Modeling of microflow during viscoelastic polymer flooding in heterogenous reservoirs of Daqing Oilfield

Abstract

Viscoelastic polymer flooding has been extensively used in oilfield development as an enhanced oil recovery method. Understanding the microflow mechanism is necessary to promote the polymer displacement effect and design the polymer flooding scheme. Previous studies have investigated polymer flooding mechanisms, such as a favorable mobility ratio and increasing sweep efficiency; however, the elasticity effect on displacement efficiency is still unclear, particularly in a heterogeneous reservoir. Class II reservoirs, which have an effective thickness between 1 and 4 m and a permeability greater than 100 mD, have recently become the target zones of polymer flooding in the Daqing Oilfield. Class II reservoirs are subdivided into type A and B oil layers. Compared with the type A oil layer, type B has a larger permeability contrast, more severe heterogeneity, and worse connectivity. There are considerable differences between types A and B in the actual development of polymer flooding as well as between type B oil layers of different oilfield blocks. Thus, it is necessary to investigate the factors that influence the micro-oil displacement mechanism. In this study, local and global micro-pore models are established based on a computed tomography scan slice of Class II reservoir cores, and mathematical models of the viscoelastic polymer and oil two-phase flow in porous media are established. The log conformation method is used, which can effectively enhance convergence, owing to a high relaxation time inducing high non-linear of equation. The volume-of-fluid method is used to track the interface between the two phases. The governing equations are solved using the OpenFOAM platform, which is open-source software written in C++. Then, the influences of pore structure and polymer elasticity on displacement characteristics are studied. The simulation results revealed that owing to the pore structure, the micro-oil displacement efficiencies of B_GI and B_PII, which belong to the type B oil layer, are lower than that of A_SIII, which belongs to type A, by 26.8% and 10.9%, respectively. The oil displacement efficiency of a commingled production comprising B_PII and B_GI is 4.0% and 15.6% lower than that of single-layer productions of B_PII and B_GI, respectively. The oil displacement efficiency increases by 6.45% when the relaxation time increases from 0.5 to 2 s and decreases when the relaxation time increases from 2 to 10 s. Therefore, by combining the micro-oil displacement efficiency and injectivity of a high-molecular-weight polymer, the optimum relaxation time is determined to be 2 s. The obtained results are significant for the design strata of a commingled production scheme and the optimization of polymer solutions in the type B oil layer of the Daqing Oilfield.