Hydrogen Sulphide Measurements In SAGD Operations-Part II

Abstract In a previous paper, a method suitable for the measurement of hydrogen sulphide concentrations in SAGD process streams was described. The method is applicable to measuring hydrogen sulphide, non-condensable gases, and steam in lines or units containing up to 70% steam, and avoids the errors inherent in stain tube and Tutweiler measurements. This paper describes the extension of the method. It is now possible to modify the method to accommodate steam concentrations as high as 98.5% at one extreme, and very dry gases at the other. In each case, modifications of the original train that are required to obtain accurate data are described. Procedures for meeting regulatory requirements, such as hydrogen sulphide and sulphur dioxide emission measurements, are also described. Introduction In a previous paper, a new method for sampling of produced from SAGD facilities was described(1). The method is capable of accurately measuring hydrogen sulphide concentrations, and was shown to be superior to the commonly used stain tube methods. The latter methods were shown to deviate from true concentrations by as much as an order of magnitude. This deviation is thought to be due to the presence of a high mole fraction of steam in the facilities under test. The method also gives the mole fraction of steam, and that of the total non-condensable gases. The standard error was calculated to be of the order of 5.3%, provided it was possible to collect a sufficiently large volume of non-condensable gas. The upper limit of the method is reached when the steam mole fraction exceeds 70%. A schematic of the sampling train is shown in Figure 1. Over the last three years, it became necessary to measure hydrogen sulphide concentrations in SAGD facilities under more extreme conditions. In the first case, a hydrogen sulphide concentration was required for a facility gas containing a mole fraction of steam in excess of 98%. In the second case, the gas in the facility was almost dry. Both cases presented significant challenges to measurement, as described in the following. Design of Original Sampling Train In the sampling train shown in Figure 1, all of the steam is forced to condense early in the train, regardless of the quality of the steam. The hydrogen sulphide is chemically trapped in the same location in the train, by use of a copper sulphate solution. Use is made of the reaction: Cu2+ + H2S →CuS + 2H+ The content of condensed water after the test is easily measured by weighing both the first impinger and the drying tube, before and after the test. The solution containing the precipitated copper sulphide is then quantitatively recovered from the impinger in the field, and the copper sulphide separated in the laboratory. Chemical dissolution of the separated precipitate then permits analysis of the total amount of copper recovered, and calculation back to the amount of hydrogen sulphide trapped. Correction for the precipitate mass is then made to the water weight.