Heat oil in a Dutch oven over medium-high heat. Add leeks and cook, stirring often, until soft, about 3 minutes. Stir in sage and continue cooking until aromatic, about 30 seconds. Stir in broth and water, increase heat to high, cover and bring to a boil. Add beans and chicken and cook, uncovered, stirring occasionally, until heated through, about 3 minutes. Serve hot.