Testing and Treating H2S and Mercaptans in Crude Oil

We recently attended a conference in Salt Lake City, where we discussed various testing methods for H2S and mercaptans. Given the significant interest in this topic, we’d like to provide an overview for those who couldn’t attend. Crude oil, a vital resource in the energy industry, often contains contaminants like Hydrogen Sulfide (H2S) and Mercaptans. These compounds not only affect the oil’s quality but also present health and safety risks. To learn more about handling these challenges, check out this article. 

A Brief Overview of H2S & Mercaptans 

Hydrogen Sulfide (H2S): 

H2S is a toxic and pungent gas found in both the formation and post-production phases of crude oil. It is present in crude oil, natural gas, and water, earning it the nickname “sour gas.” H2S is soluble in water and behaves like a weak acid, making it corrosive. 

Mercaptans (RSH): 

Mercaptans are organic molecules with a structure resembling alcohol but with a sulfur atom chained to hydrocarbons, known as thiols. There are many mercaptan species, and they are notorious for their unpleasant odor. The human nose can detect mercaptans at concentrations as low as 10 parts per billion (ppb). 

Testing Procedures 

Testing for H2S and Mercaptans is a crucial step in ensuring the quality and safety of crude oil. However, not all testing methods are created equal, and the choice of method often depends on the commercial contract. Some common testing methods include: 

Vapor Testing:  

• ASTM 5705 modified: A test that yields results in H2S concentration (ppm/v). It has been modified to test crude oil since it was originally designed for fuel oil testing. 

Liquid Testing: 

• ASTM D7621: A standard method for determining H2S in fuel oils by rapid liquid phase extraction. 
• UOP 163: A titration that measures H2S (ppm/w) and mercaptan (ppm/w) concentrations. 
• ASTM D5623: A GC method that provides H2S (ppm/w) and mercaptan speciation (ppm/w) data. 
• ASTM D130-9/D1838-16: A subjective copper strip test for specific sulfur contaminants. 

Results obtained through these tests are vital for H2S scavenging, ensuring safety, and adhering to contract specifications. Historically, there has been a lack of correlation between different test methods, emphasizing the importance of using a consistent method throughout the process. In the United States, UOP 163 and ASTM 5705 are commonly used for crude oil quality contracts. For a deeper dive into H2S test methods, explore our detailed guide here. 

Challenges with Testing Methods 

• ASTM 5705 modifications may be necessary to adapt to crude oil testing conditions, and there is no standardized temperature for testing in practice. 
• Reading stain tubes can be challenging. Moisture in the air and other contaminants in the raw crude oil may negate the accuracy of the tube. 
• UOP 163 may suffer from interference by chemical contaminants and variations in technicians’ interpretations of titration curves. 

Scavengers for H2S and Mercaptans 

To address the presence of H2S and Mercaptans in crude oil, various scavenging methods are employed. These include: 

1. Typical Amines:

• Monoethanolamine (MEA) 
• Diethanolamine (DEA) 
• N-methyldiethanolamine (MDEA) 
• Diglycolamine (DGA) 

2. Non-Regenerative H2S Scavengers:

• Solid, basic metallic compounds 
• Oxidizing chemicals 
• Aldehydes, including Formaldehydes 
• Reaction products (may include triazines) 
• Metal carboxylates/chelates 
• Other amine-based solutions 

Each of these chemical solutions has its advantages and disadvantages, making it necessary to evaluate them on a case-by-case basis. 

Alternatives to Treat H2S 

In addition to scavenging methods, nitrogen stripping is an alternative approach to treat H2S. This method involves bubbling nitrogen through a column, which attracts H2S, allowing lighter-end volumes to escape and be transported to a flare. However, this process comes with its own set of considerations, including the need for a compressor, a nitrogen membrane generation unit, stripping tower kit, and a tie-in to the flare line. Moreover, the loss of hydrocarbons at the flare may raise environmental, social, and governance (ESG) concerns. 

Why It Matters 

Understanding and effectively addressing H2S and Mercaptans in crude oil is essential for various reasons: 

• Prolonging Asset Life: Proper treatment ensures that your assets last longer, maximizing their value. 
• Expanding Markets: High-quality crude with low H2S and Mercaptan content opens doors to more markets. 
• Building Optionality: The ability to adapt to different market conditions leads to better netback prices. 
• Ensuring Personnel Safety: Protecting the health and safety of workers is of utmost importance. 
• Asset Integrity: Treating H2S and Mercaptans preserves the integrity of equipment and facilities. 

 Managing H2S and Mercaptans in crude oil is a critical aspect of the industry. By understanding the characteristics of these compounds, employing appropriate testing methods, choosing effective scavengers, and considering alternative treatments, companies can ensure safer operations and better financial outcomes. Additionally, effective odor management in refinery operations plays a vital role in maintaining environmental compliance and workplace safety. Ultimately, the equation for success in the industry is the combination of the highest quality and merchantability, resulting in the highest netback prices. 

Final Thoughts  

Understanding and complying to the commercial contract is paramount. These commercial contracts will stipulate which method is required to meet specifications, so when in doubt, have the operations team speak to the crude quality or commercial teams to understand the exact parameters to ensure the quality of the crude is being met for final delivery.