Vertically drilled oil wells are a thing of the past - modern oil exploration drilling is a three-dimensional activity, as oil companies strive to reap the maximum potential from their prospects. But directional drilling brings its own set of problems, not least being the need to convey vital information such as inclination and position of the boring tool. In addition, there are numerous other parameters, such as drill bit condition, its rotational speed, torque, temperature and so on that have to be conveyed in real time to the surface operators.
Measurement-while-drilling (MWD) tools incorporate a number of sensors to convey all this information and, as conditions prohibit the use of drill bit-to-surface cables, data is commonly transferred to the surface via an electro-acoustic method called mud pulse telemetery. Drilling mud has a variety of important functions, but its flow is also used to turn an electric generator, situated aft of the drill bit, for powering the downhole electronics. When mud flow ceases (for example, during the installation of a new drill pipe section), internal batteries supply pulse currents.
Until recently, only primary cells (lithium-thionyl chloride) were available to cope with the extreme temperatures and excessive shock and vibration that MWD tools are subjected to, but primary cells require periodic replacement. Pulling a drill string of several thousand metres just to change batteries takes time and costs a great deal of money, so the race was on to develop a rechargeable alternative that could effectively be fitted and forgotten for the duration of the drilling programme.
Saft has been a front-runner in this race and appears to have beaten off the competition with a new development that it announced late last year. The VL 25500-125 is a rechargeable lithium-ion (Li-ion) cell with an electrolyte and electrodes capable of operating at temperatures as high as 125ºC (previously only 65ºC) and a new mechanical construction capable of tolerating extreme levels of shock and vibration. This breakthrough - which Saft is understandably not willing to describe in any great detail - means that rechargeable batteries will, towards the end of this year, be deployed for the first time in MWD tools. Recharging takes place when mud flow resumes following a change of drill section, and the integral generator takes over.
Complementing this cell development is a dedicated battery design and assembly facility at Saft’s Poitiers factory for the volume production and testing of bespoke cylindrical MWD batteries called ‘staves’. The resin-potted staves, accommodating as many as 20 cells each, are arranged radially with respect to the core of the MWD tool stem (rather like bullets within a revolver barrel) and this assembly is sealed. The staves also contain all the necessary electronic balancing circuits.
In the meantime, Saft has also released a new high-temperature D-size primary lithium cell, the LSH 20-150, capable of operating at temperatures as high as 150ºC. This new cell will immediately address the battery needs of mid-temperature applications (125-150ºC) that dominate the MWD market. Field trials of the primary staves are current, whilst rechargeable staves containing the new VL 25500-125 cells are due to go into service at the end of 2008 following trials with an as yet unnamed oil well tool company.