Performance Evaluation of Long-Horizontal-Well Solvent-Assisted SAGD in Rising Stage

Abstract Solvent-Assisted Steam-Assisted Gravity Drainage (SA-SAGD) is invented by adding solvent into the injection stream. However, the high cost of the solvent makes it necessary to ensure a high efficiency of solvent transportation from the long-horizontal wellbore to the chamber edge. In this study, a novel model considering the wellbore/chamber coupling effects is established, which can accurately evaluate the performance of SA-SAGD. First, governing equations for mass flow in the wellbore are built on the basis of the mass and momentum balance determined by coupling the wellbore and steam chamber. Besides, implicit equations for phase changes were derived according to the heat-mass transfer between wellbore and chamber. Second, the model for oil flow rate in the chamber rising stage is derived by the chamber height identified by the variation in thermo-physical properties of solvent and steam in the chamber along the wellbore. Third, by applying Levenberg-Marquardt Algorithm (LMA), the mathematical model is solved The results show that: the oil rate of classic SAGD is significantly improved by the solvent of C5, while the improvement effect of C7 is quite small. As for C3-SAGD, the oil rate is even lower than that of traditional SAGD. This is mainly because C3 decreases the chamber temperature and the solubility of C3 in heavy oil is low, which results in a relatively higher oil viscosity at the chamber edge compared to other solvents. The differentials between the cases, which consider the wellbore or not, is different for different solvent injection. Specifically, oil rate differential for C3-SAGD is the highest and it is twice that for conventional SAGD which has the least differential. C5-SAGD has the second-largest differential which is followed by C7-SAGD. Currently, applying SA-SAGD in a field encounters many challenges, especially in understanding its mechanisms and improving economic efficiencies such as a conflict between the production increase and solvent cost. The newly proposed model not only sheds light on the key mechanism of wellbore/chamber coupling effects, which is usually neglected in the literature but also brings greater benefits by better designing the future SA-SAGD heavy oil recovery projects.