A Semi-analytical Approach for Estimating Optimal Solvent Use in Solvent Aided SAGD Process

Abstract SAGD process is widely used to recover heavy oil and bitumen from formations where no other recovery method is proven to be economical. It is an energy intensive process and due to economic and environmental reasons, solvents as additives to the injected steam are currently being explored to reduce energy and emissions intensity of SAGD. The Solvent Aided Process (SAP), tested in field and described in literature, is one such attempt. In SAP, a small amount of hydrocarbon solvent is introduced as an additive to the injected steam. The viscosity of the oil thus is also reduced due to solvent dilution in addition to heating. SAP can significantly improve the energy efficiency of SAGD, thus reducing the heat requirement as shown in field trials discussed elsewhere. However on the use of right amount of solvent that can result in best overall performance, there is very little discussion in the literature. Due to the high cost of such solvents there is incentive to optimize their use in SAGD. Recently a couple of authors have attempted to address the subject by, for example, using arbitrary time-dependent schemes of solvent injections and assessing their impact on results or treating the internal reservoir dynamics as black-box and using optimization methods such as genetic algorithm to estimate optimal amount of solvent. While these approaches orient us to the problem in a context specific manner, it is believed a generalized treatment to estimate optimal use of solvent requires a mechanism-based understanding. The approach presented in this paper aims to estimate the optimal solvent in SAGD context. It breaks down the process in its relevant theoretical elements, i.e., a sequential frontal advance with progressive front renewal on account of gravity drainage. This in turn facilitates investigation of a relationship between phase behavior of the solvent, its amount, and the performance of the recovery process. The results are discussed for a few light-alkane solvents. Better optimal solvent estimates promise to shed light on the supply logistics of these solvents, improve the overall economics and reduce the energy requirements of the heavy oil and bitumen recovery projects.