for Baker Hughes INTEQ. He has extensive experience with applying drilling systems worldwide. Huppertz has an MS in mining engineering from the Aachen University of Technology, Germany.
