Oil extraction is followed by oil storage. Crude oil is stored in tanks, which hold the oil for some time to stabilize the flow between the oil well and the pipeline. Even produced gas is held in storage tanks. During storage, light hydrocarbons, which are dissolved in the liquid, vaporize and get accumulated between the liquid and the tank roof. These hydrocarbons include methane, natural gas liquids (NGLs), volatile organic compounds (VOC) and other hazardous air pollutants (HAP). These vapors often escape to the atmosphere.
The underground crude oil in the reservoir has many light hydrocarbons dissolved within. When the oil is extracted and brought to the surface, it is processed at water treatment plant for oil and gas, where many of these light hydrocarbons are removed along with water using equipments such as nitrogen separation membrane, with high-pressure and low pressure separators. Then the processed oil is stored in a tank for the purpose of sale and transportation. During storage, the remaining hydrocarbons either vaporize or get flared and vent into the air. The loss of hydrocarbons can be categorized as:
Flash loss, which occur when separator or treatment equipment dumps the oil in the storage tank.
Working loss, which occur due to changing fluid levels.
Standing loss, which occur due to daily temperature changes.
The volume of vapor emission depends on many factors. Light crude oil emits more hydrocarbon vapors than heavy crude oil. Frequent oil cycle in a tank results in higher working losses as compared to tanks in which the oil is held for longer periods. The operating temperature and pressure of oil in the vessel dumping into the tank impact the flash losses.
These vapors are both valuable and dangerous. They are valuable because they are rich in Btu and have high sale value but they are dangerous to the environment. It is, therefore, essential to recover these vapors not only for economic benefits but environmental as well.
To capture and prevent the release of vapors in the environment, a competent vapor recovery system needs to be in place. For this, Crude oil vapor recovery system is deployed and a vapor recovery unit (VRU) is installed and attached to the storage tank. A VRU can capture up to 95% of Btu-rich vapors which can then be used onsite as fuel or sold in the market. Currently, between 7,000 and 9,000 VRUs are installed in the oil production sector, with an average of four tanks connected to each VRU.
The proper identification of the installation of VRUs on storage tanks cannot only help save hundreds of thousands of dollars by recovering and marketing these vapors but also reduce the emission of methane and HAP in the environment.