Clamp-on Ultrasonics Solve Storage Well Measuring Challenges

By Jack Sine | July 2012, Vol. 239 No. 7

Clamp-on ultrasonic meters provide accurate measurement for natural gas reservoir storage wells and in several other applications. Natural gas is stored during the warm months so it is available when demand increases.

The most used storage facility for natural gas is depleted gas wells. Because it is mandatory for gas producers to know the quantity of the gas being stored and the pressure, they may hire specialty companies to monitor the transfer of natural gas into and out of the reservoir wells.

Eastern Reservoir Services (ERS) is one such company that provides flowback, slickline and testing services for some of the leading gas producers, including EQT, Dominion, Columbia Gas and National Fuel.

Bram Alackness, ERS field engineer, explains, “We use a variety of monitoring systems to test the flow into and out of the storage wells. The tests are called Deliverability/Performance Tests and basically help determine at what rate the gas will flow based on varying pressures.

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The test results can also help determine well characteristics like skin factors, permeability, porosity and reservoir capacity. The client uses the data we gather to determine the performance they can expect from each well - what each well can provide under what pressures when it is time to get the gas out of the storage well. We use a variety of meters including orifice meters, turbine meters and gas flow provers.”

Alackness continues: “All have their problems, however. The most common meter type, orifice meters, are very accurate when conditions are ideal, but they can easily clog with sand and other contaminants. Moisture and wear on the orifice (also) affect accuracy.

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“We had been looking for some time for a way to be able to test areas where we didn’t have permanent installed meters. Last year I was given the assignment to identify a portable meter that didn’t involve cutting into pipe or releasing gas into the atmosphere. It would have to be battery powered and have memory capability so we could get the readings back to our data technicians.

“It soon became apparent that clamp-on ultrasonic metering was the way to go. I researched all of the meters available and narrowed it down to FLEXIM. We had a few meters in for demonstrations and it won out because it had the features we needed, including a 16-hour battery. It was also the most intuitive and fast to install,” Alackness said.

How Ultrasonic Flow Meters Work
The technique most ultrasonic flow meters use is called transit-time difference. It exploits the fact that the transmission speed of an ultrasonic signal depends on the flow velocity of the carrier medium, kind of like a swimmer swimming against the current. The signal moves slower against the flow than with it.

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When taking a measurement, the meter sends ultrasonic pulses through the medium, one in the flow direction and one against it. The exterior transducers alternate as transmitters and receivers. The transit time of the signal going with the flow is shorter than the one going against.

The meter measures transit-time difference and determines the average flow velocity of the medium. Since ultrasonic signals propagate in solids, the meter can be mounted directly on the pipe and measure flow non-invasively, eliminating any need to cut the pipe.