Offshore Polymer/LPS Injectivity Test with Focus on Operational Feasibility and Near Wellbore Response in a Heidrun Injector

Abstract

Abstract Increased oil recovery technologies are very important for the large Heidrun field located in the Norwegian Sea. Very harsh climate and stringent environmental regulations render enhanced oil recovery (EOR) projects challenging. One EOR method evaluated for the field is polymer/LPS (Linked Polymer Solution) with the objective to improve the reservoir sweep. An offshore polymer and LPS injectivity test was performed with the objectives to determine the logistical and operational feasibility of handling polymers and to observe the near wellbore response during polymer and LPS injection. Prior to the offshore test, laboratory work was performed to qualify a polymer and a LPS system. A yard test was conducted to test necessary instrumentation and mixing/pumping equipment in preparation for the offshore test. The operation was performed by mixing a 5000 ppm polyacrylamide mother solution onshore, which was then shipped and stored offshore in completion tanks on the platform. The polymer mother solution was then injected downstream the choke on the injection well at a final polymer concentration of 300 ppm and 600 ppm, respectively. The test was done with a stepwise rate increase to switch from matrix to fracture injection. Aluminum citrate (AlCit) cross-linker was added at 300:10 and 600:20 polymer-to-cross-linker ratio. A comprehensive sampling and analysis program was performed both onshore and offshore to secure high-quality polymer solution throughout the logistical chain. High-pressure sampling equipment was constructed to sample polymerized injection water at the wellhead. Furthermore, high-quality injection data was obtained through installation of a bottomhole memory gauge into the injector prior to the test. The main results from the offshore test proved the feasibility of storing, mixing, transport and injection of polymer solutions without contamination or unwanted shear degradation of the polymer. Good mixing was obtained by feeding the polymer mother solution into the injection stream. Polymer sampling at the wellhead showed good polymer quality based on viscosity and filter ratio results. Well injectivity improved under polymer/LPS injection compared to water injection due to the opening of existing fractures at lower injection rates.