Controlling Gelation Time and Microgel Size for Water Shutoff

Abstract

Abstract Injecting stable preformed microgels as relative permeability modifiers to reduce water production is an attractive new procedure to minimize the risk of formation plugging and consequently of inefficiency of in-depth treatments. This paper describes the mains results of theoretical and experimental investigations carried out to know how to control both size and conformation of microgels formed under constant shear flow. Since gelation kinetics strongly depends on cross linker chemistry, Zr K-edge XANES and EXAFS studies of zirconium speciation were performed under the complex conditions of polymergelling for water shut off. The results show that crosslinking species may bedimers, tetramers and associations of tetramers depending on pH and Zr concentration in presence of lactate. In polymer gels, Zr monomers were observed. The generalisation of our crosslinking-under-shear theory describedin this paper provides simple power laws. The microgels formed in diffusion regime are isotropic and their size decreases significantly as shear rate increases, while when formed in correction regime, they are an isotropic and their size decreases negligibly with shear rate. All experimental data are inagreement with this theory so that conclusions can be derived to optimize microgel preparation as a function of their role in the aimed application, either relative permeability modifiers for water shutoff or viscosity enhancers for polymer flooding. Introduction Controlling water production from oil producing wells become an increasingly important goal for oil industry. This is partly due to environmental regulations which act as strong incentives to reinject produced water. Among the methods proposed to reduce water production (1), injecting polymer gels iseconomical and consequently attractive. However, gel treatments are not easily controlled (2, 3). The main reason is that both petrophysical an physico-chemical characteristics of the producing layers in the vicinity of the well bore cannot be determined with the precision required to predict reliablyin-situ gelling process. In addition, the physics of "gelling under shear" (4,5) as well as the chemistry of most cross linkers in aqueous solutions is complex and still not enough well known. This is true not only for zirconium IV/lactate (4, 6, 7) but also for chromium III/acetate (8–12) systems. However, what we know very clearly today is that injecting in a porous medium a polymer solution containing a cross linker before the end of crosslinking process, results in a very severe plugging if injection is maintained for a long time. Such a plugging is theoretically expected and has been observed for both chromium (9, 10, 12) and zirconium (4) based systems.Two mechanisms, possibly acting together, have been proposed to explain such a plugging. The first one is the bridging of pore throats by microgels formedin-situ and growing in size as they move through the porous medium. Thisresults in an in-depth plugging at some distance from core entrance (9). Thesecond one is a pore throat bridging resulting from the progressive increase in thickness of a gelled surface layer (4). Its thickness increases by newcrosslinks between the free flowing macro molecules and that which are directly adsorbed or previously fixed by crosslinking. Such plugging is expected when polymer adsorbs onto the surface, which is the normal outcome with polymers having a high degree of sulfonation (14). A very strong gel is formed on thepore surface because the newly injected cross linker molecules are small enoughto penetrate inside the adsorbed layer causing the highest possible crosslink density. It should be emphasized that a strong plugging near the formation entrance prevent the polymer gel to penetrate into the formation at distances large enough for the treatment to be efficient. Consequently, formation plugging and inefficiency of the treatment are linked. It must be noted that this type of plugging is the direct consequence of the procedure currently usedin field operations. Indeed, it is usual to mix together a polymer solution and a cross linker into surface facilities and to inject immediately this "live" mixture into the well to be treated.