Dehydration of Acid Gas Prior to Injection

Abstract

Acid gas is a mixture of hydrogen sulfide and carbon dioxide with a small amount of lower molecular weight hydrocarbons, mainly methane. Acid gas is most often produced in sour natural gas sweetening units, during regeneration of the sweetening chemical, usually an amine. Acid gas liberated from rich amine leaves the amine regeneration tower at a low pressure, typically below 200 kPa, and at about 50°C, the typical temperature downstream of the regeneration tower overhead condenser. In addition, the acid gas is saturated with water. This is not produced water, which contains dissolved salts, which can cause additional problems but rather this is condensed water that should contain virtually no dissolved solids. If acid gas is to be injected into a subsurface formation such as an aquifer or a depleted reservoir, it must be compressed and sent to an injection well through a pipeline. Water present in acid gas can contribute to corrosion of the compression equipment and the pipeline, and it may facilitate hydrates formation. All main constituents of acid gas: H2S, CO2, and methane, are hydrate formers. Hydrates can form in acid gas without the presence of free water; however free water is required in the acid gas to cause corrosion of carbon steel. It is recommended to keep the relative water saturation of the acid gas mixture considerably below 100% in order to avoid condensation of water in the piping, facilities, pipeline or well.