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Abstract
Thermal recovery processes are widely applied for heavy oil and bitumen production. Unique thermal properties of water and water steam allowed efficient reduction of extremely high viscosities by several orders of magnitude and made a vast heavy oil and bitumen reserves production technically and economically feasible.

Steam effect on heavy oil and bitumen in traditional reservoir engineering for a long time has been considered as physical only, i.e. viscosity reduction, improved flow parameters, distillation effects, emulsification, etc. However multiple laboratory studies and field observations suggest that initial oil undergoes chemical alteration and gases such as H2S and CO2 could be produced in increased quantities. Estimation of H2S and CO2 production potential is important due to considerable corrosivity of these gases, associated environmental, economical and other issues.

In this study a practical approach has been developed to simulate and forecast H2S and CO2 production during thermal recovery using common reservoir simulation tools. First, analytical data was matched and then chemical reaction had been implemented to the sector model. Steam Assisted Gravity Drainage (SAGD) was chosen to demonstrate the concept of suggested approach and analyze the results.

Generated gases were considered to be soluble both in water and oil. The importance of accounting for gas solubility in water was demonstrated and discussed. Simulated volumes of H2S and CO2 are in good agreement with that observed in the field applications of steam assisted recovery methods.

Introduction
Thermal recovery is one of the most extensively used processes for heavy oil and bitumen production. Majority of ongoing thermal recovery projects involve either steam or hot water injection due to their unique thermal properties. Unparallel latent heat of condensation and heat capacity make water a very efficient mean of energy delivery into the reservoir.

Heat propagation in the reservoir results in rapid oil viscosity decrease (often by several orders of magnitude) and subsequent improvement of reservoir flow characteristics. Along with viscosity reduction multiple physical effects such as distillation, emulsification, interfacial tension change, etc. could occur.

A number of laboratory experiments1-16 and analysis of production data1,17-19 indicate that steam assisted recovery can induce complex chemical interactions between water, oil and reservoir rock. Such gases as H2S and CO2 are often among the products of reactions along with CH4 and light hydrocarbons. 
Aquathermolysis and decarboxylation are two primary mechanisms of H2S and CO2 generation in the temperature range of 200-300°C4,6,14. As steam temperature approaches 300°C thermal cracking of different oil compounds becomes increasingly important process and contributes considerably into the overall gas generation4-6,22. In this paper H2S and CO2 generation issue will be of primary focus.