(238d) 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.

From a fundamental point of view, there exists little to no data with regards to pure mercaptan components as well as mixtures containing mercaptans. In order to properly characterize the system, a variety of data including phase equilibrium, water content, hydrate formation conditions, and transport properties of mercapan + H2S, mercaptan+CO2 and blended streams would be required. A literature review for this information indicates that much of this data in either an experimental or calculated form is not available, and has not been accounted for in popular simulation engines. As such, any information determined using a simulation program can be used only as superficial at best.

With these modelling inaccuracies in mind, a simulation model has been prepared to provide an idea of the mercaptan rich stream's potential behaviour when processed for acid gas injection. While the information may have severe errors built-in from using a simulation program, the exercise does serve to reveal further issues that may arise while preparing the waste stream for disposal, including difficulties during compression and dehydration. An injection profile and compression analysis of a typical acid gas stream are also described to highlight the variation in results between both cases.