import pandas as pd
df1 = pd.DataFrame({'Well_n': ['A', 'B', 'C', 'D'],
                'Qo_rate': [200, 150, 170, 0],
                'year': [2001, 2002, 2003, 2004]})
df2 = pd.DataFrame({'Well_n': ['A', 'C', 'B', 'D'],
                'Well_ECL': ['P1', 'P3', 'P2', 'P4']})
df1.set_index('Well_n',inplace=True)
df2.set_index('Well_n',inplace=True)
joined = df1.join(df2,on='Well_n')
print(joined)

joined.set_index('year')
  Qo_rate Well_ECL