Best Practice for Measuring Vent Gas Volumes

Vapor Recovery Units (VRUs) are an essential element in an efficient production system that minimizes emissions and optimizes profits. In our blog article How to Right Size a Vapor Recovery Unit we covered the most important variables for determining what size of a VRU a well site might need.

One of the key factors we identified was expected tank vapor vent gas volumes, which is our focus in this article.

Oil & Gas Tank Vent Gas Quantification Techniques

There are two primary methods for determining vent gas volumes from above-ground crude oil and condensate storage tanks.

Emissions Models. In the past, engineers could use TANKS emissions software from the U.S. Environmental Protection Agency (EPA) to estimate volatile organic compound (VOC) and hazardous air pollutant (HAP) emissions from fixed- and floating-roof storage tanks. EPA, however, has since discontinued support for the software.

Other emissions modeling software includes E&P Tanks from Informer Technologies and TankESP from Trinity Consultants. Software modeling can be an effective tool, but the old adage of “Garbage In, Garbage Out” applies. The outputs of any model are only as good as the inputs, which puts the onus back on estimation techniques.

VRT Volumes. For operators considering replacing an existing Vapor Recovery Tower (VRT), measuring the flash gas vapor quantities captured from the VRT is a relatively easy estimation method. However, like software models, assumptions are everything.

The key assumption using this method is relative VRT efficiency. Many sources assume that VRTs operate at 70% efficiency, meaning that VRT vapor capture measurements can simply be grossed up by 30% to estimate the tank vent gas volumes (from flash gas not captured by the VRT).

The downside to this rule-of-thumb approach is that VRT efficiency can vary widely not just between well sites, but also on the same well site depending on a variety of uncontrollable factors. Factors influencing variability of flash gas volumes between well sites include API gravity of the oil as well as VRT operating temperature and pressure. On the same well site, flash gas volume can vary based not only on the VRT operating pressure and temperature, but also on changing atmospheric conditions and ambient temperatures.

If actual VRT efficiency is lower than 70%, then estimates of tank vent gas volumes will be underestimated, potentially significantly lower than actual.

Direct Measurement. Directly measuring tank vent gas volumes using sensors attached to the tank and monitored electronically is typically the most accurate method for estimating tank vent gas volumes, but not commonly used. One key caveat is to measure volumes over a full production cycle, typically 24-48 hours to take into account variability of production and include the impact of production surges (or avoid missing them).

An alternative direct measurement technique is to collect a pressurized oil sample from the separator and send it to a laboratory to measure its composition and flash rate. This method is relatively fast, cost-effective, and provides accurate data for using in emissions models. Keep in mind, however, that the sample is only accurate for the well site it was taken from!

Don’t Forget the Water Tanks

Focus on tank vent gas quantification typically focuses on crude oil and condensate storage tanks for good reason, that is where the majority of tank vapors are generated. However, it would be a mistake to ignore tank vapor gas in produced water storage tanks.

Since phase separation equipment is rarely perfect, there is almost always some crude oil or condensate in produced water tanks. Just as it does in oil storage tanks, these vapors will come out of solution over time and collect in the water tank head space. We recommend it is best practice to include produced water storage tanks when estimating tank vent gas on the entire well site.

Best Practice for Measuring Vent Gas Volumes

In our view, directly measuring gas volumes from both oil and produced water tanks is the best practice for quantifying tank vent gas volumes. It may require a little more time to collect the data, but in the long-run you the result will be a well site operating at its full potential and profitability.

Benefits of Right-Sizing Your VRU

The benefits of right-sizing your VRU to the specific well site or facility include:

• Avoid potential “over treating” by installing a VRU that is too large for the application, resulting in higher than necessary operating costs.

• Avoid installing a VRU that cannot handle peak production levels, resulting in tank battery venting and flaring emissions.

• Achieving the full potential of the well site or facility, resulting in aligning costs with volumes and maximizing profitability by capturing valuable tank vapor gas from all sources.

The Platinum Advantage

Platinum VRUs punch above their power ratings. Platinum VRUs typically move 30% more gas volume for the same horsepower rating as competing units, because of our innovative design and quality materials. Our efficient, class-leading design means that in most cases a smaller and less expensive Platinum unit will do the same job of a larger and more expensive unit to significantly capture and sell more rich-BTU gas and put out the flare.

Flexible Fleet™ lease agreements. Our Flexible Fleet lease agreements allow you to move VRUs from one site or facility to another at will and without burdensome paperwork. That allows you to move larger units from an older site further down the decline curve that produces less tank vapor to a newer drill. That allows you to tailor VRU capacity to the individual site and to your entire field.

Contact us today at sales@platinumcontrol.com to learn more about our solutions and how we can help you harness the full potential of your well site.