Prediction of Hydrogen Sulphide Production in SAGD Projects

Abstract The production of hydrogen sulphide and carbon dioxide is a feature common to all SAGD projects. Both gases are formed by a chemical reaction of steam condensate with bitumen in the SAGD steam zone in Athabasca; they are not normally present as solution gases. The production rates of these gases are therefore strongly dependent on operating characteristics of individual projects, primarily the steam pressure. The potential need for sulphur recovery plants in the larger SAGD project expansions creates the need for first order predictions of H2S production rates, so that appropriate sulphur recovery technologies can be identified. A simple method for the prediction of hydrogen sulphide production has been derived, such that, for most Athabasca projects in the 180 to 240 ºC range of steam zone temperatures, a probable production rate estimate can be read off a simple graph. The graph is built from laboratory data on aquathermolysis, first provided by Hyne, and the assumption of pseudo-zero order kinetics of the aquathermolysis reaction that generates hydrogen sulphide. A temperature dependency is predicted, and is confirmed by field measurements of projects in the 180 to 240 ºC range. Introduction The solvolytic reaction of Athabasca bitumen with steam condensate has been observed since the earliest days of piloting the recovery of this bitumen resource. The first laboratory studies were performed by Hyne et al.(1–3), who coined the now widely-used term "aquathermolysis". More detailed mechanistic studies followed from Hyne's group, and also from the group led by Strausz at the University of Alberta(4). The quantitative aspect of Hyne's work has been largely ignored, possibly because his work was, for the most part, done at 240 ºC steam temperatures, whereas the steam zone temperatures in SAGD pilots and commercial operations have varied from that. Obviously, the steam zone temperature must have an effect on production of the acid gases because the kinetics of generation are temperature dependent. A previously published method of estimating hydrogen sulphide production in SAGD is provided by the method of Thimm(5), which is based on the Henry's Law behaviour of gases in a SAGD steam zone. That method is capable of prediction of produced gas and steam zone gas compositions over time, including situations where the steam zone pressure changes or where gas is injected. However, it involves relatively lengthy computations in solution thermodynamics. For the purpose of identifying potential sulphur recovery technologies, a quicker method of estimating hydrogen sulphide production is required; at least, as an approximate guide. Moreover, it would be useful to express the hydrogen sulphide production on a mass or volume per unit bitumen basis to provide a simple guide for expansion planning. Reaction Order Although neither Hyne nor Strausz have published a reaction order, the following considerations suggest a zero order reaction in the SAGD steam zone. Consider that one might expect a reaction rate equation of the type: (1) rate = k [ sulphur ] [ steam condensate ] where the concentrations may be expressed in terms of various terms suitable for reservoir engineering.

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