A General Theory of Gas Production in SAGD Operations

Abstract

The small GOR commonly measured in SAGD projects has not previously been adequately explained, and various phenomena such as "microfingering" have been proposed to account for its presence. It is shown that the production of gases can be entirely explained by gases dissolved in the produced fluids at the temperature and pressure conditions of the SAGD steam chamber. Although methane is produced in part via bitumen, there is a significant contribution from methane dissolved in water as well. Other gases, such as carbon dioxide and hydrogen sulphide, are primarily produced by virtue of their solubility in water at the pertaining temperature and pressure. This result is a consequence of the asymptotic Henry's Law behaviour of gases in water as the critical point of water is approached. This asymptotic behaviour is shown to govern at temperatures well below the critical point, and within the temperature range of SAGD steam zones. The theoretical foundation of this work permits the estimation of gas-water equilibrium constants for the major produced gases of importance in SAGD, and thus an ultimate understanding of gas effects in the steam zone. Since the densities and K-values are constant at any given temperature, specification of saturations will give the mass distribution between the gas and water phases of any solute gas visa -vis the water draining to the production well. The distributions are given in Table 3, and the results therein permit the following calculation: Case 1: Consider a bitumen saturation of 0.5 in the drainage zone, where the total of steam and non-condensable gas saturations is taken at 0.02 or less. An hydrogen sulphide production of 25 L/tonne bitumen, and a carbon dioxide production of 2000

Hydrogen Sulphide Measurements in SAGD Operations

“…In another paper 4 , it has been shown theoretically that hydrogen sulphide from aquathermolysis in SAGD is almost entirely produced, and is produced solely via the produced water owing to its solubility at SAGD steam zone temperature. Likewise, carbon dioxide and methane are predicted to be produced via dissolution in water, although for methane a minor contribution via bitumen (i.e.…”

Henry's Law Near the Critical Point of Water-Applications In Solvent Co-Injection With SAGD

“…The solubility of gas in a liquid is governed by Henry's Law, which may be stated as as is self-evident from inspection of equation (1). In 1989, Japas and Levelt Sengers 2 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 2001, Thimm 1 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.…”

Impacts of Gas in SAGD: History Matching of Lab Scale Tests

“…The negative impacts of gas on SAGD operation have also been noticed, but usually considered minor [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. Recently, however, theoretical and numerical study had shown that the negative impacts of gas on SAGD could be critical [16].…”