The steam-assisted gravity drainage (SAGD) process is one of the key in situ recovery processes being used today to recover heavy oil and bitumen. In this process, steam injected through a horizontal well, flows convectively towards the outer edges of a depletion chamber. At the edges of the depletion chamber, the steam releases its latent heat to the cool oil sand and raises its temperature. The heated oil is mobile and flows under the action of gravity to a horizontal production well located several metres below the injection well. It remains unclear what is the exact mechanism of chamber growth. Some have suggested that in addition to heat conduction, it is by convective steam flow in the form of pointed fingers at the edges of the chamber which penetrate the oil sand. In theory published by Butler [Butler, J. Can. Petroleum Technol. 1987;26(3):70-75], it was determined that the fingers can be as long as 6 m for Athabasca bitumen reservoirs. In this research, a new theory is derived and provides predictions of the rise rate which compare better to estimates derived from field thermocouple data and physical model experimental observations than values obtained from Butler's theory. The results suggest that in the absence of mobile water, heat conduction rather than steam fingers at the chamber edge is the dominant heat transfer mechanism.Le procédé de drainage par gravité assistéà la vapeur (SAGD) est fondamental dans les procédés de récupération in situ qui sont utilisés aujourd'hui pour la récupération de l'huile lourde et du bitume. Dans ce procédé, la vapeur injectée dans un puits horizontal s'écoule par convection vers les bords extérieurs d'une chambre de déplétion. Sur les bords de la chambre de déplétion, la vapeur libère sa chaleur latente au sable bitumineux froid etélève sa température. L'huile chauffée devient mobile et s'écoule sous l'action de la gravité vers un puits de production horizontal situé plusieurs mètres sous le puits d'injection. On ne connaît pas exactement le mécanisme de croissance de la chambre. Certains suggèrent qu'en plus de la conduction de chaleur, le mécanisme de pénétration dans le sable bitumineux est basé sur la convection de la vapeur par digitationà la périphérie de la chambre. Dans un travail théorique publié par Butler [Butler, J. Can. Petroleum Technol. 1987;26(3):70-75], il aété déterminé que les doigts peuvent atteindre une longueur de 6 m pour les réservoirs de bitume d'Athabasca. Dans cette recherche, une nouvelle théorie estétablie et fournit des prédictions de la cinétique de croissance qui se comparent mieux aux estimations calculéesà partir de données de thermocouple prises dans le réservoir et des observations expérimentales de modèles physiques qu'aux valeurs obtenues avec la théorie de Butler. Les résultats suggèrent qu'en l'absence d'eau mobile, c'est la conduction de chaleur plutôt que les doigts de vapeur en bordure de chambre qui est le mécanisme de transfert de chaleur dominant.
Steam-Assisted Gravity Drainage (SAGD) is a widely used in situ recovery process for heavy oil and bitumen reservoirs. The performance of the SAGD process is tied with growth of the steam chamber which in turn depends on uniform steam delivery along well length and the underlying geology and fluid properties in the near wellbore region. If the reservoir has poor injectivity due to poor underlying geology oil production suffers. This can be avoided in by examining the interwell subcool. The subcool is the temperature difference between the injected steam and produced fluids. In this study, Proportional-Integral-Derivative (PID) feedback control has been employed to control inflow control valves settings to promote subcool to a target value. This control strategy is examined by using a PID algorithm to control SAGD in a detailed three-dimensional heterogeneous reservoir model with properties typical of an Athabasca bitumen reservoir. Specifically, the SAGD injector is divided into six intervals each with its own steam injection pressure. The interwell subcool is calculated and the PID feedback control algorithm is used to direct the subcool to a target value by changing the steam injection pressure in each well interval. The results show that this control method can be used to enhance uniform steam chamber growth and ultimately more oil production with less steam injection. The key benefit of dynamic well control is that the injection strategy is adjusted dynamically to fit the underlying geological and fluid compositional heterogeneity to obtain improved steam conformance along the wellpair. This implies that potentially a priori detailed knowledge of the geological and fluid compositional heterogeneity may not be as critical for well placement for uniform steam conformance. Introduction There are two main objectives of a thermal recovery process for heavy oil and bitumen reservoirs: first, mobilize the oil - this is often done by using high pressure steam, and second, move the mobilized oil to a production well. If one of these two objectives is not met, then the recovery process will not be technically successful. For a steam-based thermal recovery process, the first objective equates to delivering the latent heat of the steam to the oil phase in the oil sand. In general, the more efficient the heat transfer from steam to oil, the more productive and economic is the recovery process. Thus, a key goal of a steam-based recovery process is to control energy delivery within the reservoir. However, thermal production from heavy oil and bitumen reservoirs is difficult because subsurface processes such as steam flow and heat transfer in a system with both geological and fluid compositional heterogeneity tend to be difficult to control. For example, in steam-based recovery technologies such as Steam-Assisted Gravity Drainage (SAGD), displayed in cross-section in Figure 1, the steam chamber that develops in the reservoir may not be uniform along the length of the wells. For example, Figure 2 displays interpreted seismic data of the heated zones in Clearwater bitumen reservoir at the end of the steam injection period in the first three cycles of horizontal well CSS (Imperial Oil, 2006). The images reveal that steam injection is not uniform along the length of the wells. Thermocouple data also reveal that heat transfer in the reservoir is non-uniform and is controlled by the heterogeneity of the underlying geology and fluid composition (ConocoPhillips, 2008). This implies that despite the desire for uniform steam delivery within the reservoir, this is most likely not achieved in the majority of steam injection wells. Recently, an analysis by Zhang et al. (2005) of 4D and crosswell seismic and production data have shown that steam chamber growth and oil recovery are strongly influenced by reservoir geology. Steam chamber growth is especially affected by the presence of low permeability facies in the vicinity of the SAGD wellpair. Furthermore, Gotawala and Gates (2009) have demonstrated that there is a direct link between permeability heterogeneity and the evolution of a SAGD steam chamber.
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