Gelation kinetics of hydrogels based on acrylamide–AMPS–NVP terpolymer, bentonite, and polyethylenimine for conformance control of oil reservoirs

Abstract

Relatively smaller volumes of gelling systems had been used to address conformance problems located near the wellbore in oil reservoirs with harsh temperature and salinity conditions. These gelling systems were formulated with high concentrations of low-molecular-weight acrylamide-based polymers crosslinked with polyethylenimine (PEI). However, for in-depth conformance control, in which large gelant volumes and long gelation times were required, lower-base polymer loadings were necessary to ensure the economic feasibility of the treatment. In this study, a gelling system with high-molecular weight 2-acrylamido-2-methylpropane sulfonic acid (AMPS), N-vinyl-2-pyrrolidone (NVP), acrylamide terpolymer, and PEI, with the addition of bentonite as a filler, was formulated. The influence of the gelant formulation and reservoir conditions on the gelation kinetics and final gel strength of the system was investigated through bottle tests and rheological tests. The addition of clay in the formulation increased the gelation time, thermal stability, and syneresis resistance, and slightly improved the final gel strength. Furthermore, samples prepared with polymer and PEI concentrations below 1 wt %, natural bentonite, and PEI with molecular weight of 70,000 kg/kmol and pH of 11: (i) presented good injectivity and propagation parameters (pseudoplastic behavior and viscosity ~25 mPa·s); (ii) showed suitable gelation times for near wellbore (~5 h) or far wellbore (~21 h) treatments; and (iii) formed strong composite hydrogels (equilibrium complex modulus ~10–20 Pa and Sydansk code G to H) with low syneresis and good long-term stability (~3 to 6 months) under harsh conditions. Therefore, the use of high-molecular-weight base polymer and low-cost clay as active filler seems promising to improve the cost-effectiveness of gelling systems for in-depth conformance treatments under harsh conditions of temperature and salinity/hardness.