The key factor in the Li-ion cell life is operating temperature. At 255°F (125°C) the cell capacity will drop by 25% after 35 cycles. At 229°F (110°C) the graph is much flatter, with 75% capacity remaining after 90 cycles. (All figures courtesy of Saft).

MWD tools incorporate a variety of electronic sensors to detect such things as the tilt and position of the well bore, rotational speed, torque and weight on bit, vibration, temperature, and type of formation. Since there can be no direct cable link from surface to the drill bit, the electronics have to be powered by onboard battery systems. The data collected is relayed to the surface either by mud pulse telemetry — the most common technique — or electro-magnetic telemetry.

Demanding application

MWD is a highly demanding application that creates a challenging environment for the onboard batteries. They must operate over a wide temperature range — from well below 30°F (0°C) at the surface in Arctic oil and gas exploration projects to well over 210°F (100°C) during drilling — while enduring very high vibrations and provide complete reliability for long periods. This is especially important as each time the bottomhole assembly has to return to the surface the downtime costs can be high. Replacing a failed MWD battery can add significant costs to the operation.

For some time, specialized primary lithium cells have remained the optimum choice for batteries capable of providing reliable, cost-effective, operation in MWD conditions. However, Saft has now developed the world’s first Li-ion (lithium-ion) cell capable of operating at temperatures of up to +255°F (+125°C). This major leap in Li-ion operating temperature, from the previous current maximum of +148°F (+65°C), opens up new horizons for MWD tool developers and manufacturers.

Why use a rechargeable battery?

The potential advantages of using rechargeable batteries in MWD tooling are best illustrated by considering the various sequences in the duty cycle of the a current primary (non-rechargeable) onboard battery:

- During drilling operations there is a continuous mud flow. The battery delivers a low idle current to the MWD tool.

- When a stand of drill pipe has to be added drilling is halted and mud flow stops. During this quiet period the survey sensors in the MWD tool are activated. The battery delivers a relatively high pulse current for a few seconds followed by a lower pulse current for several minutes.

- Drilling is restarted. Mud flow resumes and the battery delivers a pulse current for a few minutes as the data is transmitted to the surface by mud or electromagnetic telemetry. The current demand from the MWD tool then drops back to idle until the next survey is required.

Typically, this duty cycle requires a primary battery to be sized to deliver six, 10-minute pulse cycles per hour over a drilling operation lasting around 80 hours. At the end of the operation the spent battery will need to be replaced. If, for any reason, the drilling operation has to be stopped early such as to replace a drill bit and the MWD tool is returned to the surface, then the battery must still be replaced. This is to ensure that there is a sufficient safety margin of battery power to maintain operation of the MWD tool through to completion. A primary battery might have to be discarded with half of its capacity unused.

An attractive alternative under development is to substitute the primary battery with a power system comprising a generator turbine propelled by the mud flow that charges a rechargeable battery. Now, rather than acting as the main source of power, the onboard battery functions as an energy buffer.

During the survey period, the operation of an MWD tool equipped with a rechargeable battery is similar to a tool fitted with a primary battery — the survey is carried out during quiet periods, as the new stand is added, and the battery discharges. The major difference is that when the drilling starts, the telemetry pulses transmit data to the surface, the mud flow spins the turbine and the battery then recharges.

Instead of sizing a battery to deliver all the power needed for the MWD tool for an 80 hour tour of duty, the designer can now specify a much smaller and lighter battery that only has to support around six to 12 cycles before it is recharged.

Furthermore, the rechargeable battery will last for many operations before it needs replacing. This could eliminate completely the need to withdraw an MWD tool to replace a spent battery, with the benefit of improved continuity for the drilling operation. It is also no problem if a halt in the operation means that the MWD tool has to be brought up early, as the battery will not now need to be replaced.

Adapting Li-ion technology

The fast-charging, deep-discharge and high cycling capability of Li-ion electrochemistry are ideally suited for MWD tools. Also, since Li-ion batteries first became commercially available in the early 1990s, they have established an excellent reputation for delivering high levels of reliability in the extremely arduous conditions found in military and spaceflight applications.

There was one major challenge that needed to be overcome, since the maximum safe operating temperature of existing Li-ion cell designs was +148°F. For MWD tools, the cell design has been refined to raise this threshold to +255°F.

Cell design considerations

The increase in operating temperature has been achieved by focusing on three aspects of a cylindrical Li-ion cell:

Electrolyte. Normally, the organic electrolyte is expected to provide operation over a range from -41/-59°F (-40/50°C) to +148°F. This has been replaced by a specialized electrolyte that operates from 30°F to +255°F.

Separator. The cell separator is a thin plastic membrane that separates the positive and negative electrodes while still allowing ion exchange to take place. In a standard cell this would provide a safety shutdown at +237°F (+135°C). A new separator has been developed and alternative safety devices introduced.

Electrodes. New lithium oxide positive and carbon negative electrodes that remain stable at high temperatures have been developed.

In addition to meeting the temperature requirements, a new rugged design of stainless steel can and internal electrical connections have been developed to meet vibration and shock criteria. These include withstanding 750 G peak shocks, a 20 Grms random vibration profile and a linear sine sweep at 30 G peak.

Battery life optimization

Li-ion cells in an MWD application do not suffer from sudden death. Instead, there is a gradual decrease in cell capacity restored by charging over a period of time. This is related to operating factors such as temperature, depth of discharge (DOD) and number of cycles. So the cell life is determined as the percentage of the initial capacity that remains.

For example, at 100% DOD (when the cell is discharged completely down to its minimum operating voltage) at 255°F the cell capacity will drop to 75% after 35 charge/discharge cycles. At 25% DOD at the same temperature the cell will deliver 200 cycles before it drops to 75% capacity.

The key factor in the Li-ion cell life is operating temperature. In practice, we anticipate that Li-ion batteries will mainly be operated at the cooler MWD temperatures of 193°F to 229°F (90°C to 110°C). This means that they will offer extended life as well as a substantial safety margin should they experience a hotter environment for short periods of time, such as when drilling through a pocket of hot gas.

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