Hydrogen Sulphide Measurements in SAGD Operations

Abstract A new method of measuring hydrogen sulphide concentrations in gas streams, suitable for SAGD operations, is reported. The method is independent of the steam content up to 70 mole %, the composition, temperature and pressure of the gas, and has been tested and validated in the field. The method has a standard error of about 5%. By contrast, the stain tube (Draeger) measurements performed routinely on various gas streams at SAGD pilots are shown to be correct only within an order of magnitude of the reading by the new method. Factors between two to ten were common between stain tube readings and the corresponding true readings taken immediately after or before the stain tube readings. The results have implications for plant safety procedures. It is suggested that certain plant streams be sampled only by operators wearing self-contained breathing apparatus; stain tube readings of the past have suggested this to be not necessary. There are further implications for understanding of partitioning of produced hydrogen sulphide in process streams. Introduction The production of hydrogen sulphide and carbon dioxide together with other minor gases in thermal recovery projects such as Steam Assisted Gravity Drainage is a common observation. The process giving rise to these gases is a high temperature hydrolysis of aliphatic sulphur linkages in the bitumen, dubbed "aquathermolysis" by Hyne et. al.1,2,3 For Arthabasca, the amount of hydrogen sulphide typically produced per tonne of bitumen varies between 6 and 75 litres. Considerably more carbon dioxide is produced, usually in the range 900-10,000 litres per tonne. It is shown elsewhere4 that such production rates of hydrogen sulphide can easily give rise to H2S concentrations of about 20,000 ppm in the produced gases of a SAGD production project. Hydrogen sulphide measurement has been usually performed by means of stain tubes. The reason is that laboratory analysis of gas from a normal stainless steel gas cylinder will not give meaningful results. It has therefore been customary to measure the main gases from a cylinder in the laboratory, but to measure H2S concentrations in the field by means of stain tubes. This technique, while probably suitable for the natural gas industry, is problematic in SAGD work. In reference 4 it is shown that hydrogen sulphide is highly soluble in steam condensate at temperatures of interest in SAGD. Moreover, the solubility goes through a minimum at approximately 180 °C. As a produced gas line or facility with a significant steam pressure is being sampled, the quality of the steam must necessarily change in the sampling line and in the stain tube itself. Hydrogen sulphide will re-partition accordingly. It is improbable that hydrogen sulphide is quantitatively determined by the stain tube from both phases. Therefore it is doubtful whether stain tube measurements are reliable in this situation. DESIGN OF SAMPLING TRAIN A sampling train suitable for the purpose at hand would ideally be easy to assemble from conventional sampling equipment, but would still be able to determine the composition of gas in terms of both water vapour, hydrogen sulphide and non-condensible gas content.