Sour gas is a term used to refer to gas which contains hydrogen sulfide in concentrations greater than four parts per million. The term “sour gas” is usually used with reference to natural gas, although it can be used to discuss other gases as well. Gas with impurities such as hydrogen sulfide needs to be treated before it can be safely used. The gas refining process includes a step known as “sweetening” to remove hydrogen sulfide and other materials.
When natural gas is initially accessed in the field, it usually contains an assortment of impurities, which can depend on where natural gas drilling sites are located. These impurities must be removed at a refinery to ensure that the gas performs in a stable and predictable way when it is used. In the case of sour gas, the hydrogen sulfide gives the gas a distinctively strong odor which makes it easy to identify, and the sweetening process removes much of the odor.
As sour gas is drilled and transported to a refinery for processing, care must be taken, because the hydrogen sulfide can be corrosive. Specialized pipes and equipment are needed to avoid adverse reactions during transport which could pose a safety risk. Once sour gas arrives at the refinery, it can be put through a series of processes to sweeten it. Typically, hydrogen sulfide isn't the only impurity in the gas, with sour gas often containing carbon dioxide as well.
A closely related term is “acid gas.” Acid gas is natural gas which has a high concentration of acidic gases. It must also be processed before it can be safely used. However, acid gas and sour gas are not the same thing. That said, there is often a great deal of intersection between sour and acid gases, as impurities in natural gas supplies do not neatly confine themselves to different areas.
Drillers and refinery facilities can conduct tests to learn about the composition of their raw natural gas and determine the most appropriate treatment methods for the gas. During the refining process, the contents of the natural gas are standardized so that it can be used in any system designed to utilize natural gas for heat or energy. These systems are usually not designed to cope with trace impurities, making it critical to deliver a consistent and reliable product to avoid damaging natural gas systems or posing a safety risk to users of such systems.