In last two decades, the steam assisted gravity drainage (SAGD) has been successfully implemented for increasing the oil recovery from bitumen reservoirs (Butler et al. 1981, Butler and Stephens 1981). This paper describes a detailed study of compositional variation in a SAGD process.

A series of 2D numerical simulations are conducted based on the generic Athabasca type reservoirs. The fluid model provided in this study developed from an extensive work on the experimental analysis and fluid characterization modeling of the Athabasca and light solvent mixtures. In the equation of state (EOS) model, Bitumen is represented by 5 pseudo-fractions ranging from light, medium and heavy to extra-heavy components. Moreover the steam based Kvalues of bitumen fraction is developed and considered in this study.

The change in the compositional variation in the edge oil-flow zone are briefly studied in this work. Based on the analysis of compositional variation, we found the presence of vaporizing-condensing mechanism (VCM) as the second order effect that helps to increase the oil recovery from a SAGD process. The viscosity profile at the edge oil-flow zone and at the residual oil behind the interface is changed when the VCM is presented. The VCM effect can be more significant if the SAGD process is examined under different type of heavy oil and bitumen samples. The effect of the oil viscosity heterogeneity (the oil viscosity increases with depth) on the performance of SAGD process is studied. The assumption of uniform oil viscosity throughout the bitumen (heavy oil) reservoir may not be representative of the measured oil viscosity in the field. The presence of the highly nonlinear vertical composition and fluid viscosity gradient in bitumen deposits is mainly due to severely biodegraded oils. Extensive degradation creates a highly nonlinear vertical viscosity variation with a large viscosity gradient at the bottom of the formation. We develop an empirical correlation which account for the nonlinearity of the viscosity variation versus depth.

The novel of this present work is introducing VCM as the second mechanism which has second order effect in increasing heavy oil recovery. Lack of accurate EOS model or inadequate number of pseudo components in the EOS model may neglect the VCM term and lead to underestimate the ultimate oil recovery e.g. ~10% of oil recovery is only gained by VCM for heavy oil with API of 14.

The Effect of Bitumen characterization and viscosity heterogeneity in a SAGD process is extensively studied in this work. Moreover, the results show that the effect of VCM in increasing oil recovery for the viscosity gradient reservoir is even more significant compared with the homogeneous viscosity. Certainly, the results of this work is essential to understand the true physics of the SAGD process in the compositional variation of the heavy oil reservoirs.

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