The importance of battery selection for demanding downhole electronics applications, in particular measurement-while-drilling tools, is discussed by Saft’s Wayne Pitt.
As oil companies strive to reap the maximum potential from their prospects, modern oil exploration drilling has become a multi-dimensional project.
Directional drilling is increasingly being used to fully exploit oil reservoirs. Remaining reserves are often difficult to access and can only be reached by angling the tool horizontally. When drilling at multiple angles, it is imperative that measurement-while-drilling (MWD) tools reliably convey vital data to surface operators.
MWD tools incorporate a variety of electronic sensors, such as accelerometers and magnetometers that provide the operator on the surface with real time information about the tilt and position of the wellbore, as well as details about the conditions at the drill bit, such as rotational speed, torque/weight on the bit, vibration, temperature, and type of rock formation. The electronics are completely powered by onboard battery systems and the data is typically transmitted by mud pulse or electromagnetic telemetry.
Demanding application
MWD is a highly demanding application that creates a challenging environment for the batteries. They must be able to operate over a wide temperature range – from well below 0°C at the surface in Arctic oil and gas exploration projects to well over 100°C during drilling – while enduring very high vibrations (20 g rms). Another requirement is complete reliability and long life time, typically from a few hours to more than 20 hours. Each time the bottom-hole assembly has to return to the surface it costs tens of thousands of dollars in downtime, so premature withdrawal to replace a failed MWD battery adds significant costs to the operation.
During drilling operations, there is continuous mud flow and the battery delivers a low idle current to the MWD tool. Drilling is often stopped and then restarted, resulting in frequent battery replacement. If, for any reason, the drilling operation has to be stopped early to replace a drill bit, the MWD tool must return to the surface and the battery must be replaced to ensure there will be a sufficient safety margin of battery power to maintain operation of the MWD tool through to completion.
Often, a primary battery might have to be discarded with much of its capacity unused.
Recharge while drilling
Until recently, specialized primary lithium batteries were the only product capable of providing reliable, cost effective operation in harsh MWD conditions. Saft offers primary lithium batteries for the oil & gas market and also recently launched the world’s first rechargeable lithium-ion (Li-ion) cell capable of operating at temperatures up to 125°C under drilling conditions. This is a significant increase in the previous Li-ion operating temperature – 65°C – opening new horizons for MWD tool manufacturers. For the first time, MWD tool developers can incorporate a highperformance rechargeable battery into their designs. This development eliminates the need to withdraw a MWD tool for replacement of a spent battery, with the benefit of improved continuity for the drilling operation.
Saft’s VL 25500-125 C-size and new VL 32600-125 D-size cells are intended for use in constructing batteries to be integrated into sophisticated MWD tools that incorporate onboard alternator technology, driven by the mud flow, to power their electronic systems. When the mud is flowing, the battery will be charged. When the flow stops, such as when drilling is halted, the battery will be discharged to provide power for the MWD electronics. When the mud flow restarts, the battery is recharged.
The fast-charging, deepdischarge and high-cycling capability of the Li-ion electrochemistry will enable the MWD tool to remain in continuous downhole operation.
The C-size cell functions as an energy buffer and is commonly used in oil & gas drilling applications, while the higher power D-size cell was designed for oil exploration operations.
Mechanical integrity
The Li-ion cells are integrated into customized, rugged, cylindrical MWD battery staves.
A key part of the stave design is to provide complete mechanical integrity, even under extreme temperatures, vibration and pressure. This involves careful selection of the construction materials and specialized manufacturing techniques, such as the crossply, tape-wrapping process. The staves also incorporate electronic controls, such as diodes to protect primary cells or balancing circuits to manage rechargeable cells. OE
About the author
Wayne Pitt is marketing & business development manager for the Saft Specialty Battery Group. He worked with a number of international companies in the electrical engineering and electrochemical energy systems fields before joining Saft 12 years ago, and has since served the company in various applications engineering, design, marketing and commercial roles.
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