Effects of salinity and temperature on drag reduction characteristics of polymers in straight circular pipes

Abstract

It has been nearly 60 years since Toms [Toms, B.A., 1949 first discovered the tremendous reduction in wall shear stress caused by the addition of small amount of linear macromolecules to a flowing fluid. This drag reduction phenomenon corresponds to an increase in pumpability of a fluid by the additive. Therefore the application of this phenomenon can be found in many areas of the oil and gas industry; including well completion, workover, drilling operations, and hydraulic fracturing. The turbulent flow prevails in such operations. The eddy currents and velocity fluctuations in turbulent motion produce increased pressure drops and thus, limit the maximum obtainable fluid flow rate. This results in a problem particularly in hydraulic fracturing where higher flow rates are desired. Employing the commercially available drag reducing fluids can overcome this limitation. Salinity and temperature are among the factors that affect the extent of drag reduction. Other factors include polymer type and concentration, molecular weight and distribution, solvent chemistry, pH, ionic strength, molecular conformation, flow geometry, and degree of mechanical shearing. Despite the widespread use of brine solutions as base fluids in fracturing the deep hot formations, the effects of temperature and salinity on the drag reduction characteristics of polymers have received the least attention. Most studies on drag reduction have considered either water or salt-free base fluids at ambient temperatures. Therefore, the aim of the present study is to experimentally investigate the effects of salinity and temperature on the extent of drag reduction of polymers flowing in straight circular pipes. Two commonly used anionic polyacrylamides; Nalco ASP-700 and ASP-820 are used in this study: ASP-700 is an analog of partially hydrolyzed polyacrylamide (PHPA), while ASP-820 is a sulfonate-containing polyacrylamide. A 1/2-in. OD flow loop with a 15-ft straight tubing is used. Flow tests are conducted to determine the optimum concentrations of ASP-700 and ASP-820 polymers. It is found that the optimum concentration for both polymers is 0.07% (vol.). To investigate the effect of temperature, tests are conducted with fresh water at 72, 100 and 130 °F. Additional tests are conducted at ambient temperature and with 2% KCl brine and synthetic sea water to investigate the effect of salinity on drag reduction of ASP-700 and ASP-820. It is found that both polymers are quite effective drag reducers in circular straight pipes. Drag reduction in the range of 30–80% is noticed. Both temperature and salinity have significant effect on their drag reduction characteristics. Due to differences in their composition and potentially molecular weight variances, both polymers show different characteristics in the presence of salt and at elevated temperatures. Within the conditions studied, ASP-700 is found to be better than ASP-820 at providing drag reduction. Correlations to predict drag reduction at either temperature or salinity are developed. These results are presented and discussed in detail.