Evaluation of Polymer Flooding in a Layered Reservoir With Crossflow, Retention, and Degradation
Authors: Vela Saul , Peaceman D.W. , Sandvik E.I. 
Published: December 2007

Oil recovery by polymer flooding is influenced by reservoir heterogeneity not only through its effects on crossflow, but also through the dependence of polymer retention, inaccessible pore volume, and polymer retention, inaccessible pore volume, and polymer shear degradation on permeability. This polymer shear degradation on permeability. This paper describes a simulator that accounts for all paper describes a simulator that accounts for all these factors and includes experimental data, obtained using field and Berea cores, that demonstrate the permeability dependence of these factors.

Use is made of a two-dimensional reservoir simulator with the added feature of point-tracking capability. For each of the different layers, one chain of points is identified with the front of the polymer bank and another chain with the rear of the polymer bank and another chain with the rear of the bank. Retention and inaccessible pore volume can be accounted for through a factor that is the ratio of the velocity of the moving points to the wetting-phase velocity. Rheology and degradation are introduced by alterations in wetting - phase viscosities at the leading edge and at the trailing edge of the bank. Each layer can have a different resistance factor, retention, inaccessible pore volume, and shear degradation. The flow equations used in the simulator, as well as the method of exploitation of the point-tracking feature, are given in detail.

A number of cases are presented in which the sensitivity of oil recovery to variations in polymer retention, degradation, inaccessible pore volume, and reservoir heterogeneity was studied. Results show that oil recovery is sensitive to these variables and that using less realistic assumptions can give significantly different results.

Introduction

For many reservoirs, polymer flooding is an attractive alternative to conventional waterflooding, since relatively minor modifications need be made to a waterflood to enable polymer injection and recovery of additional oil. Addition of polymers to flood water can increase oil recovery not only by improvement in vertical and areal sweep, but also by alteration of the water-oil fractional flow properties toward more efficient oil displacement. properties toward more efficient oil displacement. A commonly accepted technique for evaluating polymer flood potential for a particular reservoir is polymer flood potential for a particular reservoir is by simulation of waterfloods and polymer floods under varying conditions and comparison of resulting oil recoveries. For this simulation technique to realistically portray reservoir response to polymer, it must include all significant properties of the polymer-reservoir system. Several authors have polymer-reservoir system. Several authors have described polymer characteristics important to evaluation of a flood prospect, as well as techniques for their simulation.

The method for simulating polymer flooding presented here, while similar in many ways to presented here, while similar in many ways to previous techniques, differs in several respects. previous techniques, differs in several respects. Bondor et al. solve the fluid flow equations numerically in three dimensions, consider three-phase flow, and simulate thickened water as a fourth phase. They do not, however, simulate the dependence of retention and resistance factors on permeability, nor do they consider degradation. Jewett and Schurz describe a layered model with retention and resistance factors dependent on permeability, but do not include capillarity, permeability, but do not include capillarity, crossflow, or gravity effects.

POLYMER FLOW TESTS POLYMER FLOW TESTSPolymer flow test measurements included retention, resistance factor during polymer flow, and residual resistance factor to brine following polymer. A partially hydrolyzed (about 20 percent), polymer. A partially hydrolyzed (about 20 percent), high-molecular-weight (about 5.5 × 10) polyacrylamide was chosen for the tests. Preliminary polyacrylamide was chosen for the tests. Preliminary measurements in Berea outcrop material, as well as other reported results, indicated that a polyacrylamide of relatively high molecular weight polyacrylamide of relatively high molecular weight would be required to attain significant resistance factors in the highest-permeability cores. Significant effects of brine salinity level upon polymer flow properties have been reported. properties have been reported. SPEJ

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