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

Abstract A 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.) aperture, by 2-ft-long, by 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 roughly 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 hrs, 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 gel exhibited significant disproportionate permeability reduction (DPR) effects during oil and brine flow through gel-filled fractures. The magnitude of the DPR effect decreased with increasing flow rate (and differential pressure). The effect also decreased with increasing number of flooding cycles with brine and oil.