SDRL, SDLP, NODL - Why crude oil is stabilized?


why crude oil is stabilized?

Once degassed and dehydrated–desalted, crude oil is pumped to gathering facilities to be stored in storage tanks. However, if there are any dissolved gases that belong to the light or the intermediate hydrocarbon groups it will be necessary to remove these gases
along with hydrogen sulfide (if present in the crude) before oil can be stored. This process is described as a ‘‘dual process’’ of both stabilizing and crude oil sweetening.

In stabilization, adjusting the pentanes and lighter fractions retained in the stock tank liquid can change the crude oil gravity. The economic value of the crude oil is accordingly influenced by stabilization. First, liquids can be stored and transported to the market more profitably than gas. Second, it is advantageous to minimize gas losses from light crude oil when stored.
This chapter deals with methods for stabilizing the crude oil to maximize the volume of production as well as its API gravity, against two important constraints imposed by its vapor pressure and the allowable hydrogen sulfide content.
To illustrate the impact of stabilization and crude oil sweetening on the quality of crude oil, the properties of oil before and after treatment are compared as follows:
(a) Before treatment
Water content: up to 3% of crude in the form of emulsions and from 3% to 30% of crude as free water
Salt content: 50,000–250,000 mg/L formation water Gas: dissolved gases in varying amounts depending on the
gas–oil ratio (GOR)
Hydrogen Sulfide: up to 1000 ppm by weight
(b) After treatment (dual-purpose operation): Sour wet crude must be treated to make it safe and environmentally acceptable for storage, processing, and export. Therefore, removing water and salt, is mandatory to avoid corrosion; separation of gases and H2S will make crude oil safe and environmentally acceptable to handle.
Water content (B.S.&W.): 0.3% by volume, maximum
Salt content: 10–20 lbs salt (NaCl) per 1000 barrels oil (PTB)
Vapor pressure: 5–20 psia RVP (Reid vapor pressure)
H2S: 10–100 ppmw
Crude oil is considered ‘‘sweet’’ if the dangerous acidic gases are removed from it. On the other hand, it is classified as ‘‘sour’’ if it contains as much as 0.05 ft3 of dissolved H2S in 100 gal of oil. Hydrogen sulfide gas
is a poison hazard because 0.1% in air is toxically fatal in 30 min.
Additional processing is mandatory—via this dual operation—in order to release any residual associated gases along with H2S present in the crude. Prior to stabilization, crude oil is usually directed to a spheroid for storage in order to reduce its pressure to very near atmospheric.


STABILIZATION

the traditional process for separating the crude oil–gas mixture to recover oil consists of a series of flash vessels
[gas–oil separation plant (GOSP)] operating over a pressure range from roughly wellhead pressure to nearly atmospheric pressure. The crude oil discharged from the last stage in a GOSP or the desalter has a vapor pressure equal to the total pressure in the last stage. Usually, operation of this system could lead to a crude product with a RVP in the range of 4 to 12 psia. Most of the partial pressure of a crude comes from the low-boiling compounds, which might be present only in small quantities—in particular hydrogen sulfide and low-molecular-weight hydrocarbons such as methane and ethane.
Now, stabilization is directed to remove these low-boiling compounds without losing the more valuable components. This is particularly true for hydrocarbons lost due to vent losses during storage.
In addition, high vapor pressure exerted by low-boiling-point hydrocarbons imposes a safety hazard. Gases evolved from an unstable crude are heavier than air and difficult to disperse with a greater risk of explosion.
The stabilization mechanism is based on removing the more volatile components by (a) flashing using stage separation and (b) stripping operations.
As stated earlier, the two major specifications set for stabilized oil are as follows:
The Reid vapor pressure (RVP)
Hydrogen sulfide content
Based on these specifications, different cases are encountered:
Case 1: Sweet oil (no hydrogen sulfide); no stabilization is needed. For this case and assuming that there is a gasoline
plant existing in the facilities (i.e., a plant designed to recover pentane plus), stabilization could be eliminated, allowing the stock tank vapors to be collected [via the vapor recovery unit (VRU)] and sent directly to the gasoline plant,