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RFID-Enabled Valves Promise to Maximize Oil Well Output
To control valves and other downhole tools, an operator drops passive RFID tags, encoded with instructions, into a well.
In Europe and North America, Petrowell carried out multiple tests with Marathon Oil Co., which helped develop some of the technology and is a partner on several of the patents. During those tests, Petrowell mounted a battery-operated RFID reader onto a single valve. Operators placed handfuls of loose, glass-encapsulated RFID tags into the well, which flowed down vertically and out horizontally, communicating with the interrogator as they passed through the valve.
"By controlling valves," Purkis explains, "the system equalizes the flow of oil across the reservoir so that we get more oil—and get it quicker." The tags deliver command code messages to specific tools in the hole, and the codes instruct specific valves to take no action, or to take action, such as opening or closing a particular amount. "The message to tool number six may be 'shut' or 'open' or 'close 10 percent,'" he says.
According to Purkis, the system is designed in a redundant fashion—multiple tags encoded with the same instructions are thrown down the hole—in case some of the tiny RFID tags are unreadable, or break once inside the hole. In addition, checks are built into the system so that once a tag passes through a valve, it cannot pass back up through that same valve and cause unwanted reactions from the tools.
"The worst thing that could happen," Purkis says, "is a rogue RFID tag makes its way through the system and activates a tool it shouldn't, or at the wrong time. This could cause a blowout condition, and then a fire." To keep this from occurring, and to gain the necessary security permits, users encode time and sequence stamps onto the RFID tags before they are dropped into the hole. In this way, the tags' commands expire after a specified period of time, and the interrogators react only to new tags. Once the tags reach the end of the well, they become lost there, or else embedded in the rock.
The tags measure either 4 millimeters (0.2 inch) in diameter and 24 millimeters (0.9 inch) long, or 2 millimeters (0.1 inch) in diameter and 8 millimeters (0.3 inch) long, and are purchased from Texas Instruments and modified by Petrowell. They are inserted into the well by hand, though the company is currently designing a tag injector that would put them in automatically.
Read rates are determined by how fast tags travel down the hole, Purkis notes, and by the amount of flow turbulence. The read rate averages approximately 60 to 70 percent for a single tag, but operators can insert 10 identical tags simultaneously to raise the rate to nearly 100 percent. For instance, operators send down 10 tags containing an instruction to "close valve six by 5 percent." The change in valve settings, he says, is obvious in the flow at the top of the well.
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