In recent years, substantial progress has been made in the theoretical treatment of hydrocarbon dissolution in water, near the critical point of water (374 ° C). At these temperatures, water becomes a solvent for gases including the lower hydrocarbons, and possibly higher hydrocarbons.
The SAGD process is currently the only viable method for in-situ recovery of Canada's Athabasca oil sands deposit, a deposit of high viscosity oil in unconsolidated sand. Recent studies have sought to understand modifications at lower steam pressures and gas injection. Most recently, the idea of solvent co-injection has been under discussion.
In the present paper, the predictive capabilities that have been developed for gas production in the SAGD process are studied in conjunction with the chemical kinetics and mechanisms of solvolytic reactions. The reactions that produce hydrogen sulphide and carbon dioxide, generally referred to by the name "aquathermolysis", are in fact solvolytic reactions by their nature.
The results of this work suggest strongly that the production of the acid gas gases, hydrogen sulphide and carbon dioxide, will be suppressed in SAGD operations if a solvent is coinjected. The work has implications for the need for sulphur recovery plants in SAGD projects that are considered for solvent co-injection.
In 2001, Thimm1 proposed that gas production in SAGD proceeds via a dissolution mechanism. Gases are dissolved in the produced liquids, and break out of solution in the wellbore and facilities. There has been no case reported so far where it is necessary to assume free gas production in SAGD in order to account for observed gas production or composition. The rationale is as follows:
The distribution coefficient (K-value) of a solute gas in equilibrium with a solvent is given by
Equation (1) (Available in full paper)
where yi and xi are the mole fractions of solute i in the gas and liquid phase respectively. Henry's Law may be stated as
Equation (2) (Available in full paper)
In this form, the unit of the Henry's Law coefficient is that of pressure, as is evident from inspection. For the purpose of this work, all Henry's Law constants are given in units of mPa. The equation shows that the K-values are related to the Henry's Law constant.
Henry's law coefficients for gases in solvents normally follow a power law known as the Valentiner equation T, T(2), T(3), However, at elevated temperatures, this equation begins to fail at about 175 °C, and could only be used for the lowest steam pressure situations. Above this temperature, deviations become progressively larger, because an asymptotic behaviour of the Henry's Law constant near the critical point of water makes an increasingly important contribution. Above °1750C, the specific volume of water begins to fall significantly from the normal 55.56 mole/L, and Harvey and Levelt Sengers2 have shown a linear relationship between
Equation (3) (Available in full paper)
in the range °1750C and the critical point of water at °3740C.
