Polymer Gels Formulated With a Combination of High-and Low-Molecular-Weight Polymers Provide Improved Performance for Water-Shutoff Treatments of Fractured Production Wells

Abstract

SummaryA laboratory study has shown improved performance for fracture-problem water-shutoff polymer gels that are formulated with a combination of high-and low-molecular-weight (MW) polymers. These gels are intended for application to fractures or other high-permeability anomalies that are in direct contact with petroleum production wells. More specifically, we focused on evaluating the mechanical strength and improved performance of these water-shutoff gels for use when exceptionally large fracture apertures or large drawdown pressures are encountered. During our study, the gels were injected into laboratory-scale fractures while the gel was in a partially formed state. The flooding-experiment study involved the placement of partially formed chromium(III)carboxylate/acrylamide-polymer (CC/AP) gels in 1-to 4-mm (0.04-to 0.16-in.) apertures, by 2-ft-long × 1.5-in.-height fractures where the fracture walls were 700-md unfired Berea sandstone.During the injection of a 1.5% high-MW and 2.0% low-MW polymer-gel formulation, the partially formed gel fluid exhibited an effective viscosity of approximately 500 cp during placement in a 1-mm (0.04-in.) -aperture fracture, and the matured gel exhibited exceptionally good fracture-plugging characteristics. The gel withstood 52-psi total differential pressure across the fracture length (26-psi/ft pressure gradient) for 24 hours, while permitting no detectable brine flow through the gel-filled fracture. Subsequently, when the differential pressure was increased to 175 psi (88-psi/ft pressure gradient), the gel rendered a brine permeability-reduction factor in the fracture of 30,000. When placed in a 4-mm (0.16-in.)aperture fracture, a 25-psi/ft critical pressure gradient was required to render first and limited brine flow through the fracture containing gel of the same composition. After exceeding the critical pressure gradient, the stabilized permeability-reduction factor imparted by the gel to brine flow in the fracture was 260,000. When increasing the brine flow rate through a gel-containing 4-mm fracture from 500 to 8,000 cm3/hr (superficial velocities of 260 to 4,100 ft/d in the open fracture), the stabilized permeability-reduction factor decreased from 100,000 to 39,000.The high-and low-MW CC/AP gels exhibited disproportionate permeability-reduction (DPR) effects during oil and brine flow through gel-filled fractures. However, the gels of this study are probably better characterized as total-shutoff or sealing agents (not DPR agents) because of the large permeability reduction imparted to oil flow through the gel-filled fractures.