Mechanical Degradation of Partially Hydrolyzed Polyacrylamide Solutions in Unconsolidated Porous Media

Introduction A number of recent papers have addressed the problem of mechanical degradation during injection problem of mechanical degradation during injection into oil reservoirs for secondary or tertiary recovery applications. Ref. 6 introduces and tests a mechanism for mechanical degradation of partially hydrolyzed polyacrylamide solutions and develops a procedure for predicting loss of mobility control in practical situations. The correlation of experimental degradation data on which this procedure depends is based on results of flow procedure depends is based on results of flow through consolidated sandstones only. Porosity was not a variable. Since many applications involve unconsolidated reservoirs, this paper investigates the effects of porosity, permeability, length, and flow rate on mechanical degradation of partially hydrolyzed polyacrylamide solutions in unconsolidated sand packs. A new correlation fitting both types of porous media is developed. The aforementioned correlation (Fig. 4 of Ref. 6) for screen-factor loss in saline polyacrylamide solutions depended on porosity through the correlating group, epsilonLD 1/3. However, the generality of the correlation with regard to porosity dependence was untested, since all the media used to induce degradation (mostly Berea outcrop sandstone) had a porosity of about 24 percent. Subsequent porosity of about 24 percent. Subsequent investigations have been conducted in sand packs with 600-ppm polyacrylamide concentrations in 3.0-percent NaCl plus 0.3-percent CaCl2 to test the porosity dependence and provide more realistic mechanicaldegradation data for application to unconsolidated reservoirs. EXPERIMENTAL PROCEDURE The polymer used was from the same commercially available stock used in Ref. 6, having an estimated average molecular weight between 5 and 7 million and a 20-percent degree of hydrolysis. Sand was packed by sifting into a brine-filled lucite cell designed to eliminate effects of possible degradation caused by a plastic retaining screen at the outlet face. Sand-grain density was assumed to be 2.65 gm/cc, and porosities were determined from weight/volume measurements of sand packed in a brine-filled graduated cylinder. Various sand-grain size fractions were obtained by dry-sieve separation on three different sand sources. The sand packs are described in Table 1. Notice that Sand Packs 1, 3, and 4 were obtained from narrow size ranges, while Sand Pack 5 was a deliberate, broad distribution. RESULTS Experimental screen-factor and viscosity losses induced by flow through the sand packs are analogous to those in Ref. 6 for consolidated sandstones; however, the curves are shifted to larger fluxes (volumetric flow rate divided by cylindrical cross-sectional area) because of higher permeabilities. Plotting screen-factor losses as a permeabilities. Plotting screen-factor losses as a function of the correlating group, epsilonLD 1/3, yields the curves in Fig. 1. The consolidated-sandstone correlation curve from Ref. 6 is reproduced here for comparison. Screen-factor losses resulting from mechanical degradation in unconsolidated porous media occur at larger values of epsilonLD 1/3 than in consolidated sandstones and are not well correlated; that is, this correlating group does not allow all screen-factor-loss data to converge on a single curve. SPEJ P. 172