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

摘要

A laboratory study has shown improved performance forfracture-problem water-shutoff polymer gels that areformulated with a combination of high and low molecularweight(Mw) polymers. These gels are intended forapplication to fractures or other high permeability anomaliesthat are in direct contact with petroleum production wells.More specifically, we focused on evaluating the mechanicalstrength and improved performance of these water-shutoff gelsfor use when exceptionally large fracture apertures or largedrawdown pressures are encountered. During our study, thegels were injected into laboratory-scale fractures while the gelwas in a partially formed state. The flooding-experiment studyinvolved 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.-heightfractures where the fracture walls were 700 md unfired Bereasandstone.During the injection of a 1.5% high Mw and 2.0% lowMw polymer gel formulation, the partially formed gel fluidexhibited an effective viscosity of roughly 500 cp duringplacement in a 1-mm (0.04-in.) aperture fracture, and thematured gel exhibited exceptionally good fracture-pluggingcharacteristics. The gel withstood 52 psi total differentialpressure across the fracture length (26 psi/ft pressure gradient)for 24 hrs, while permitting no detectable brine flow throughthe gel-filled fracture. Subsequently when the differentialpressure was increased to 175 psi (88 psi/ft pressure gradient),the gel rendered a brine permeability reduction factor in thefracture of 30,000. When placed in a 4-mm (0.16-in.) aperturefracture, a 25 psi/ft critical pressure gradient was required torender first and limited brine flow through the fracturecontaining gel of the same composition. After exceeding thecritical pressure gradient, the stabilized permeability reductionfactor imparted by the gel to brine flow in the fracture was260,000. When increasing the brine flow rate through a gelcontaining4-mm fracture from 500 to 8,000 cm3/hr(superficial velocities of 260 to 4,100 ft/d in the openfracture), the stabilized permeability reduction factordecreased from 100,000 to 39,000.The high and low Mw CC/AP gel exhibited significantdisproportionate permeability reduction (DPR) effects duringoil and brine flow through gel-filled fractures. The magnitudeof the DPR effect decreased with increasing flow rate (anddifferential pressure). The effect also decreased withincreasing number of flooding cycles with brine and oil.