North Sea and GOM operators were early leaders in the adoption of Rotary Steerable Systems (RSS) when the tools entered the market in the late nineties. Increasing rig costs and the need to reach ever more complex targets were the driving forces behind this adoption. However, with announced deepwater rig dayrates already topping $500,000 in 2006, it appears that downhole reliability is playing an increasing role in the drive to reducing drilling costs.
There is no doubt that when a RSS works well, it drills a smooth hole. Similarly, there is little doubt that when the well path needs complex geometry, it may not be possible to make the necessary turns without an RSS. But, what happens when that same RSS tool has to be pulled because the complex electronic package has failed, when it can not build angle at the required rate, or if something mechanical packs-in or falls off?.
MOTOR/VGS COMBINATION
North Sea companies like BOL, Conoco Phillips, CNR, Devon, Encana, Gaz de France, NAM, Nexen, Shell, Statoil, Talisman, Wintershall, etc., appear to have found the answer and regularly run a motor/Variable Gauge Stabilizer (VGS) combination when the well profile is less complex. Historically, three out of every four correction slides with an RSS are for inclination. Adding a VGS to a positive displacement motor (PDM) allows the driller to correct for inclination changes without having to orient and slide. The motor/VGS becomes, in effect, a low-cost rotary steerable system.
Faced with rising rig rates, operators are again looking at the motor/VGS, because it is less expensive to run than an RSS. Motor prices have halved over the past six years and, although VGS prices have remained firm, it is fair to say that a motor/VGS today costs no more than a motor run did at the end of the last century.
Motors have continued to develop, have become more powerful and have gained in downhole reliability. It is interesting to note that while Baker Hughes Inteq (BHI) promotes the virtues of its RSS, it has also continued to develop its PDM fleet. Their newest addition is the X-treme motor, i.e., a special performance motor for high-profile, performance applications that is reported to handle higher temperatures, greater torque, WOB and more aggressive drilling fluids. BHI has recognized that the motor still has a fundamental part to play in the directional market. They, too, see the operator becoming more discerning of where and how they run downhole packages.
Similarly, National Oilwell Varco (NOV), showed its new two-speed, downhole-switchable motor at last year's OTC. All the big motor providers have announced significant motor footage drilled and motor revenue increases over the past 12 months, over and above the global drilling activity increase.
A similar evolution has occurred with the VGS. Hydraulic activated tools have replaced the original weight-set actuation tools. With continuing advances in motor technology, a larger VGS gauge change was required. The new tools offer a choice of blade positions, while reliably indicating blade gauge from any depth. If a PDM could drill for 600 hours or more, then a VGS had to reliably operate and signal gauge change to the surface over that same time. The new VGS tools had to be user friendly, so the directional man could run them without a specialist engineer.
CASE HISTORY
The requirement to improve both performance and reliability persuaded Statoil to look at the motor/VGS when planning the drilling program for their South PARS field, offshore Iran. An extension of Qatar's giant North field, South PARS covers an area of 500 sq m with gas located 3,000 m below the seabed, Fig. 1.
[FIGURE 1 OMITTED]
Required to batch drill all their 12 1/4-in. sections, Statoil wanted tools that were reliable. When BHI was awarded the directional contract, it decided to run its new motor. But, due to the irregular formations and the need to keep sliding to a minimum, the company required a VGS tool to provide the required build, hold and drop with the same assembly. The company also wanted a tool that would signal reliably from depth, no matter the length of time in-hole. The tool also had to be simple to use, as rig space was at a premium. Above all, the VGS had to be reliable. Because Statoil wanted to batch-set their 12 1/4-in. sections, both motors and VGS tools would remain on the rig for making multiple downhole runs.
Having run Hydrastab H-VGS tools extensively on BOEs Britannia platform in the North Sea, BHI approached Toolbox Drilling Solutions and asked if it could provide 11-in. to 11 5/8-in. to 12 1/4-in. tools for the PARS project.
Author: Jon Symons
Previous article: 10 November 2006
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