With the steady increase in the number of directional drilling operations over the past 3 decades, return on investment relies increasingly on directional drilling technologies. Timely transmission of key data from the bottom hole assembly to the rig floor is critical for the efficiency of the drilling operation and the quality of the borehole - both in structure and in placement relative to the payzone. Simply speaking, two main forms of telemetry for directional drilling exist today - Mud Pulse Telemetry and Electromagnetic (EM) Telemetry.
Mud Pulse telemetry relies on the flow of hydraulic fluid - "mud" - that flows down the center of the drill pipe and back up its annulus both cooling the bit and carrying cuttings away. A mud pulse tool restricts that flow in a controlled fashion to create pressure pulses that propagate to surface where they can be detected with pressure sensors in the fluid flow.
Mud pulse tools come in a variety of forms. Poppet-style Mud Pulse tools are the most common. These include a small poppet valve assembly that opens or closes creating pressure pulses that propagate to the surface. The mud pulses encode data by varying the time between pulses in a method known as time shift keying. This is a time-proven method of telemetry, however it has some drawbacks compared to other methods Poppet-style mud pulsers suffer from low pressure amplitudes making it difficult to detect signals on surface and reducing bandwidth or data rates. Poppet-style mud pulsers also suffer from a high rate of clogging under moderate to high concentrations of lost circulation material (LCM).
On the other hand, Rotary Mud pulsers employ a valve assembly that rotates an obstruction into or out of the path of the mud flow in order to create pressure pulses. This method is more robust to clogging and has been proven to transmit successfully in very high LCM conditions - "mud so thick you can walk on it," or more precisely 80+ lbs of LCM per barrel. It also offers higher pressure amplitudes, making detection easier and increasing bandwidth and data rates. However, the performance of Rotary mud pulsers has been constrained based on the high instantaneous current draws they demand when the rotor is moving. This current can instantaneously draw down the terminal voltage of the toolstring's battery, causing issues for other systems sharing that buss, prematurely ageing the battery, and limiting the useable lifetime of the battery. In extreme cases, this current draw can cause catastrophic failures of the battery resulting in loss of the tool downhole or loss of the borehole altogether. On surface, similar failures can lead to dangerous conditions for users to say the least. All of these factors limit the performance that we can derive from our Rotary mud pulsers. Performance requirements and battery life must be weighed against reliability, safety, cost and complexity considerations when choosing a battery for these situations.
Nanoramic's FastCAP ultracapacitor (also known as supercapacitor or electrochemical double layer capacitor) technology offers an alternative method - high power, ruggedized ultracapacitors that provide the tool designer a way to decouple the instantaneous power demands of a Rotary mud pulser from other considerations. The result is safer, more cost-effective solutions that allow us to access higher levels of mud pulse telemetry performance. For example, the data below shows high instantaneous currents from a common Rotary Mud Pulser in Green. On the left, the blue line shows the effect on a high power downhole battery's voltage. On the right, the blue line is now stable thanks to Nanoramic FastCAP ultracapacitors. The implications are "beginning of life performance through the useable life of the battery," more cost-effective utilization of the battery, the ability to deploy safer, higher energy, and less costly low or moderate rate batteries where high rate batteries were previously needed, and perhaps most significantly, opportunities to increase the torque and power of the rotor for higher data rates and resilience to LCM.
Read more about this technology in our free white paper Ultracapacitors for Pulsed Power Systems.