Equation of State Models Phase Behavior of Systems Under Reservoir Conditions Available to Purchase

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 218145, “Modeling Phase Behavior of Solvents/Water/Heavy Oil Systems Under Reservoir Conditions With the PC-SAFT Equation of State,” by Yunlong Li, SPE, Desheng Huang, and Xiaomeng Dong, SPE, University of Regina, et al. The paper has not been peer reviewed.

In this work, a perturbed-chain (PC) statistical associating fluid theory (SAFT) equation of state (EOS) has been developed to characterize heavy-oil-associated systems containing polar components [e.g., dimethyl ether (DME) and water] and nonpolar components [e.g., carbon dioxide (CO₂) and nitrogen (N₂)] with respect to their phase behavior and physical properties. The proposed model shows its superior performance over the widely used Peng-Robinson EOS with a root-mean-squared relative error (RMSRE) of 2.93% for the predicted saturation pressure (P_sat) of the aforementioned systems. The theoretical model proposed in this study reproduces accurately experimentally measured phase behavior and physical properties under reservoir conditions.

The primary objective of this work is to develop a PC-SAFT EOS to accurately quantify the phase behavior and physical properties of heavy-oil-associated systems consisting of diverse components, including polar elements and nonpolar components. Experimentally, constant composition expansion (CCE) tests have been performed to measure P_sat, phase volume, and phase compositions within CO₂/heavy oil systems, N₂/heavy oil systems, and DME/heavy oil systems in the absence and presence of water. Theoretically, the PC-SAFT EOS is integrated to reproduce accurately the measured P_sat and physical properties observed in the aforementioned systems by using the single-carbon-number (SCN)-type approach along with temperature-independent binary interaction parameters (BIPs).

For the CCE tests, a heavy oil sample with a molecular weight of 482 g/mol was used. Synthetic brine was formulated with 2 ppm of dissolved mineral salt. The purities of CO₂, N₂, and DME used in the study were 99.998 mol%, 99.998 wt%, and 99.5 wt%, respectively.

The six CCE tests included the measured P_sat and phase volumes sourced from both the present work and previous investigations. These CCE tests contained two CO₂/heavy oil systems, two N₂/heavy oil systems, and two DME/heavy oil systems in the absence and presence of water. The CCE experiments were conducted at temperatures of up to 433.15 K and pressures not exceeding 20 MPa.

PC-SAFT EOS.

The PC-SAFT EOS is articulated as a summation of residual Helmholtz free-energy terms, originating from diverse molecular interactions within a scrutinized system.

The truncated version of the PC-SAFT EOS (tPC-PSAFT EOS) stands as a straightforward yet precise engineering model. Like the PC-PSAFT EOS, the tPC-PSAFT EOS uses the formulas introduced in the literature to manage dipolar and quadrupolar interactions, ensuring computational accuracy; however, it further simplifies certain aspects tailored to industrial needs.

The application of these models to mixtures involves the use of suitable mixing and combining rules for various parameters. Different mixing rules are available for the same parameters, each of which has its own advantages, disadvantages, and scope of applications. The choice of mixing rules is crucial for a given mixture.