Offshore Nanofluid Injectivity and Huff-n-Puff Field Trials in Japan

Summary Enhanced oil recovery using nanoparticles (Nano-EOR) is an improved waterflooding assisted by nanoparticles dispersed in the injection water (nanofluid). Numerous laboratory studies have revealed the effectiveness of Nano-EOR. However, the comprehensive study, including the field pilot of Nano-EOR is still extremely limited. In this paper, we present the overview of laboratory studies towards the field application and the insights obtained from the field injectivity trial with Huff-n-Puff, which is the first offshore nanoparticle injection in Japan. Laboratory tests (e.g., core flooding, compatibility tests, and water injection tests) were conducted according to the workflow proposed by Kaito et al. (2020) using the target reservoir fluids. The Huff-n-Puff nanofluid injectivity trial was designed and performed based on the laboratory tests. The objective of this pilot trial was to investigate the effects of the nanoparticles on the well and to validate the potential of oil increment. In the injection stage, the concentration of nanoparticles in the nanofluid injected into the target reservoir was maintained at 0.5 wt.%, which was the concentration optimized in a series of laboratory studies, by the chemical pump. The injection fluid was sampled and analyzed appropriately during the injection period to confirm that the target concentration was achieved. Bottom hole pressure was also measured to obtain the injectivity index. The production was performed by gas lift operation after the soaking stage. The Huff-n-Puff nanofluid injectivity trial was conducted twice during two years. A total of 7.0 tons and 8.1 tons of nanofluid were injected, respectively. While it was confirmed that the target concentration of nanoparticles was attained, the injectivity index was decreased in both trials. It is possible that the decrease in the injectivity was caused by the unique phenomenon of nanoparticle traffic jam. On the contrary, an increase in oil production was observed in the production stage. This observation suggested that the injected nanoparticles mobilized the residual oil existing near the wellbore and recovered it. The injectivity index, which was decreased due to nanofluid, was recovered by flow-back, i.e., production at the “puff” stage. This paper provides a comprehensive discussion about the decrease in injectivity and the increase in oil production observed in a series of field trials. In addition to that, lessons learned from the field pilot are identified and discussed to further enhance the Nano-EOR performance in tough offshore environments.