New Size-Controlled Microgels for Oil Production

Abstract

Abstract New size-controlled microgels formed by crosslinking polymers under shear flow are very promizing for various applications in oil production. Indeed, when produced by using a proper polymer/crosslinker system and under the conditions needed to obtain the desired properties, these microgels should be quasi-ideal products. They are expected to control water mobility at long distances from the wells to improve sweep efficiency and reduce selectively permeability to water for water production control. For this latter application, injecting stable, preformed microgels eliminates the risks inherent to in-situ gelling which is a technique now recognized as being very difficult to control. This paper reports the results of new lab experiments conducted to complete our theoretical description of the crosslinking-under-shear process and to test the properties of these microgels in porous media. The actual properties of these microgels are compared to theoretical predictions. The results provide new theoretical insights into microgel formation and show that such microgels 1) have sizes measured directly by Photon Correlation Spectroscopy which are satisfactorily predicted by our model 2) adsorb quasi-irreversibly, forming adsorbed layers having a thickness equal to two times their viscometric radius of gyration, thus, are capable of controlling permeability more efficiently than the polymer alone 3) can be injected in porous media without any plugging tendency 4) have small internal rigidity as suggested by elastic modulus measurements and thus, they should be ideal disproportionate permeability modifiers 5) have viscosity higher than polymers in the dilute regime and extremely high in the semi-dilute regime, and 6) are stable, showing no tendency to re-form larger microgels when ageing, in presence of a suitable stabilizer. Introduction Reducing water production is an important goal for the oil industry, particularly because of new environmental regulations imposing severe limitations for the disposal of produced water. Among the methods available for reducing water production, injecting a polymer solution together with a crosslinker is currently used due to its low cost (1,2). However, this method based on in-situ gelling is very difficult to control because gelation is a process very sensitive to physico-chemical conditions such as pH, salinity and temperature as well as shear rates (3–6). Since these parameters are often very poorly known around the well bore, it is very difficult to understand why certain well treatments fail or succeed so that the possibilities of improving this technique remain questionnable. Controlling water mobility at reservoir scale to improve sweep efficiency is usually achieved by dissolving anionic polyacrylamides in injection water (7). Recent pioneering work (8) showed that the addition of zirconium to hydrolyzed polyacrylamides could create microgels that provide a better mobility control of water than the polymer alone. Interestingly, zirconium is safer than chromium for an environmental aspect. Lately (5,6) we proposed a new technique to produce microgels which consists in crosslinking polymer under shear. By this way our theory predicted that the microgel size was controlled by the shear stresses.