Abstract

In the last ten years, substantial progress has been made in the theoretical treatment of gas dissolution in water, near the critical point of water (374 ° C). An asymptotic behaviour of Henry's law for a variety of gases (the noble gases, methane, ethane, nitrogen, oxygen, carbon dioxide and hydrogen sulphide) already governs the behaviour of these gases at temperatures above approximately 180 ° C, well below the critical point. This behaviour has been used to develop a theory of gas production in SAGD, according to which gas production in SAGD largely occurs via dissolution of gas in the steam condensate.

Further recent developments in this area of solution thermodynamics have extended the knowledge of this phenomenon to various light hydrocarbons, from butane to dodecane, and the aromatics such as benzene and toluene. In the present paper data for the distribution coefficients (K-values) of light hydrocarbon solvents in water are presented, and the application for prediction o solvent returns in SAGD enhanced by solvent co-injection is discussed. Optimal choice of solvent composition to maximize solvent returns is possible.

Introduction

In 2001, Thimm1 proposed that gas production in SAGD proceeds via a mechanism whereby gases are dissolved in the produced liquids, and enter the wellbore in that form. No case has been reported so far where it has been necessary to postulate free gas entering the SAGD production well in order to explain the observed GOR and produced gas compositions. Gases are considered to separate from the liquids in the wellbore and facilities.

The solubility of gas in a liquid is governed by Henry's Law, which may be stated as (Equation 1) (Available in full paper)

The Henry's Law constant is related to the more conveniently used distribution coefficient, (Equation) (Available in full paper) as is self-evident from inspection of equation (1). In 1989, Japas and Levelt Sengers2 provided the theoretical basis for observations made at that time and since, that the distribution coefficients for gases in general, for all solvents-gas pairs, declined as the critical point of the solvent was approached. In fact, Japas and Levelt Sengers showed that al values of K approach unity towards the critical point.

In the case of water, this asymptotic behaviour governs gas solubilities above 180 ° C or so, and considerably earlier in the case of some gases. Harvey and Levelt Sengers3 published a power law for a variety of gases in water, valid between the triple point and the critical point of water: (Equation 2) (Available in full paper) For the case where the solvent is water, all values of density, and also the fugacity term, in this equation may be found in or derived from steam tables. The values of A, B and C are published by Harvey and Levelt Sengers. Other authors have published coefficients of the same kind, notably Suleimenov & Krupp4 for hydrogen sulphide. Thimm1 has shown that this approach, when applied to simulated oil and water production forecasts in SAGD, or to actual production data, yields GOR and produced gas composition data in good agreement with actual results.

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