That window-rattling percussive sound is getting louder, as an orchestra of service companies, operators, consultants and other interested parties beat the oilfield automation drum. Decibels aside, McKinsey & Company’s observations are also a useful description of its timbre:
“The rapid progress of technology, such as big data and analytics, sensors, and control systems, offers oil and gas companies the chance to automate high-cost, dangerous, or error-prone tasks. Most oil and gas operators are starting to capture these opportunities and would do well to accelerate their efforts. Companies that successfully employ automation can significantly improve their bottom line.
“While automation offers many potential benefits in the upstream value chain of exploration, development and production, some of the biggest opportunities are in production operations, such as reducing unplanned downtime. Given the oil and gas industry’s substantial increases in upstream capital investment, optimizing production efficiency is essential. Automation creates several opportunities to that end: maximizing asset and well integrity, increasing field recovery, and improving oil throughput.”
No argument there, but, as the ancient proverb warns, “there’s many a slip ‘twixt the cup and the lip.” As increasingly sophisticated gadgetry goes downhole, in the service of automation, one of those banana peels could be an economical and reliable downhole power source. What’s automation without control, and control means having these gadgets do something useful when you tell them. And, in some cases—sliding sleeves come to mind—this action should be repeatable and reversible. Needless to say, sensing and data reporting are essential and also require energy.
Currently available power sources for downhole use have limitations. Devices like springs, propellants and rupture disks are useful and effective for one-time events. Turbines require flow. Batteries are life-limited and can be difficult or impossible to replace downhole.
“There is a critical need for robust and reliable downhole power generation and storage technologies, in order to push the boundaries of downhole sensing and control. Downhole power harvesting is an enabling technology for a wide range of future production systems and applications, including self-powered downhole monitoring, downhole robotics, and wireless intelligent completions.”
So say Ahmad, et al, in their 2015 paper, “Piezoelectric Based Flow Power Harvesting for Downhole Environment.” The paper intends to analyze available ambient energy sources in the downhole environment, and various energy-harvesting techniques.
Downhole energy sources. The authors survey ambient energy sources present in the downhole environment: [These] include mechanical energy of the production flow, and thermal energy inside the well. Conversion of thermal energy into useable electrical energy is not practical, as thermoelectric generators require a temperature difference to operate. The temperature is almost constant in the horizontal section of a well, while the temperature gradient in a vertical section is too low (0.025° C/m) to be useful.
The industry also has looked into the possibility of using a radioisotope thermoelectric generator (RTG) in a well [Tosi 2013]. It uses an array of thermocouples to convert the heat released by the decay of a suitable radioactive material into electricity by the Seebeck effect. One end of the thermocouples is connected to radioisotope, with the outer end of each thermocouple connected to a heat sink. Creating a heat sink in the downhole environment is very challenging because of the presence of high temperature and thermal energy.
Therefore, the most practicable solution for long-term reliable downhole power generation is scavenging the mechanical energy of the production flow. Despite the large amount of mechanical energy present in the flowing fluid, downhole energy generation provides the challenge of long-term robustness.
Energy harvesting methods. Mechanical energy harvesters scavenge energy from vibrations, mechanical stress or strain of the surface that the sensor is deployed on. In case of downhole application, the energy harvester has to be in contact with flowing production fluid, which will produce the required vibrations or mechanical stress/strain.
Piezoelectric. A piezoelectric energy harvester converts mechanical energy into electrical energy by straining piezoelectric material. If these materials are subjected to mechanical strain, it causes charge separation across the device, producing an electric field and, consequently, a voltage drop proportional to the applied strain.
Magnetostrictive. These materials are used to harvest ambient vibrations, based on the Villari effect. These materials deform when placed in a magnetic field and, conversely, a change in magnetic field occurs if the material is strained.
Magnetic induction. A magnetic field can be used to convert mechanical energy to electrical energy. When a coil attached to an oscillating mass traverses a magnetic field that is established by a stationary magnet, the magnetic flux around coil changes, which induces a voltage according to Faraday’s law. The authors present a single-phase flow harvesting system, based on vortex-induced vibrations by employing a bluff body in fluid flow, and using a piezoelectric device.
They conclude that a robust energy harvester capable of power generation in the range of 100s of milliwatts can enable self-powered downhole sensing and control, if combined with a reliable high-temperature rechargeable battery.
They note that new R&D initiatives are required to develop wideband flow power harvesters and rechargeable batteries for harsh downhole environment. WO
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