Characterization of Partially Formed Polymer Gels for Application to Fractured Production Wells for Water-Shutoff Purposes

Abstract

Abstract A laboratory study was conducted to characterize water-shutoff polymer gels that are injected in the partially formed (partially matured) state into fractures (or other high permeability anomalies) that are in direct contact with production wells. Partially formed (<8-hr-old) 1X (0.5% polymer) chromium(III)-carboxylate/acrylamide-polymer (CC/AP) gels showed much lower (as much as 100 times less) effective viscosities (17 to 30 cp) during placement in a 1-mm-wide fracture than "fully formed" (>15-hr-old) gels with the same chemical composition. Thus, partially formed gels exhibit substantially higher injectivities and lower placement pressures. This feature is of major importance during field applications where pressure constraints limit rates and volumes during gel injection. For gelants and partially formed gels that were 5 hours old or less, the rates of gelant leakoff through fracture faces were very low [about 0.013 ft3/ft2/d (ft/d)]. Thus, field applications that inject relatively small volumes of gelant or partially formed gels will generally experience small gelant leakoff distances, and the leakoff substance will not significantly inhibit oil from entering the fractures. During first brine injection after gel placement and maturation in 1-mm-wide fractures, the pressure gradient required to first breach the gel increased significantly with increasing polymer concentration in the gel — ranging from roughly 5 psi/ft for 1X (0.5% polymer) partially formed gels to 99 psi/ft for 3X (1.5% polymer) partially formed gels. For 1X gels, the breaching pressure gradient was greatest (~9 psi/ft) when the gel was aged from 12 to 24 hours before injection. Prior to exceeding the breaching pressure gradient, no detectable brine flowed through the fracture. During the limited brine flow after gel placement, most (>90%) of the gel remained in the fracture and did not "washout." The stabilized residual resistance factors (permeability reduction factors) for the first brine flood through the fracture (following gel placement and maturation) ranged from 750 to 22,000 – increasing with increasing polymer concentration and gel strength. The large stabilized (final and equilibrium) residual resistance factors for brine flow through the gel-filled fracture resulted from the brine flow occurring through relatively small channels (wormholes) residing in the gel. For the 1X gel, the stabilized permeability reduction factors (for brine flow in a gel-treated fracture) were comparable for formulations injected in the gelant state, the partially formed state, and the "fully formed" state. The CC/AP gels exhibited disproportionate permeability reduction during brine and oil flow through gelfilled fractures. During one experiment with the 1X gel, brine permeability in the fracture was reduced 166 times more than that for oil. In this case, brine was flooded first, followed by oil. For the 1X and 3X gels, the permeability reduction factor for oil flow remained constant (within experimental error) during four cycles of brine and oil injection. In contrast, the permeability reduction factor for brine decreased more than a factor of 10 during these cycles.