Ian Phillip Benson

New Insights Into Steam/Solvent-Coinjection-Process Mechanism

We present results of a detailed investigation of the steam/ solvent-coinjection-process mechanism by use of a numerical model with homogeneous reservoir properties and various solvents. We describe condensation of steam/solvent mixture near the chamber boundary. We present a composite picture of the important phenomena occurring in the different regions of the reservoir and their implications for oil recovery. We compare performances of various solvents and explain the reasons for the observed differences. An improved understanding of the process mechanism will help with selecting the best solvent and developing the best operating strategy for a given reservoir. Results indicate that as the temperature drops near the chamber boundary, steam starts condensing first because its mole fraction in the injected steam/solvent mixture (and hence its partial pressure and the corresponding saturation temperature) is much higher than the solvent’s. As temperature declines toward the chamber boundary and steam continues to condense, the vapor phase becomes increasingly richer in solvent. At the chamber boundary where the temperature becomes equal to the condensation temperature of both steam and solvent at their respective partial pressures, both condense simultaneously. Thus, contrary to steam-only injection, where condensation occurs at the injected steam temperature, condensation of steam/solvent mixture is accompanied by a reduction in temperature in the condensation zone and the farther regions. However, there is little change in temperature in the central region of the steam chamber. The condensed steam/solvent mixture drains outside the chamber, leading to the formation of a mobile liquid stream (drainage region) where heated oil, condensed solvent, and water flow together to the production well. The condensed solvent mixes with the heated oil and further reduces its viscosity. The additional reduction in viscosity by solvent more than offsets the effect of reduced temperature near the chamber boundary. As the steam chamber expands laterally because of continued injection and as temperature in the hitherto drainage region increases, a part of the condensed solvent mixed with oil evaporates. This lowers the residual oil saturation (ROS) in the steam chamber. Therefore, ultimate oil recovery with the steam/solvent-coinjection process is higher than that in steam-only injection. The higher the solvent concentration in oil at a location, the greater is the reduction in the ROS there. Our explanation is corroborated by the experimental results reported in the literature, which show smaller ROS in the steam chamber after a steam/solvent-coinjection process. A lighter solvent has a lower viscosity, a higher volatility, and a higher molar concentration of solvent in the drainage region. Thus, a lighter solvent causes a greater reduction in the viscosity of the heated oil and also leads to a lower ROS. Therefore, the lightest condensable solvent (butane, under the conditions investigated) provides the most favorable results in terms of enhancements in oil rate and oil recovery. This is different from the prior claims in the literature. Introduction Steam-assisted gravity drainage (SAGD) has become the preferred technology for exploiting the huge resource base of bitumen. There are more than 10 commercial SAGD projects in Canada. The field performance indicates that the process offers high production rate and high ultimate recovery. However, it requires a large volume of steam injection. The observed steam/ oil ratio (SOR) in the field is in the range of 3 to 5 cold water equivalent (CWE) bbl/STB (Jimenez 2008). A large usage of steam can affect the project economics adversely and also can have a detrimental impact on the environment. There have been numerous studies that aim to improve SAGD’s performance. These involve variations in the well configuration or changes in the operating methodology. In particular, steam/solvent coinjection appears promising. In this process, a small amount of vaporized but condensable hydrocarbon solvent is added to steam (Nasr et al. 2003). Laboratory investigation (Nasr and Ayodele 2005, 2006) and field trials (Table 1) have demonstrated that compared with steam-only injection, adding solvent to steam results in higher oil rate, reduced SOR, and higher ultimate recovery. Table 1 presents a brief summary of the previous field trials of steam/solvent coinjection. Although none of the pilots [except the LASER (liquid addition to steam for enhanced recovery) pilot by Imperial Oil] has been conducted long enough to provide conclusive results, they generally indicate much-improved performance. This technique may prove invaluable in overcoming some of SAGD’s shortcomings. Steam/solvent coinjection is a complex process. A successful and profitable field implementation requires a judicious decision about solvent type, solvent concentration, and the operating strategy. Selection of the optimum set of parameters is difficult because of a large number of variables involved and their nonlinear effect on the economic performance (Edmunds et al. 2009). A thorough understanding of the oil-recovery mechanism is required for an improved design of the process and to gain maximum benefits of the technology. This topic has attracted much research interest in the industry as well as in academia. Ardali et al. (2012) present a detailed review of prior studies. Although the effect of solvent addition on viscosity reduction is well described in the literature (Gates 2007; Deng et al. 2010), no other mechanistic details have been reported. There is a misunderstanding that a solvent with vaporization temperature comparable to that of steam will condense together with steam at the chamber boundary (Nasr et al. 2003). The misunderstanding of steam/solvent condensation leads to an incorrect selection of the most appropriate solvent. Furthermore, there is little information available about the formation of a mobile liquid stream (or drainage region) outside the steam-chamber boundary and the phenomena occurring in that region. Most importantly, the mechanism of increase in ultimate oil recovery by solvent addition to steam is not well understood (Nasr and Ayodele 2006). It often leads to an inappropriate representation of ROS in numerical-simulation studies and may result in erroneous conclusions. This study fills some of the gaps to develop an improved understanding of the steam/solvent-coinjection process. We have carried out a detailed investigation of the process mechanism by use of a fine-grid numerical model with homogeneous reservoir properties and various solvents. We describe condensation of the Copyright VC 2013 Society of Petroleum Engineers