Shear Effects on Polyacrylamide/Chromium (III) Acetate Gelation

Abstract

Summary In practice, gel formulations used in water shutoff and conformance improvement treatments are subjected to hours of strong shearing in surface and well equipment (i.e., pumps, tubings, etc.) and in the near-wellbore part of the formation before being left under quiescent conditions (well shut-in). The purpose of rheological measurements presented in this paper is to assess the influence on polyacrylamide/chromium (III) gelation of shearing sequences representative of those occurring in the field. The gel properties investigated are the gelation time and final yield stress, which are related, respectively, to the pumping time and to the maximum differential pressure the gel can withstand in the reservoir (matrix or fracture). Two different polyacrylamide/chromium (III) formulations were analyzed, one of which, involving a high-molecular-weight polyacrylamide, is adapted for plugging fractures, and the other one, involving a low-molecular-weight polyacrylamide, is usually employed for plugging porous matrices. These formulations had previously been used in some specific field applications. The experiments consisted of a series of viscosity, storage modulus and yield stress measurements on the gelling solutions, with the purpose of quantitatively characterizing the gelation kinetics and final strength of the gel formed. The results demonstrate that the effect of shearing is to restrain gelation and, in particular, to limit the viscosity rise due to the formation of aggregates of crosslinked polymers (microgels). The gelation time, defined in relation to this viscosity rise, increases with shear rate for the high-molecular-weight (non-Newtonian) formulation, but is unaffected by shear for the low-molecular-weight (Newtonian) formulation. Experiments have also been conducted in which an extended initial period of strong shearing was followed by another extended period of rest. The final gel strength of both formulations, evaluated from yield stress measurements, turned out to be independent of the shear rate history.