Evaluation of Polymer Flooding in a Layered Reservoir With Crossflow, Retention, and Degradation

Abstract

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 shear degradation on permeability. This 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 0 f 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 wettingphase 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, inaccessibl e 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.

Aperture-Tolerant, Chemical-Based Methods to Reduce Channeling

“…In addition to the expense of requiring large gelant volumes, this situation may be limiting because many polymer-based gelants cannot penetrate into tight rock. 18 On a positive note, the case for Entry 10 (using gel with 0.4% HPAM in 40-md Berea) exhibited F rrw = 2,110, F rro = 2.0, and F rrw /F rro = 1,050. These values would be quite acceptable for applications in either fractured or unfractured wells.…”

“…21,14,15,[18][19][20] In other words, these materials damage lowpermeability rock more than high-permeability rock. Depending on the magnitude of this effect, these polymers and gels can harm production flow profiles in wells.…”

“…16 Also, suspensions of gel particles and adsorbed polymers provide resistance factors and residual resistance factors that increase with decreasing permeability. 14,15,[18][19][20][21] Not surprisingly, suspensions of small gel particles are more effective at restricting flow through small pores than through large pores. Straight-forward calculations using the Darcy equation reveal that this behavior can be detrimental for sweep improvement-both during polymer flooding and gel treatments.…”

“…For adsorbed polymers and suspensions of gel particles, resistance factors (F r , apparent viscosities in porous media relative to brine) and residual resistance factors (F rr , permeability reduction values) increase with decreasing permeability. [14][15][16][18][19][20][21][169][170][171] In other words, these materials reduce the flow capacity of low-permeability rock by a greater factor than high-permeability rock. Depending on the magnitude of this effect, these polymers and gels can harm injection or production flow profiles in wells, even though the polymer or gelant penetrates significantly farther into the high-permeability rock.…”

“…During this series of floodingexperiment studies, one must also be concerned about the magnitude of residual resistance factors as a function of rock permeability. 15,[18][19][20][21] This latter comparison should be made using cores that were completely filled with polymer or gel (i.e., NOT using misleading parallel corefloods). If residual resistance factors in low-permeability rock are significantly greater than in high-permeability rock, polymer or gel treatments will impair sweep efficiency (see Fig.…”