Abstract
This paper gives some recommendations of the pertinent rheological measurements for an efficient gel screening in water shut off and conformance improvement treatments. The proposed methodology is based on measurements of 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).
A laboratory study has been carried out with two polyacrylamide/chromium (III) formulations, 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 have been used in some specific field applications. A series of viscosity, storage modulus and yield stress measurements on the gelling solutions are presented in this paper, with the purpose of quantitatively characterizing the gelation kinetics and final strength of the gel formed. The proposed methodology is utilized for investigating in detail the influence of shearing on the gelation time and final gel strength. In practice, the gel formulation is in fact subjected to hours of strong shearing in the surface facilities (i.e., pumps, tubings….) and in the near-wellbore part of the formation before being left under quiescent conditions (well shut-in).
The results demonstrate that shearing has for effect 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, strongly 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.
