Limitations and Challenges Associated with the Disposal of Mercaptan-Rich Acid Gas Streams by Injection – A Case Study

As oil and gas reserves become depleted, producers are increasingly driven to process difficult or unconventional petrochemical fuel sources to sell to the market in order to meet rising demand. One such source is the production of sour gas wells that contain high levels of mercaptan (thiol) species. In sour gas processing applications, a waste acid gas stream (ie. a stream containing high concentrations of hydrogen sulphide and carbon dioxide) is produced as an effluent stream in the gas sweetening unit. Generally speaking, when these volumes are relatively low (less than 5 MMSCFD or less than 20 t/d sulphur equivalent), a suitable method of waste handling of the stream is by injection to a wellbore formation. Acid gas injection is a mature technology with over 50 applications in Western Canada and another 20 around the world. In the application where mercaptan-rich sour gas wells are produced, the resultant effluent stream from a gas sweetening process will be one that contains high levels of H2S, CO2, mercaptans and hydrocarbons. While investigating the ability to dispose this stream in a similar manner to that of a typical acid gas injection scheme, a number of challenges have been identified. These challenges stem from three main sources. First, little experimental data is available with regards to phase equilibrium, water content, hydrate formation conditions, and transport properties of both pure mercaptan species and mercaptan mixtures. Second, current simulation programs used for process design can employ poor assumptions or contradictory data in generating the models. And third, even when these programs are used to generate results around the injection scheme, several additional issues are revealed which requires attention. These issues are investigated in a hypothetical case study, comparing the injection of a mercaptan-rich acid gas stream to a conventional acid gas stream.