How ground-level point sensors detect emissions from tank tops

By: Lukasz Zielinski, Scientific Advisor, Schlumberger-Doll Research; and Drew Pomerantz, Emissions Technology Manager, SLB End-to-end Emissions Solutions

Point sensors have become a popular technology for detecting and quantifying methane emissions from oil and gas operations. Using this method, an array of sensors are installed on a facility, often around the fence line. If methane is emitted, wind will carry that methane off the facility, and the point sensors are positioned to intercept the emitted plume. The closer to the plume center the sensors are installed, the more methane molecules there will be to detect, and the more sensitive the network will become.

A key question when mounting point sensors is what height off the ground to install them. Of course, point sensors should be at the height with the greatest methane concentration, but what height is that?

Several factors are at play: methane can be emitted at different heights, often on top of large objects like tanks; if the methane is at high pressure, then momentum can carry the emitted jet up or down. Conventional wisdom might also suggest that, being lighter than air, methane would rise due to buoyancy.

Considering all those factors, some companies prefer to install a three-dimensional block of sensors including towers of multiple sensors at different heights. But is that extra effort really necessary? Others prefer to install sensors at the height of the highest emission point, expecting buoyancy to lift all the emissions to that level. But does it? Or will some go elsewhere and not be counted?

Into the wind tunnel

Scientists here in the SLB methane team wanted to understand how methane plumes rise and fall in the air so we can identify the best height to install point sensors. To develop that understanding, the team carried out a two-pronged approach.

First, they performed experiments in one of the world’s largest boundary layer wind tunnels at the University of New Hampshire’s Flow Physics Facility. In those experiments, methane was emitted in one section of the tunnel, carried by the wind down the tunnel under carefully controlled conditions, and measured across the tunnel at various distances from the release point. The methane was released at different flow rates, under a range of tunnel wind speeds, and with variable-sized obstacles in its path. 

Second, the team simulated the flow of methane plume using computational fluid dynamics (CFD), which is a branch of fluid mechanics that simulates fluid flow and is used in various applications such as designing airplane wings.

One example of the results of those measurements and calculations is shown below. This example studied methane emission from the top of a tank. In the wind tunnel, methane was released from the top of a 3D-printed cylindrical block via a small pipe through its center. Likewise, in the CFD simulation, the block was modeled as a solid object impenetrable by the wind.

Both the experiment and the calculation showed the same surprising phenomenon: rather than rising up due to buoyancy and initial jet momentum, the plume was pulled down and rapidly dispersed downwind of the tank.  

Weak buoyancy …

How can it be that a gas lighter than air and emitted with a positive upward momentum moves down, not up, under these conditions? 

The explanation is twofold. First, it turns out that the buoyancy effect is rather weak. If you consider a parcel of air in a typical methane plume, that parcel contains mostly air with relatively small amounts of methane. 

For example, the SLB methane point instrument is designed to detect 1 part per million of methane in the air, reflecting the relatively small concentrations of methane present in a plume at any distance away from the source. Because the concentration of methane in the plume is low, the effect of buoyancy is small.

The second reason is the fluid dynamical effect aptly called downwash. When the wind blows across a solid object like a storage tank, air can move above the tank and around the tank but obviously not through tank. That creates a low-pressure zone directly behind the tank, which pulls in air from the top and the sides of the tanks. 

… meets downwash dominance

So, as the plume travels over the edge of the tank, it gets pulled down toward the ground and recirculated as if it were released from the ground level at the downwind edge of the tank.  It turns out that downwash is typically the dominant effect, overcoming the weak impact of buoyancy and initial jet momentum, and causing methane plumes to move down. 

On a practical level, downwash means that point sensors installed close to ground level can detect methane emitted from elevated locations, particularly if the methane is emitted from the top of wide objects, like tanks, that create significant downwash.

The team has measured the downwash effect across a variety of wind tunnel experimental conditions and cemented our understanding of the effect by developing CFD models validated with the experimental results. 

That understanding is used to develop improved correlations between the concentration of methane measured by the point sensor and the rate at which methane is being emitted, allowing our methane point instrument to quantify methane emissions accurately, even for emissions coming from above the sensor height.  

The same scientific insight is also included in the prevailing meteorological conditions (PMC) algorithm that evaluates the facility layout and local wind conditions to optimize the location and heights of point sensor installation, enabling the methane point instrument to effectively monitor emissions with a small number of optimally located devices.

Not so simple tech

Point sensors appear to be simple instruments—they are small devices that operate in the background, sniffing the air and finding emissions on their own. But under the hood, they are powered by sophisticated science and engineering.  

The downwash effect is one example: a combination of wind tunnel experiments and CFD simulations shows how a methane plume can come down, even though methane is lighter than air. Understanding the downwash effect lets us detect methane emissions from elevated sources, quantify elevated methane emissions accurately, and position point sensors optimally to get the largest methane signal from the smallest number of sensors.

Check out our methane point instrument and the science behind it.