Optimization of Hydrolyzed Polyacrylamide/Chromium (III)-Acetate Gel-Plugging Process after Preflush Crosslinker in Fractured Extralow Permeability Reservoir at Moderate Temperature

Summary Hydrolyzed polyacrylamide/chromium III [HPAM/Cr (III)]-acetate gel treatment is an effective way for conformance control and water shutoff in various mature reservoirs around the world. However, it encounters severe challenges in the fractured extralow permeability reservoirs with the performance varying between success and failure when channeling caused by through-type fracture exists. The through-type fracture channel that connected injection to production is formed by the connection of hydraulic and natural fractures. This research takes the extralow permeability reservoir in the Ordos Basin as the background, and under the characterization of HPAM/Cr (III)-acetate gel, the effect of a preflush crosslinker on improving gel-plugging performance was studied via experiment, and the corresponding gel-plugging process was optimized. Experimental results showed that the preflush crosslinker could effectively improve the blocking strength and stability of HPAM/Cr (III)-acetate gel for through-type, large-opening fractures. Moreover, a high-quality “gel wall” was formed based on the preflush crosslinker; it worked as a barrier within the fracture and was the key to successfully blocking the millimeter-opening fracture. Under the experimental conditions, the optimized plugging process was as follows: The crosslinker was preflushed 24 hours in advance, and the gelant was injected in three slugs, with the volume of the first slug being 0.5 pore volume (PV). A field trial conducted in Ansai Oil Field demonstrated the potential of HPAM/Cr (III)-acetate gel and its plugging capability of optimized plugging method based on the preflush crosslinker to block through-type water channeling. This research provides valuable experimental data and theoretical guidance for conformance control and water shutoff of HPAM/Cr (III)-acetate gel treatment in fractured extralow permeability reservoirs.