By Scott Keller, CEO/CTO, SignalFire Remote Telemetry www.signal-fire.com scott.keller@signal-fire.com
Spurred by the growth of hydraulic fracking within the United States, flare gas regulations have become tighter, requiring companies to more closely monitor the total flow of gas on their sites.
The release of gas at a well in upstream oil and gas operations is a common occurrence, primarily conducted for safety and/or economic reasons. When oil is extracted from a well, any present gas is separated from the oil and either routed to a pipeline or flared (burned) at the site. As the gas is less valuable than the liquids in many cases, collecting it is economically unfeasible or the required pipeline infrastructure is yet to be built out. In these situations, oil producers choose to burn gas in a flare system to dispose of it. This is known as gas flaring.
Gas flaring or venting also ensures that natural gas can be disposed during an emergency or shutdown situation. Even a well that sends gas to a pipeline may need to flare it if the pressure in the pipeline exceeds a threshold value. This scenario occurs if the processing plant at the end of the pipeline shuts down for some reason. In this case, the pipeline will “back up” and wells will need to flare the gas locally or shut-in. Where gas cannot be stored or piped, the risk of fire and explosion must be reduced by either flaring or venting.
State and Governmental Regulations for Gas Emissions
In April 2012, the U.S. Environmental Protection Agency issued federal rules requiring companies to create a plan to monitor and control emissions by January 2015. These regulations are aimed at newer (hydraulically fractured) wells.
States (notably Wyoming and Colorado) also have local requirements on gas emission monitoring and control. While not every operation is in compliance, more and more companies are moving toward monitoring the amount of gas flared or expelled.
As gas is considered a greenhouse source, these regulations are being passed in order to capture and understand the amount of gas being flared to the atmosphere. The concept is that monitoring will eventually lead to a better understanding of the amount of gas flared to the environment by different companies in certain regions of the country. Results will be used to put more pressure on capturing this gas and use it as an energy source.
Monitoring Methods
Different challenges exist when measuring and/or monitoring flare gases related to changing gas composition, flow variation, and hazardous locations. EPA regulations also require a 5% measurement accuracy. These challenges must be taken into consideration when determining which technology to use for monitoring gas flow. Several technologies can be used:
- Thermal Mass Flow Meter introduces heat into the gas flow stream (usually an insertion probe), and measures how much heat dissipates using one of more temperature sensors. As the mass flow increases, more heat is dissipated thus the flow can be determined.
- Ultrasonic Flow Meter measures the time-of-flight of a pulse and calculates velocity from the timing.
- Optical Flow Meters use laser beams to measure gas flow by sensing the velocity of microscopic particulates in gas.
- Pressure Differential (such as Orifice Plate, Venturi, Pitot Tube) use pitot tubes to measure gas pressure (like an airplane’s airspeed indicator) or the differential pressure across a restrictor plate.
Each of these has these technologies has its own benefits and issues. See Table below.
While there is no one ideal technology that addresses every situation, in general, flare applications where a modest amount of gas is expelled and cost is an issue, thermal sensors are often the best choice. In larger flow applications where cost is not as much of an issue, ultrasonic sensors are often used. In these cases, the sensors can have custody accuracy.
Whatever technology is used, the monitoring system needs to be able to totalize the daily gas amount (Standard Cubic Feet) over a settable contract (24 hour) period, so a settable real-time clock should be part of the equipment. See Pic 1 for an example of a how a module can be used to provide instantaneous totals of gas flow rates to help address flare gas regulations.
Conclusion
While most small (well pad) flare systems currently are not monitored, regulations will accelerate the need to monitor gas flow. In turn, it is likely that gas flow monitoring will drive the push for capturing this gas as it has value. While a number of technologies exist for monitoring flare gas (with thermal being the most prevalent), the push for monitoring compliance will, most likely, stimulate the growth of more technologies to address this application.
| Flow Meter Technology | Cost | Pros | Cons |
| Thermal | Medium | Easy installation, accurate enough, adequate range, measures mass flow | Composition change and moisture = inaccuracy |
| Ultrasonic | High | High accuracy, large range, immune to composition of gas | Cost, complex installation, need pressure sensor to compute mass flow |
| Optical | High | Easy to install, good accuracy, large range, immune to gas composition | Cost, relies on particles in gas stream |
| Delta P | Medium | Common technology used on other flow applications | Restrictor plate is not recommended for safety applications like this |
| Pilot/Venturi | Low | Cost | Low accuracy, low turndown, not good at low flows |
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