Drainage of Aqueous Foams: Generation-Pressure and Cell-Size Effects

Abstract

Introduction The physical characteristics of aqueous foams make them excellent candidates for drilling fluids in geothermal formations. A major concern in this application is the foam-forming ability of a surfactant system in the harsh geothermal environment. While investigating the stability of various foams in an autoclave. we observed a dramatic increase in drainage times with increasing generation pressure, even though the liquid volume fraction of the foam was held constant. This effect appears to result from a decrease in cell size with increasing autoclave pressure and therefore is an artifact of the foam generation technique in which the pressure path of the foam changes with autoclave pressure. A theoretical model of drainage predicts that drainage times should increase with decreasing cell size. as observed. These results emphasize that proper control of cell size can have a substantial effect on the apparent stability of a foam. Experimental Procedure The foam is generated in the pressurized autoclave (Fig. 1) by metering surfactant solution and gas through a foam generator consisting of a tube tightly packed with stainless steel wool. A pressure 250 psig (1. 72 MPa) greater than the autoclave pressure pressure 250 psig (1. 72 MPa) greater than the autoclave pressure was used to supply the gas and surfactant solution in all experiments. This technique permits the generation of foam at the desired temperature and pressure with control on the liquid volume fraction. In the autoclave, foam is dispensed into a 10-oz. (311-g) covered glass container. A quartz viewing port permits visual observation. The relative foam stability is assessed by measuring the gravity drainage of liquid from the foam into a graduated cylinder. The time required for 25% and 50% of the original liquid to drain is recorded. Results Initial experiments at room temperature yielded surprising results. For foams produced at autoclave pressures ranging from 1 to 20 atm (0. 1 to 2. 0 MPa), the drainage times increased with pressure by an order of magnitude, as shown in Fig. 2, even though the liquid volume fractions of the foams were essentially identical. Nitrogen gas and an alpha olefin sulfonate foamer were used in these tests. The drainage experiments were repeated for several good foamers of various surfactant types (anionic, amphoteric, cationic, and nonionic) and gas combinations (air, argon, and nitrogen) at 1 and at 20 atm (0. 1 to 2. 0 MPa) (Table 1). As shown, the same systematic increase in drainage times with increasing pressure also was observed in these experiments, suggesting that the effect was not dependent on the chemistry of the gas/liquid interface in the foams. Because of gas compressibility, and the varying pressure profiles in the foam generator, the influence of pressure on the foam structure. specifically the cell size, was explored. Foam Drainage An effect of cell size on foam drainage has been discussed in the literature. SPEJ P. 152