August 23, 2016 by Amy Bowman
The Vast Limitations of Vapor Recovery
Methane and Hydrocarbon Emissions
The Vast Limitations of Vapor Recovery
Properly engineered and operated, Vapor Recovery Units (VRU)’s are utilized to reduce the amount of Volatile Organic Compounds (VOC) being emitted into the atmosphere, including methane, ethane, and vaporized Natural Gas Liquids (NGL). The sole purpose of a VRU is to capture a “hydrocarbon cloud” and re-compress these vaporized hydrocarbons into a high pressure gas pipeline. While the process sounds simple enough these units are extremely ineffective due to the presence of the heavier NGL’s that re-liquefy when compressed. The reason for the term “natural gas liquids” is because these VOC’s consist of mainly butane, propane, and pentanes that are a liquid at high pressure and a gas at low pressure. When NGL’s are produced and separated at high pressure these compounds will remain with the oil and are therefore dumped into the atmospheric storage tanks dissolved in the oil. Due to the fact that there is now no pressure to keep the NGL’s a liquid they naturally flash and vaporize into a gas in the oil storage tanks. Also, unknown to many outside the oil and gas industry, due to the nature of hydrocarbons, when under pressure, a large volume of methane and ethane are dissolved in these NGL’s that also vaporize when the pressure is released. It is also worth noting that while this problem exists during oil and gas production it also occurs throughout the midstream and downstream natural gas industry. If the gas has not been mechanically processed and stabilized to separate these compounds, when the gas stream is compressed and or re-pressurized, these liquids are created and must be separated.
The reason VRU’s have been accepted by the oil and gas and midstream industry is because they are relatively inexpensive and would “appear” to solve the problem but actually only reduce the emissions. Also, before the protection of the environment became the driving issue in eliminating hydrocarbons being emitted into the atmosphere the objective was to reduce waste. The most valuable gaseous hydrocarbons are the heavier gasses, which we will describe later in more detail, butane, propane, and pentane. Depending on gas prices, in many cases it was considered economically feasible to capture these gases, as they contain two to three times more energy and therefore more valuable than methane and ethane. Safety has also been a driver in the capture and or elimination of these gases as the heavier components are very slow to dissipate into the atmosphere like methane and ethane. Due to this fact, these gases tend to collect at ground level causing breathing problems for the facility operators and increasing potential of a devastating explosion.
As stated above, these heavier gases are more valuable because gas is purchased on a MMBTU basis. For reference the BTU average of methane and ethane is 1390/scf and the BTU average of NGL’s are 4381/scf. During the capture and re-pressurization of the hydrocarbon cloud, the most valuable liquids are dropped out of the gas through the change in pressure and are re-routed into the oil tanks to again flash off and potentially vaporize into the atmosphere.
Average composition of gas stream vaporized from a shale oil storage tank, with no VRU in
operation, average MMBTU, and average GWP per Mscf:
1.50% Carbon Dioxide
2,045,273 BTU per Mscf
821 lbs CO2 equivalent GWP per Mscf
Inherit flaws of the VRU process, of which there is no resolution; limit the recovery of the methane, ethane and vaporized NGL’s to approximately 50%. The reason for the dramatic inefficiency is due to the fact as describe above, when the VRU compresses the vaporized gas cloud the heavier components, the NGL’s are again liquefied, separated from the gas stream and therefore dumped back into the oil storage tanks. While the volume is reduced, below is the average composition of actual re-vaporized gas stream.
Average composition of gas cloud vaporized from a shale oil storage tank, with a VRU in
operation, and average MMBTU:
0.66% Carbon Dioxide
2,541,429 BTU per Mscf
732 lbs CO2 equivalent GWP per Mscf
As described above, there are several problems in the operation of a VRU, the inefficiencies do not end there. In order for a VRU to operate properly a positive pressure must be maintained at all times on the oil storage tanks. The reason this pressure must be maintained is so that oxygen does not get into the storage tank and contaminating the vaporized gas stream. A gas stream which contains oxygen is very corrosive in the gas pipeline causing gas leaks but it can also explode when compressed. During the operation of an oil and gas production the hatches are opened to gauge tanks and the gas is blown into the face of the operator and into the atmosphere.
When oil is loaded into a transport truck the tank must again be gauged by the truck driver and the hatch must be left open so that a vacuum is not created on the tank causing it to collapse. Unfortunately, due to operator error, the hatches are rarely closed and again these gasses are emitted into the atmosphere with or without a VRU.
Public health and safety must also be discussed, as this is the first concern when discussing hydrocarbons being vaporized into the atmosphere. As mentioned earlier, the majority of the gases escaping into the atmosphere are heavier than air and can therefore travel a surprising distance before dissipating. When someone mentions “smelling gas” from a production facility in the majority of cases what they smell is not methane or ethane it is the NGL’s because these gases remain close to the ground. At the front of the line on this concern is the facility operators and others that must occupy the location. There is always the presence of hydrocarbons that are inhaled by those in the area which can create respiratory issues as well as other health concerns. Next on the list of those being exposed are the people living in close proximity of the production facility and or those who are required to be in the area and are exposed to the gas issues as are facility operators. Those traveling through the area are also put at risk, although briefly, the exposure can initiate health issues such as an allergy or an asthma attack.
The objective of this paper is to begin an honest discussion, which detail these problems. We know there are environmental specialist as well as oil and gas industry experts who are willing to work together to solve these problems. All of us at One Step In hope you will join us in this discussion.
Wiley Rhodes, Founder