It has long been known that heavy oil and bitumen recovery by SAGD and CSS processes is accompanied by significant production of acid gases, as well as solution gas. Since the old laboratory studies of aquathermolysis from the 1970's and 1980's, there has been considerable development in the knowledge concerning production of methane and acid gases from Athabasca, Cold Lake, Peace River, Venezuela and Utah oil sands. It is found that both GOR and gas composition may vary with the deposits concerned. There is considerable divergence of opinion about the chemical origin of some gases, notably carbon dioxide. This affects the methods of control that may be available in operating individual reservoirs, and the matter is discussed. Also discussed will be the means of taking advantage of aquathermolysis phenomenon. It has been shown that both the GOR and gas composition of a SAGD project can be calculated from first principles. This permits estimation of the daily throughput of hydrogen sulphide and therefore allows a prediction or control of the requirements for sulphur recovery. The control of scale in SAGD plants has also been achieved by application of the current state of knowledge. Finally, there are implications for hydrogen sulphide release during loss of well control, an important regulatory aspect.
Six 3-substituted picolinic acids were synthesized and decarboxylated in buffered aqueous solutions of ionic strength 1.0 at 150 and/or 95°C. 3-Amino-and 3-hydroxypicolinic acids appear to decarboxylate by initial protonation, but the others fit the requirements of the Hammick mechanism for picolinic acid decarboxylation. Both electron-withdrawing and electron-releasing 3-substituents accelerate decarboxylation in picolinic acids but inhibit decarboxylation in their anions. Acceleration in the acids is considered to be the result of interference by 3-substituents with coplanarity of the carboxyl group and the aromatic nucleus. This reduces the order of the bond between the carboxyl group and the ring, thus facilitating bond breaking. In decarboxylation of the anions water appears to play a critical role, since picolinate ions have not been observed to decarboxylate in any other solvent, including ethylene glycol. It is proposed that water forms a hydrogen-bonded bridge between carboxylate oxygen and aromatic nitrogen, so that as the carbon-carbon bond breaks a nitrogen-hydrogen bond is formed. This may provide a lower energy path through an ylide intermediate than would be required if the picolinate ion were to decarboxylate to a 2-pyridyl carbanion. Chem. 55, 1342Chem. 55, (1977. On a synthetise six acides picoliniques substitues en position 3 et on les a dCcarboxyles dans des solutions aqueuses tamponnes de force ionique 1 .O, a 150 et/ou 95'C. I1 semble que les acides amino-3 et hydroxy-3 picoliniques se decarboxylent par une protonation initiale; toutefois les autres presentent les caracteristiques necessaires pour le mecanisme propose par Hammick pour la dCcarboxylation de I'acide picolinique. Les substituants en position 3 qui attirent les electrons ainsi que ceux qui repoussent les electrons accelerent la decarboxylation des acides picoliniques mais inhibent la decarboxylation de leurs anions. On considere que I'accelCration dans les acides provient d'une interference, par les substituants en position 3, sur la coplanarite du groupe carboxyle et du noyau aromatique. Cet effet reduit l'ordre de la liaison entre le groupement carboxyle et le cycle et facilite ainsi le bris du lien. Dans le cas de la decarboxylation des anions, il sernble que l'eau joue un r6le critique puisque l'on n'a jarnais observe la decarboxylation des ions picolinates dans d'autres solvants, m&me i'ethylene glycol. On suggere que l'eau forme des ponts hydrogene entre I'oxygkne du carboxylate et I'azote du cycle arornatique provoquant la formation d'un lien azote-hydrogene au moment oh le lien carbone-carbone se brise. Ce processus pourrait fournir une voie requtrant une energie plus basse, via un ylide intermediaire, que celle qui serait requise si l'ion picolinate se decarboxylait par I'intermidiaire d'un carbanion pyridyle-2.[Traduit par le journal]
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractIn a previous paper, a general theory for gas production during Steam-Assisted Gravity Drainage (SAGD) has been presented 1 . The main gases formed during "aquathermolysis" reactions, carbon dioxide and hydrogen sulphide, tend to be produced primarily via the produced water.The present paper reports a numerical analysis of the theory, and provides the results of a first simulation that describe the gas production history during SAGD. The numerical analysis includes hydrogen sulphide, carbon dioxide, and methane solution gas.Gases were included in the numerical analysis by using Kvalues derived from new theory. This theory accounts for the asymptotic behaviour of gases in solution as the critical point of water is approached.Simulation results corroborate the initial simple analysis, and agree with field observation. Minor differences are discussed.
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
The injection of non-condensable gases (NCG's) with steam in SAGD is a concept that has been much discussed in the literature, and tested via laboratory studies, and in a small number of field pilots, the most comprehensive being the UTF Phase B. There has been considerable discussion, both positive and negative, regarding the behaviour of these gases in the steam chamber, and their impact on ultimate bitumen recovery. Benefits to SAGD productivity have been suggested by some, while other authors have projected impacts on bitumen recovery. The authors have reviewed the results of methane injection in several projects, and used thermodynamic theory to develop a method to effectively model this behavior via simulation. While conventional simulators use the Peng-Robinson Equation of State, the effectiveness of EOS's become challenging at high temperature. In addition, the effects of recent developments in high temperature solution thermodynamics become significant with increasing temperature in SAGD. AOSC has developed a method of modeling the effects of NCG in a steam zone, via the development of a new set of equilibrium values for methane exchange between the oil/gas phase, water/gas phase and oil/water phases. This paper will present the results of these simulations and predicted impact of methane injection on SAGD performance, as well as implications for other NCG's. Introduction The simulation of the behaviour of non-condensable gases (NCG) in SAGD schemes has so far met with limited success. While methane, the main solution gas in Athabasca bitumen, can be modeled to some extent, greater difficulty has been encountered with the other two main produced gases in SAGD, carbon dioxide and hydrogen sulphide. These are not solution gases and must be introduced in some other way before they can be partitioned by simulator routines. Further, at high temperatures in a steam zone, neither has a significant solubility in Athabasca bitumen. Thimm 1a,b has suggested that gas production in SAGD is best understood in terms of dissolution of gases in the produced liquids. The commercial simulators do not account for this phenomenon, which is controlled by high temperature solution thermodynamics. In addition, literature reports suggest a wide difference in predictions from simulators on one hand, versus laboratory data and field results on the other, in the few cases where gas has been co-injected with steam. Background Some evidence has been published in the past to suggest that gases which accumulate in the steam zone tend to migrate to the top of the steam zone, and provide a barrier to heat loss to the overburden. Butler in 19972 was the first to propose this effect, largely on the basis of laboratory measurements and subsequent evaluation of the thermal data. Butler's primary concern appears to have been that conventional SAGD is most suitable for thick pay zones, and that the effective barrier to heat loss provided by a gas blanket would extend the economics of SAGD to thinner reservoirs. Evidently, Butler was also hoping for a Steam and Gas Push (SAGP) effect to improve production rates, and recommended the field testing of the SAGP scheme.
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