Selection of a drilling rig capable of meeting well plan/program demands is a critical decision. Many economic scenarios come under consideration and play a critical role in the final decision-making process. The recent trend of building bigger and more capable drilling facilities has placed the upgrading of existing equipment on the back burner.
The application of extended reach drilling (ERD) techniques has become increasingly popular in the past decade. The concept gave birth to a new generation of drilling tools and techniques, which eventually pushed existing surface facilities to the limit. Drilling rig selection became more critical because of greater well depths being achieved. One of the results was the trend to bigger and better rigs, specifically drillships and semisubmersibles.
Vastar Resources, in association with Helmerich and Payne IDC (H&P), recently went against this trend to jointly develop a platform rig upgrade program, and drill an ERD well 28% beyond the original capability of the rig and 13 days under the originally estimated drilling time curve.
In March 1999, Vastar proceeded with plans to drill a well into a "bright spot" identified in seismic cross section. The project, called Avenger, required a 15,240 ft stepout at 12,701 ft true vertical depth (TVD) from the platform. This required a wellbore nearly 20,466 ft long. Vastar considered two choices to drill the well:
- Use a larger API platform rig, currently not under contract with Vastar.
- Upgrade and use a smaller rig that was currently under contract with Vastar.
The second option was considered the most viable.
H&P has drilled for Vastar and its predecessor Arco in the South Pass 60 field for over 22 years with several different large capacity API-type rigs. The self-erecting rigs 101 and 108 were developed for Arco to use in the South Pass 60 field to facilitate rapid moves from one platform to another. These moves use rig cranes and avoid expensive derrick barge costs for the operating company. With good weather, these rigs can be moved in less than 10 days.
Rig 108 was built in 1991 and has continuously worked for Arco and Vastar, making five rig moves to date. There were three key reasons Vastar decided to attempt the Avenger ERD well with this rig:
- Rig crews with over 327 years of cumulative H&P work experience. The Vastar drilling foremen added a combined 25 years of drill ing experience to the team.
- Mobilizing a larger API type rig would be much more expensive.
- Rig 108's past performance and the crews' safety record led to a low average cost/ft and a high level of confidence from Vastar. The Rig 108 crew worked 991 consecutive days without a lost time accident and 232 days without an OSHA recordable accident at completion of the Avenger well.
There were many technical challenges in the Avenger well program. To use Rig 108, equipment needed to be upgraded and/or replaced, new technology (both in drilling practices and equipment) had to be implemented and a detailed well plan had to be engineered. Upgrades to the rig included:
- More horsepower added - a 5th SCR bay and a temporary 5th engine.
- More hydraulic capability added - third mud pump, new pulsation dampeners, and used 5-1/2 in. drillpipe.
- Better solids control - new gumbo machine, 4 cascading shakers, de sander, and mud cleaner.
- More torque and hoisting capability - up graded top drive, put high-torque motors on drawworks, used synthetic oil-based mud, and used non-rotating drill pipe rubbers.
- Bigger hole sizes required a 16 3/4-in. 5,000 psi BOP stack in addition to a 13 5/8-in. 10,000 psi stack.
- Longer trips substantiated the use of power slips (round-trips required laying down pipe).
- Preventive maintenance-related issues included a new crane, new choke manifold, reconditioned BOP equipment, reconditioned generators, overhauled engines, and traveling/handling equipment inspections.
Quantifying well risk
Different risk estimation processes are used by operators for pre-drill planning purposes, minimum rig requirements, and drillstring planning. Each operator applies respective limits and guidelines for evaluation purposes. Risk is quantified into a number, often called a mechanical risk index, or MRI, the magnitude of the number reflecting a degree of difficulty for the project in question.
Vastar includes in this index parameters such as directional stepout, true vertical depth, proposed hole sizes, mud type and weights, casing strings, and others. Wells previously drilled from South Pass 60 E platform averaged a MRI of 474. The Avenger project resulted in a MRI of 2,550, implying that the project would be over five times more difficult than the preceding wells on the South Pass 60 E platform.
Vastar and H&P began planning the rig upgrades and well program after deciding that the upgrades to Rig 108 would be sufficient to overcome Avenger's technical obstacles. Upgrades took place during the rig move from "D" platform, resulting in minimal downtime and expense.
Rig 108 spudded the Avenger well in early March 1999. Total depth was reached 57 days and 20,466 ft later. This accomplishment was ahead of schedule and was achieved with no safety or environmental incidents.
The temporary 5th engine (Cat D-399) and third mud pump (G.D. PZ-11) were used along with 5 1/2 in. drill pipe and synthetic oil based mud to achieve the greater hydraulic horsepower (hhp) demanded from the Avenger well plan. The mast was fully racked with drill pipe at a hole depth of 17,000 ft, requiring laying down drill pipe during trips out of the hole. This added considerably to trip time and pipe handling, but was not catastrophic to achieving goals set forth on the drilling-time curve.
The mast capacity of Rig 108 is rated at 750,000 lb with 500,000 lb of setback. A heavy string of 11 3/4-in., 65.5 ppf casing, weighing in total 914,445 lb in air, was run at 13,961 ft. Using mud buoyancy (10.4 ppg) at a hole angle of 58°, the string was landed weighing only 400,000 lb on the weight indicator.
By using a well-prepared plan and excellent drilling engineering practices, Vastar and the Rig 108 Team delivered this well safely and under budget. A high degree of teamwork on the part of Vastar, H&P, and all the service companies involved was necessary to achieve this milestone.
Vastar and H&P would like to thank everyone on the Rig 108 Team for their efforts on the Avenger project.