Mechanical Degradation of Polyacrylamide Solutions in Core Plugs From Several Carbonate Reservoirs

Abstract

Summary This paper presents results of laboratory studies on mechanical degradation of partially hydrolyzed polyacrylamide (HPAM) solutions in core samples from several different carbonate reservoirs. Studies in carbonates are of particular importance because several of the larger polymer projects are in carbonate reservoirs and because data on carbonate rocks have not previously been available. Many of the data in the literature were obtained with solutions of polymer-in-brine. Because HPAM polymers are generally more efficient in fresh waters, lower-salinity waters were used for the bulk of the present study. Investigated variables included polymer molecular weight, polymer concentration, and salinity of the aqueous solvent. HPAM samples used were commercial products of various molecular weights. Results were compared with correlations previously developed for sandstone core plugs and unconsolidated sands. The extent of induced degradation was monitored by comparison of viscosities and screen factors before and after flow through the core plugs at various flow rates. During injection of low-salinity polymer solutions through low-permeability carbonate core plugs, the extent of induced mechanical degradation was higher than values estimated with existing correlations obtained with higher-permeability porous media. Severe nonuniformities in the low-permeability carbonate core caused difficulties in our attempts to relate the degradation of existing correlations. Thin sections of the core materials were very helpful in the qualitative understanding of the differences in injectivity and mechanical degradation for cores from the various reservoirs. Cores with relatively uniform porosity performed differently from those that were very nonuniform, even though the permeabilities were similar. The induced mechanical degradation of the polymer reduced the viscoelastic and pseudoplastic nature of the HPAM solutions. As expected, the extent of mechanical degradation increased as the polymer molecular weight increased. Residual viscosities and screen factors of the sheared solutions, however, were often greater for the higher-molecular-weight HPAM polymers. Higher salinities or calcium concentrations of the solvent resulted in increased degradation. Thus, use of low-salinity water or softened fresh water can reduce the level of irreversible shear degradation.